a simulation investigation of stucco cladding wall system

A SIMULATION INVESTIGATION OF STUCCO CLADDING WALL SYSTEM VAPOR TRANSPORT PERFORMANCE IN A COLD CLIMATE:

PHASE III: HYGROTHERMAL RESPONSE TO INTERIOR WATER SOURCES AND THE DRYING POTENTIAL OF DRAINAGE PLANES

FINAL REPORT

Principal Investigator: Louise F. Goldberg, Ph.D (Eng)

Building Physics and Foundations Research Programs College of Design University of Minnesota.

Project Manager: Steven Pedracine

MN Lath and Plaster Bureau St. Paul, MN

Date: February, 2007 Revision: B

ACKNOWLEDGEMENT AND CERTIFICATION The research described herein has been performed with funding provided by the Minnesota Lath and Plaster Bureau. While this support is gratefully acknowledged, the Principal Investigator assumes complete responsibility for the contents herein.

EXECUTIVE SUMMARY This report is a sequel to the report describing phases I and II of the “Vapor Transport Performance in a Cold Climate” research project. As such, it needs to be read in conjunction with the previous report as descriptions of the methodologies used, boundary conditions and other background information are not repeated herein. This report investigates the following two aspects of stucco cladding system performance:

• The hygrothermal impact on the wall system of an internal moisture source adjacent to the exterior surface of the water resistive barrier (WRB).

• The potential of using an air gap between the WRB and the stucco as a means of drying internally wetted stucco and, in so doing, decoupling the stucco moisture source from the framing system.

The first aspect was studied using the WUFI-2D (version 2.1) simulation program with all its attendant limitations as described in the phase I and II report. Given these limitations, the internal moisture loading was accomplished by setting the initial moisture content of the stucco adjacent to the WRB at a saturated state at the beginning of the simulation year on July 1. The moisture performance was inferred from the resulting relative humidity and moisture content transient temporal profiles as well as from the annual performance summary data used in Phases 1 and II.

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The second aspect was studied using the computational fluid dynamics module of the ALGOR finite element program. The velocity profiles within the air gap were simulated as a function of gap width, top and bottom slot height as well as gap configuration. Three gap configurations were investigated, namely a uniform thickness extending over the full wall height; an extruded polystyrene layer at the bottom of the wall on the interior side of the air gap; and, the same configuration except with the insulation layer on the exterior side of the gap. The simulations of a stucco wall system subject to a transient interior wetting event show that this moisture loading yields a phenomenology that is different from that which occurs with exterior only wetting events that were addressed in phases I and II of this research. While the performance of the base stucco system nominally is adequate under single event transient internal moisture loading, the data indicate a cause for concern as to what could happen with repeated transient internal moisture loading events. Decreasing the vapor coupling between the wet stucco moisture source and the framing cavity (such as replacing 2 layers of grade D building paper with one layer of no. 15 felt) while not changing the rest of the system seems to improve the performance. Increasing the coupling between the framing cavity (such as replacing a warm-side polyethylene vapor retarder with a PA-6 membrane) improves the hygrothermal performance in some respects while exacerbating it in others. Hence implementing both of these modifications together offers the potential of effectively managing internally wetted stucco systems. Taken as a whole, the results do offer some clues as to possible mechanisms for field-observed failures that often defy conventional forensic analysis (such as a patch of sheathing mold in the center of a windowless wall). Using a vented drainage gap of adequate width offers an alternative approach to effectively decoupling the wetted stucco vapor source from the wall cavity system that could be used with a warm side polyethylene vapor retarder. In particular, a drainage gap with 12 in. of extruded polystyrene insulation installed on its bottom interior face shows promise of yielding flow rates adequate to achieve sufficiently robust drying of internally wetted stucco when driven by horizontal temperature gradients alone. However, implementing such a system is likely to engender strong resistance from the stucco community owing to the complexities of achieving a structurally robust stucco system with a relatively large (0.5- 1”) gap between the WRB and the stucco. Conversely, however, current practice with drainage air gaps in the 1/8-1/4” range with bottom slots (or weep screeds) only are not effective as ventilation planes and it is erroneous to claim otherwise.

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A. INTRODUCTION This report is the sequel to a previous report that covered phases I and II of the research project (Goldberg, 2006). As such, this report should be read in conjunction with its predecessor as the detailed information on the simulation methods, weather and material property data contained in the first report are not repeated here. Phase III of the research project was devoted to the following two aspects of the hygrothermal performance of stucco clad wall systems in a cold climate: a. The hygrothermal impact of internal moisture sources The object was to determine the hygrothermal response of the wall system to wetting of the stucco cladding as a result of bulk water rundown within the drainage gap between the water resistive barrier (WRB) and the cladding. Generally, the bulk water rundown can manifest in one of three forms, namely, continuous, plug and free-surface flows. All three will wet the stucco from the inside out, but the last form, namely free-surface flow, has the lowest average velocity (largest residence time) and thus the greatest potential for being absorbed into the stucco. In practice, free-surface flows are the most prevalent (the anecdotally oft-reported absence of any visible liquid appearing at the weep screed). Thus once the stucco has become wet, the hygrothermal response of the wall system will determine whether the stucco can dry out without producing condensation related failures caused by exterior to interior vapor flux, particularly during the cooling season. b. The drying potential of the drainage plane Given the presence of a drainage plane, even of negligible width that can be effective in draining continuous flows at least1, the question arises as to whether such a gap can also function as a drying plane by introducing a buoyant, natural convection air flow into the gap. This entails having the gap open at the top and the bottom to the ambient surroundings via slots in the stucco surface. In order to answer this question, a series of computational fluid dynamics analyses of a few selected drainage gap / air slot width combinations together with two alternate gap configurations have been performed during the cooling season. These simulations give a sense of whether using the drainage plane as a drying mechanism for dissipating internal water sources is a physically reasonable proposition. B. PHASE III OBJECTIVES 1. Using the methods of phases I and II, the impact of an internal water source (that is, the stucco

system becoming wet by absorbing water from the drainage plane) on the condensation / vapor transport performance of the wall system was evaluated using the WUFI-2D hygrothermal simulation program and the following phase II configurations and boundary conditions (BC): • Variation Base-A-DD (2 layers of 60-min. grade D building paper WRB):

TMY exterior BC, moderate and severe interior BC 2002 exterior BC, moderate and severe interior BC

• Variation 2-A (1 layer no. 15 felt WRB): TMY exterior BC, severe interior BC

1 The ability of typically small drainage cavities that exist between the WRB (say, two layers of Grade D building paper) and the stucco cladding to drain plug flows is very questionable given the impact of surface tension on decohering the flow plug that then degenerates into free surface flows. Small gap drainage planes with at least one highly sorbent bounding surface are not effective in draining free surface flows.

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• Variation 4 (OSB sheathing): TMY exterior BC, severe interior BC • Variation 6-A (plywood sheathing): TMY exterior BC, severe interior BC • Variation 10-A (PA-6 interior membrane): TMY exterior BC, severe interior BC

Given the inability of WUFI to explicitly model bulk water flow, the methodology adopted to evaluate the impact of an internal water source was to arbitrarily set the initial moisture content of a thickness of stucco adjacent to the WRB to a saturated condition. Three depths of saturation were evaluated, namely 25, 50 and 100% of the stucco thickness. Clearly, this is excessive as the actual depth of saturation likely will never reach even the lower 25% limit. However, this is a valid parametric evaluation of the internal wetting performance in that if failures from this kind of moisture loading are not found within the context of the WUFI modeling assumptions even with exaggerated amounts of internal wetting, then this failure mode is unlikely in reality as well. In all cases simulated, the phase II material properties were used as these properties generally were shown not to produce condensation failures when subject to exterior moisture loads. The simulation for each configuration and BC combination commenced on July 1 with the stucco moisture content saturated for 25, 50 and 100% of the stucco cladding thickness from the WRB inwards (as discussed above). In all cases, the EIFS base and finish coats2 were dry initially. The transient time evolution of the relative humidity and moisture content profiles in the wall system revealed whether interior sourced moisture can be a cause of wall system failure as a function of the starting absorbed moisture content. The overall annual moisture performance in terms of the system performance criteria (as defined in section F.1 of the phase I and II report) gave an indication of the long-term systemic effects.

2. The natural convection behavior of the air gap system was investigated using a Computational Fluid Dynamics (CFD) simulation code. A pseudo-two-dimensional solid model of the wall system including the air gap (that is, a vertical cross section through the wall with a perimeter length of 1 in.) was prepared that allowed the vertical air gap thickness as well as the height of the slots at the top and bottom of the wall system to be varied. The system was based on the baseline stucco wall system (variation Base-A-DD) with a vertical cross-section at the center of a stud pocket in order to establish valid thermal conditions within the air gap. Steady-state simulations were carried out for two sets of cooling season interior and exterior temperatures derived from the TMY/Severe exterior/interior climate, one with the exterior temperature greater than the interior and vice versa. The simulations yielded the velocity profiles within the drainage plane air gap as well as the net volumetric flow rate through the gap per unit length of perimeter. The simulations were repeated for three air gap widths in order to ascertain the impact of drainage plane width on flow rate. In addition, two additional drainage plane configurations were investigated to ascertain their ability to amplify to the flow rate magnitude.

2 In typical stucco applications, the base coat is not applied. However, the phase II material properties used (Kumaran, et al, 2002) do not separate the base and finish coats material properties and hence the combined properties have been used as published.

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C. THE HYGROTHERMAL IMPACT OF INTERNAL MOISTURE SOURCES C.1 METHODOLOGY The methodology used in phase III for the first objective (hygrothermal impact of internal moisture sources) was the same as that used in phases I and II and is described in sections C, D, F.1 and F.2 of the phase I and II final report (Goldberg, 2006). In particular, the material properties are given in table F.6 and the boundary conditions are discussed in section C. The 2-dimensional simulation domain cross-sectional schematic is shown in figure C.1 and was the same as that used in phases I and II. There were more hygtothermal performance (temperature, relative humidity and moisture content) monitoring locations in phase III as shown in figure C.2. The annual moisture performance was evaluated using the same criteria as used in phase II (Goldberg, 2006, section F.1 and tables F.1 through F.5), namely:

• Relative humidity quantification of the existence of condensation • Condensation plane surface wetting • Annual wetting/drying stability • Dry basis moisture content quantification of the degree of saturation relative to the maximum

moisture content (derived from a literature consensus) In addition, graphs showing the time series profiles of the moisture content at the center of the sheathing and at the center of the initially wet portion of the stucco together with a that of the relative humidity on the insulation side of the interior vapor retarder allowed the transient performance of the system in drying the interior sourced moisture load to be assessed.

Figure C.1 2-dimensional simulation domain schematic

Figure C.2 Hygrothermal performance monitoring locations

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C.2 RESULTS The same methodology used in phases I and II to discuss the hygrothermal results is carried forward to the discussion below. Each set of results is prefaced by a table describing the wall configuration layers from the exterior on the left to the interior on the right. If a particular layer contains parallel heat flow paths, then its descriptive column is divided into rows. Thereafter, follow a series of tables for each of the simulated climates with a column devoted to each of the initial stucco layer water saturation thicknesses. The base results for the 0 saturation depth case are taken from the phase II simulations performed previously with the sampling period aperture shifted from January 1 through December 31 to July 1 through June 30. It must be emphasized that the Phase II computations were not repeated. The tables are succeeded by a set of smoothed time series profiles showing the transient response of the system in terms of the moisture content at the sheathing and initially saturated stucco segment centers as well as the relative humidity on the cavity side surface of the interior membrane. Each graph spans a period that terminates when the 3 wetted stucco profiles approximately converge to a common profile. Each set of tables and graphs is followed by a discussion.

Tables C.1 Base-A multiple climate simulation summary Layers Stucco Stucco Interface Exterior

Sheathing Insulation Interior

Membrane Interior Sheathing

A. 1.42 mm - NRC2002 EIFS base+finish coats

A1. 139.7 mm - NRC2002 fiberglass Phase II

material props.

B. 16.64 mm - NRC2002: Portland cement stucco

0.68 mm - NRC2002 60 minute Grade D Paper (2 layers)

19.8 mm - NRC2002 Wood Fibreboard

A2. 139.7 mm - NRC2002 spruce

1 mm – IBP PE-Membrane (equivalent spec)

12.7 mm - NRC2002 Interior gypsum + 1 coat primer + 2 coats latex paint

Exterior climate: TMY Interior boundary conditions: phase I severe Climate TS

Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : -0.06 marginal : +5.6 yes : +4.6 yes : +3.2

Sheathing interior surface condensation - max. RH (%) no – 72.8 no – 73.9 no – 78.2 no – 80.4

Sheathing interior surface dry basis maximum moisture content (%) 3.0 3.3 4.6 5.2

Insulation / interior membrane condensation - max. RH (%) @ time (h) no – 90.0 @ 8730 no – 90.2 @ 115 no – 90.4 @ 115 no – 90.6 @ 139

Insulation / interior membrane maximum condensation (g/ft2) @ time (h) 1.35 @ 8730 1.47 @ 115 1.60 @ 115 1.73 @ 139

Stable stud center wetting/drying cycle : relative change (%) yes : -0.08 yes : -11.9 yes : -11.4 yes : -10.32

Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : -0.39 yes : +0.68 yes : +0.56 yes : +0.60

Stud / interior membrane dry basis maximum moisture content (%) 6.4 6.5 7.1 7.6

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Exterior climate: TMY Interior boundary conditions: phase I moderate Climate TM Criteria Initial stucco saturation thickness (%)

0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : - 0.17 yes : +2.7 yes : +2.4 yes : +1.4






Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : -0.59 yes : -9.7 yes : -8.3 yes : -7.0


Exterior climate: 2002 Interior boundary conditions: phase I severe Climate 2S

Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : - 0.11 no : 15.4 no : 12.7 marginal : 8.6






Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : - 0.28 yes : -1.6 yes : -1.5 yes : -1.6


Exterior climate: 2002 Interior boundary conditions: phase I moderate Climate 2M

Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : -0.22 no : 12.1 marginal : 9.27 marginal : 6.3



Insulation / interior membrane condensation - max. RH (%) @ time (h) no – 89.7 @ 8755 no – 90.0 @ 116 no - 90.2 @ 116 no – 90.6 @ 139



Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : -0.44 yes : -9.5 yes : -8.5 yes : -7.3


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Discussion of Tables C.1 With the exception of the sheathing wetting/drying stability, the baseline configuration with phase II material properties shows adequate overall drying behavior in response to the imposed transient loading. The sheathing wetting/drying stability in all cases except the TMY / moderate boundary condition shows that the severity of the instability decreases with increasing initial saturation depth. Given that this occurs with maximum sheathing interior surface moisture contents that are well within the maximum limits, the data indicate that single episode transient events are not a cause for concern. However, superficially, the results also create an uncertainty as to what might happen with repeated episodic wetting events, for example, wetting from the drainage plane on a weekly basis for a couple of months during the cooling season. This query is partially addressed by the 2002 / severe climate in which there are annual wetting/drying stability failures at both 25% and 50% initial depth saturations. The inverse relationship between wetting/drying stability and saturation depth is quite intriguing but subtle since the change in moisture content over a year is relative to the maximum moisture content during the year (see Table F.3 in Goldberg, 2006). In other words, as the saturated depth decreases, the maximum annual moisture content becomes smaller and the indicated instability becomes larger. This effect also is partly a consequence of the transient nature of the simulation. Once the wall system reaches steady state (for the given transient saturation depth loading on July 1), a stable wetting drying cycle is likely, although at far higher maximum sheathing moisture contents. The question as to whether these higher sheathing moisture contents could reach failure levels at steady state remains open.

SHEATHING MOISTURE CONTENT TRANSIENT RESPONSEVariation: Base-A Exterior climate: TMY Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000 3500

Julian Time from 7/1 - 0h00 (h)

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Initial stucco saturationdepth from interior surface

0% (from phase II) 25% 50% 100%

All profiles smoothed with negative exponential curve fitting.

Figure C.1 Base-A sheathing moisture content transient response for climate TS

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SHEATHING MOISTURE CONTENT TRANSIENT RESPONSEVariation: Base-A Exterior climate: 2002 Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000 3500


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0% (from phase II) 25% 50% 100%


Figure C.2 Base-A sheathing moisture content transient response for climate 2S

INITIALLY SATURATED STUCCO MOISTURE CONTENT TRANSIENT RESPONSE

Variation: Base-A Exterior climate: TMY Interior climate: phase I severe

0 50 100 150 200 250 300 350 400 450 500


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25% 50% 100%


Figure C.3 Base-A initially saturated stucco moisture content transient response for climate TS

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INITIALLY SATURATED STUCCO MOISTURE CONTENT TRANSIENT RESPONSEVariation: Base-A Exterior climate: 2002 Interior climate: phase I severe

0 50 100 150 200 250 300 350 400 450 500


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Figure C.4 Base-A initially saturated stucco moisture content transient response for climate 2S

BATT / VAPOR RETARDER INTERFACE TRANSIENT RELATIVE HUMIDITY RESPONSE


0 500 1000 1500 2000 2500 3000 3500


0.0

0.1

0.2

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0.4

0.5

0.6

0.7

0.8

0.9

1.0

Rel

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0% ( from phase II) 25% 50% 100%


Figure C.5 Base-A vapor retarder relative humidity transient response for climate TS

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Variation: Base-A Exterior climate: 2002 Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000 3500


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0R

elat

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Hum

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0% (from phase II) 25% 50% 100%


Figure C.6 Base-A vapor retarder relative humidity transient response for climate 2S

Discussion of Figures C.1 – C.6 Figures C.1 and C.2 reveal that the wetting transients in the exterior sheathing moisture contents are well damped with the initial wetting spike in the moisture content at 100% saturation depth decaying within 500 hours (approx. 21 days). Clearly this is sufficient time for mold activity to take place, particularly in the presence of repeated wetting events given the other necessary conditions for each occurrence (adequate temperature and nutrients). The anomalous spike in the 50% saturation depth profile at hour 1400 is likely a result of a localized numerical instability in the simulation code, since if it were a real effect, it should manifest in at least the 100 % saturation depth profile as well. The effect of the transient dissipates completely after about 3800 hours (5 months). Thus, in theory, the effects of an interior wetting event in September could persist into February, in other words, allowing the elevated moisture to be locked into the sheathing by freezing. Upon thawing in the spring, the elevated moisture content could be exacerbated by new interior wetting episodes, thus setting up the potential for mold related failures. It can be speculated that perhaps this is one of the mechanisms contributing to some of the counter-intuitive and even mysterious stucco wall system failures that have been anecdotally reported in Minnesota. The moisture content responses of the initially wet stucco shown in figures C.3 and C.4 show monolithically decreasing moisture contents that reach substantial convergence within 300 hours (12.5 days). The increase in initial moisture content response (after the simulation start-up transient that is suppressed by the exponential smoothing dies out) increases non-linearly with saturation depth. This indicates that the stucco dries preferentially to the interior. The relative humidity perturbation on the cavity side of the interior membrane is relatively small and only persists for the 100% saturation depth where it remains fairly constant until about 1000 hours, again, further evidence that the stucco dries preferentially to the interior.

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Tables C.2 Variation 2-A single climate simulation summary Layers Stucco Stucco Interface Exterior





material props.


0.72 mm - NRC2002 15 lb felt (1 layer)






Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : -0.13 yes : +3.9 yes : +3.9 yes : 3.3



Insulation / interior membrane condensation - max. RH (%) @ time (h) no - 90.0 @ 2392 no – 90.1 @ 115 no – 90.2 @ 115 no – 90.4 @ 139



Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : -0.31 yes : +1.4 yes : +1.1 yes : +1.7


Discussion of Tables C.2 Replacing the 2 layers of 60 min. grade D building paper in the base case with a single layer of 15 lb felt, produces no wetting/drying stability anomalies for the TMY / Severe climate (in comparision with the marginal behavior shown for the 25% initial saturation depth for the same climate in Tables C.1). This is to be expected in view of the lower permeance of the no. 15 felt.

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SHEATHING MOISTURE CONTENT TRANSIENT RESPONSEVariation: 2-A Exterior climate: TMY Interior climate: phase I severe

0 1000 2000 3000 4000 5000 6000 7000 8000


14

16

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20

22

24

26

28

30S

heat

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Cen

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oist

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Con

tent

(kg/

m3 )


0% (from phase II) 25% 50% 100%


Figure C.7 Variation 2-A sheathing moisture content transient response for climate TS

Discussion of figure C.7 Noticeable in figure C.7 is the relatively long persistence of the 100% saturation depth transient relative to the 25 and 50 % depths so that a significant moisture content elevation is evident to 5500 hours (about 7.5 months) before monotonic convergence commences. This also appears to signify that the primary stucco drying mechanism is to the interior, even over a prolonged period during which the initial vapor pressure gradient (exterior larger than interior) is likely to reverse. A comparison of figures C.7 and C.1 further emphasizes this observation given the larger availability of the wall cavity with Grade-D to absorb moisture originating from the wet stucco. Thus providing a ventilated drainage plane (discussed in section D) may alleviate this phenomenology considerably.

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INITIALLY SATURATED STUCCO MOISTURE CONTENT TRANSIENT RESPONSEVariation: 2-A Exterior climate: TMY Interior climate: phase I severe

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700


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25% 50% 100%


Figure C.8 Variation 2-A initially saturated stucco moisture content transient response for climate TS


Variation: 2-A Exterior climate: TMY Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000 3500 4000


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

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0% (from phase II) 25% 50% 100%


Figure C.9 Variation 2-A vapor retarder relative humidity transient response for climate TS

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Discussion of Figures C.8 and C.9 The transient behavior of the initially saturated stucco moisture content and the cavity side interior membrane relative humidity is similar to that of the baseline case. Once again, the elevated RH persists only for the 100% saturation case through about 700 hours while the moisture contents substantially converge within 300 hours. This highlights another effect of transient moisture transport in absorbent cladding systems with semi-permeable WRB’s, namely, the imbalance in vapor transport rates through the WRB that increases as their permeability decreases. In particular, the rate of wetting from the exterior is far higher than the rate of drying (compare figures C.8 and C.3 with essentially the same stucco drying rate with figures C.1 and C.7 that show a far longer persistence of elevated sheathing moisture content with a lower permeability membrane). Historically, the knee-jerk response to this has been to increase the permeability of the WRB (hence the preference for grade-D building paper) rather than the physically more reasonable response of isolating the sheathing from the moisture source to the greatest extent possible by converting the WRB to a true water separation plane (WSP)3 (Goldberg and Huelman, 2005) while simultaneously maximizing the drying potential of the system on the warm side of the WSP to the interior. This basic approach that is embodied in the foundation system performance criteria of the Draft Minnesota Energy Code (chapter 1322, dated 7/17/06) that has been experimentally validated for foundation systems actually applies universally to the entire building envelope.

Tables C.3 Variation 4 single climate simulation summary Layers Stucco Stucco Interface Exterior





material props.



12.7 mm - NRC2002 OSB





Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : 0.92 yes : +0.92 yes : +0.99 yes : +0.86






Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : +3.3 yes : + 3.3 yes : 3.4 yes: +3.1


3 Defined in Section N1102.2.4 of chapter 1322 (7/17/06) as “A single component or a system of components creating a plane that effectively resists capillary water flow and water flow caused by hydrostatic pressure and provides a water vapor permeance of 0.1 perms or less to retard water vapor flow by diffusion.”

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Discussion of Tables C.3 In this variation, the wood fiberboard sheathing is replaced with OSB while retaining two layers of grade D building paper as the WRB. The net effect on the annual hygrothermal performance is the same as replacing the grade D with no. 15 felt insofar as the OSB sheathing serves as a vapor retarder with a permeability 17 times less than fiberboard. Thus like Tables C.2, Tables C.3 show no failures for the TMY / severe climate over an annual period.

SHEATHING MOISTURE CONTENT TRANSIENT RESPONSEVariation: 4 Exterior climate: TMY Interior climate: phase I severe

0 1000 2000 3000 4000 5000 6000


44

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0% (from phase II) 25% 50% 100%


Figure C.10 Variation 4 sheathing moisture content transient response for climate TS

Discussion of Figure C.10 Figure C.10 shows similar phenomenology to Figure C.7 that generally is attributable (as discussed above) to a lower permeability connection to the saturated stucco. In this case, for the 100% initial saturation depth level, elevated sheathing moisture contents persist to 4800 hours, however at higher average levels (about 62 kg/m3 for OSB/grade-D compared with about 26 kg/m3 for fiberboard / no. 15 felt).

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INITIALLY SATURATED STUCCO MOISTURE CONTENT TRANSIENT RESPONSEVariation: 4 Exterior climate: TMY Interior climate: phase I severe

0 100 200 300 400 500 600 700 800 900 1000


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Initia

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(kg/

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25% 50% 100%

All profi les smoothed with negative exponential curve fi tting.

Figure C.11 Variation 4 initially saturated stucco moisture content transient response for climate TS


Variation: 4 Exterior climate: TMY Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000 3500 4000 4500


0.0

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Figure C.12 Variation 4 vapor retarder relative humidity transient response for climate TS

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Discussion of Figures C.11 and C.12 The drying response of the stucco in variation 4 is not very different from either that of variation 2 or the baseline case, and substantial convergence also is reached within 300 hours. Figure C.12 shows a period of elevated cavity side membrane relative humidity of about 700 hours that is similar to that of variation 2 and about 300 hours less than the base variation.

Tables C.4 Variation 6-A single climate simulation summary

Layers Stucco Stucco Interface Exterior Sheathing

Insulation Interior Membrane

Interior Sheathing



material props.



12.7 mm - NRC2002 plywood





Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes : -0.57 marginal : +6.3 marginal : +7.0 marginal : + 6.1




Insulation / interior membrane maximum condensation (g/ft2) @ time (h) 1.61 @ 1482 1.61 @ 115 1.85 @115 2.02 @ 139


Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : - 0.35 yes : 1.1 yes : 1.0 yes : 3.2


Discussion of Tables C.4 In variation 6-A, the wood fiberboard sheathing is replaced with plywood with a permeability that is 10 times lower but also 62% greater than OSB. This intermediate coupling of the wet sheathing to the cavity system produces the worst simulated annual transient moisture performance for the TMY/severe climate with all 3 saturation depths producing marginal annual sheathing wetting/drying stability and the only instance of marginal condensation on the cavity side of the interior membrane for a 100% initial saturation depth. This could be interpreted to mean that intermediate moisture coupling is worse than relative decoupling (variations 2-A and 4) or relative coupling (base variation) from a transient moisture performance perspective. However, this nominally conflicts with field observations that usual report superior performance for CDX compared with OSB, for example. Perhaps this has more to do with the moisture durability of plywood compared with that of OSB rather than the moisture transport properties of the materials.

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0 500 1000 1500 2000 2500 3000


38

40

42

44

46

48

50

52

54

56

58

60

62S

heat

hing

Cen

ter M

oist

ure

Con

tent

(kg/

m3 )


0% (from phase II) 25% 50% 100%





0 200 400 600 800 1000 1200 1400 1600


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ally

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urat

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tucc

o C

ente

r Moi

stur

e C

onte

nt (k

g/m

3 )


25% 50% 100%



20

BATT / VAPOR RETARDER INTERFACE TRANSIENT RELATIVE HUMIDITY RESPONSEVariation: 6-A Exterior climate: TMY Interior climate: phase I severe

0 500 1000 1500 2000 2500 3000


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Rel

ativ

e H

umid

ity


0% (from phase II) 25% 50% 100%



Discussion of Figures C.13 - C.15 The persistence of the elevated sheathing moisture content for the initial 100% saturation depth in figure C.13 is intermediate between that of figure C.1 (base) and figure C.10 (variation 4, OSB sheathing) extending for about 1000 hours before monotonically decreasing to convergence with the 25 and 50 % initial saturation depth profiles. The saturated stucco moisture content damps to convergence within about 300 hours as with all the other variations. The period of elevated RH on the cavity side of the interior membrane also extends out to 1000 hours (same as the base variation).

21

Tables C.5 Variation 10-A single climate simulation summary Layers Stucco Stucco Interface Exterior





material props.





1 mm – IBP PA-6 membrane (equivalent spec)



Criteria Initial stucco saturation thickness (%) 0 25 50 100 Stable sheathing wetting/drying cycle : relative change (%) yes :-0.08 marginal : +6.5 marginal : +6.2 yes : + 4.3





Stable stud center wetting/drying cycle : relative change (%) yes : -0.05 yes : - 9.5 yes : -9.0 yes : - 7.7

Stable stud / interior membrane wetting/drying cycle : relative change (%) yes : -0.16 marginal : 7.0 marginal : +6.9 marginal : +6.1


Discussion of Tables C.5 Variation 10-A is the same as the base variation except that the polyethylene interior membrane is replaced with PA-6 (RH dependent permeability). Somewhat surprisingly, the annual transient performance is worse than the base variation for the TMY/severe climate with marginal sheathing wetting/drying stability for both the 25 and 50 % initial saturation depth cases (compared with only the 25 % case for the base variation). In addition, variations 10-A produces marginal wetting/drying stability at the stud/PA-6 interface for all three initial saturation depths. One can conclude from this that the increased drying potential to the interior provided by the PA-6 produces an increased vapor flux from the wet stucco through the cavity to the interior so producing the elevated sheathing and stud/PA-6 interface moisture contents. As alluded to above (in the discussion of Figures C.8 and C.9), a possible remedy is to provide a water separation plane on the exterior of the sheathing so essentially decoupling the wood components from the moisture source. This would realize all the interior drying advantages of the PA-6 while mitigating the effects of elevated exterior moisture loading.

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0 250 500 750 1000 1250


14

16

18

20

22

24

26

28

30

32

34

36

38

She

athi

ng C

ente

r Moi

stur

e C

onte

nt (k

g/m

3 )


0% (from phase II) 25% 50% 100%





0 100 200 300 400 500 600 700 800


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BATT / VAPOR RETARDER INTERFACE TRANSIENT RELATIVE HUMIDITY RESPONSEVariation: 10-A Exterior climate: TMY Interior climate: phase I severe

0 250 500 750 1000 1250 1500


0.4

0.5

0.6

0.7

0.8

0.9

1.0

Rel

ativ

e H

umid

ity


0% ( from phase II) 25% 50% 100%



Discussion of Figures C.16 - C.18 One of the consequences of the increased permeability of PA-6 under elevated RH conditions is evident in Figure C.16 in which the initial wetting transient of the sheathing is damped out within 280 hours compared with about 500 hours for the base case owing to the elevated drying vapor flux to the interior. The saturated wetted stucco moisture content profiles converge within 300 hours in concordance with all the other cases simulated indicating that the stucco drying rate appears to be essentially a constant in the process. The difference between the variations is where the drying vapor flux ends up, either in the sheathing, the studs, on the interior membrane surface or in the interior space. The cavity side interior membrane RH is lower than all the other variations and the degree of perturbation in the smoothed profiles shows the much greater degree of vapor coupling between the interior space and the wall cavity interior afforded by the low permeability of the PA-6 at relative humidities above 60%. However, as expected, the initial elevated RH transient at 100% initial stucco saturation depth persists for just 313 hours compared with 1000 hours for the base case with polyethylene. This demonstrates the advantages of an elevated interior drying potential. C.3 CLOSURE Taken as a whole the transient simulations show that for an interior stucco system wetting episode initiated on July 1 the following phenomenology occurs:

• Substantial drying occurs to the interior • Increasing the decoupling of the sheathing and cavity from the moisture source improves the

hygrothermal performance • Increasing the coupling between the wall cavity and the interior in conjunction with a vapor

coupled moisture source and wall cavity improves the hygrothermal performance in some respects but makes it worse in others.

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Hence combining the decoupling of the vapor source on the exterior of the sheathing and the coupling of the vapor sink on the interior of the framing cavity can yield the optimum results provided that the interior vapor coupling does not overwhelm the safe moisture sequestration capacity of the sheathing and framing members. This is consistent with the hygrothermal phenomenology shown in the phase I simulations of Variation 20 (tables E.39). D. THE NATURAL CONVECTION BEHAVIOR OF VENTILATED DRAINAGE GAPS As the results of section C have suggested, decoupling the internally wetted stucco moisture source from the framing system via a water separation plane (WSP) offers a means of mitigating the negative impact of a drainage plane on the wall system hygrothermal performance. If this decoupling is combined with significant drying potential to the interior so that incidental bulk water leakage across the WSP can be safely managed, then a WSP can be a practical solution. However, in cases when drying to the interior is excluded by the “standard” construction practice of installing a warm-side polyethylene vapor retarder between the framing cavity and the interior finishing system, then eliminating drying to the exterior as well via a WSP can lead to rather dramatic mold and rot failures. Hence in situations that employ a warm-side polyethylene vapor retarder as well as a permeable WRB such as two layers of grade-D building paper (the base configuration discussed above), another means of decoupling the stucco moisture source from the framing cavity is to install a discreet drainage gap between the stucco and the WRB and vent it so that water vapor from the drying stucco can be conveyed safely out of the system via natural convection. There are several requirements for such a system to work effectively as follows:

• The natural ventilation must be supported by a horizontal temperature gradient alone regardless of whether that gradient is positive from the exterior to the interior or vice versa.

• The drainage gap must be sufficiently wide to sustain a large enough volumetric air flow rate per unit length of stucco cladding perimeter.

• Slots must be provided at the top and bottom of the drainage cavity that are adequately sized to permit the induction of buoyant cavity flow natural ventilation.

• The top and bottom ventilation slot separation distance should not be so large as to choke the flow as a result of excessive pressure drops. Clearly, the separation distance is a function of the gap width and top and bottom slot heights.

D.1 METHODOLOGY The performance of a ventilated drainage gap system was evaluated using the computational fluid dynamics modules of the ALGOR finite element simulation program. A 3-dimensional solid model of a 1 in. length of wall perimeter was prepared at the center of a stud pocket based on the baseline stucco wall system (Base-A-DD) as shown in figure D.1.

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structure (top plate)

structure (bottom plate)

insulation (unfaced batt)

interior sheathing (gypsum)

interior membrane (polyethylene)

exterior sheathing(fiberboard)

stucco interface or WRB(2 layers grade D)

stucco (scratch andbrown coats)

EIFS finish

drainage gap

top ventilation slot

bottom ventilationslot (weep screed)

Figure D.1 Cross-section through solid model of wall section

All the simulations were carried our for a fixed ventilation slot vertical separation distance of 96 in. with gap widths of 15/64 in (6mm), 0.5 in. and 1 in. No attempt was made to assess the impact of ventilation slot separation distance on the buoyant flow performance, although an optimized separation distance is likely (as a function of gap width). This will have to await future research. The smallest gap width of 6 mm comes from an EIFS study (gap between foam and trowel-applied weather resistive barrier) performed by Oak Ridge National Laboratory (Pedracine, 2007). Two sets of cooling season steady-state temperature boundary conditions were selected from the TMY / Severe boundary conditions set as follows:

• Interior: 21.89°C Exterior = 19.6°C • Interior: 21.32°C Exterior = 35.00°C

26

The first set of boundary conditions represents a situation in which there is a very small temperature difference with the interior warmer than the exterior while the second set shows the reverse gradient with the exterior significantly warmer than the interior. These two conditions were sufficient to demonstrate the ventilation capability of the drainage slot in principle regardless of the magnitude of the temperatures. Clearly as the temperature difference approaches zero, the magnitude of any buoyant flow decreases concomitantly. While transient analyses can provide useful additional data on the dynamics of the buoyant flow, they do not provide any additional information about the feasibility of the concept and hence were not performed given the limited scope of the available research budget. It is important to note that the simulations do not include the effects either of wind loading or of pressure gradients produced by exterior temperature stratification. It is in theory possible to produce a flow in the drainage gap by virtue of a lower wind stagnation pressure closer to the ground (because of horizontal boundary layer effects) than at the top of the wall, however, this is an episodic event and does not address whether the drainage gap can function reliably as a drying mechanism under the most prevalent horizontal temperature gradient conditions only. A total of five drainage gap configurations were evaluated. The first 3 configurations (A, B and C) investigate the baseline system shown in figure D.1 each with a different gap width / slot height combination. Configuration D (shown in figure D.2) investigated the performance enhancement accruing from adding a 12” tall section of 1” thick extruded polystyrene insulation to the bottom sheathing face (that is, the interior face of the drainage gap). In configuration E (figure D.3), the insulation was moved to the stucco cladding face of the drainage gap (or its exterior face).

12" height of 1"extruded polystyreneattached to frame

1" drainage gapwidth

25/32" drainage gapwidth

Figure D.2 Cross-section through solid model of wall section bottom: configuration D

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28


12" height of 1"extruded polystyreneon 0.5" verticalfurring strips


EIFS finish extendingover polystyrene

Figure D.3 Cross-section through solid model of wall section bottom: configuration E

Clearly configurations D and E are presented as illustrative examples only to investigate in principle whether adding insulation to the bottom of the cavity improves the slot flow as is suggested by purely theoretical considerations. There are several practical difficulties in implementing such strategies in the field in a cost-effective manner that would need to be resolved, particularly with configuration E (such as, the necessity for an EIFS base coat plus reinforcing over the polystyrene that would create a color difference, the possibility of cracking at the stucco interface, susceptibility to physical damage, etc). There are fewer issues with placing the insulation on the interior as shown in configuration D (such as framing structural concerns and supporting the stucco with a non–uniform air gap), however, these issues can be more readily addressed. D.2 RESULTS The results for all five configurations are tabulated in Table D.1. The table shows the geometrical parameters of the drainage gap system, the imposed boundary conditions, the net volumetric flow rate in the drainage gap and its direction (upwards or downwards) as well as the average horizontal velocity at the inlet and outlet slots.

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Table D.1 Convective flow performance of drainage gaps Slot Configuration Boundary Temperatures Config-

uration Index

Drainage Gap Width

(in.)

Inlet/Outlet Slot Height

(in.)

Insulation Exterior(°C)

Interior (°C)

Net Flow Rate (mm3/s.cm [ft3/min.in])

Average Inlet/ Outlet Slot Velocity

(mm/s, ft/min)

Net Flow Direction Within Gap

19.6 21.89 0.121 [6.502 x 10-7]

0.00435 [8.563 x 10-4] upwards

A 15/64 (6 mm) 0.25

none

35.00 21.32 0.550 [2.957 x 10-6]

0.0198 [3.894 x 10-3] downwards

19.6 21.89 0.450 [2.424 x 10-6]

0.00811 [1.596 x 10-3] upwards

B 0.5 0.375

none

35.00 21.32 1.601 [8.618 x 10-6]

0.0290 [5.709 x 10-3] upwards

19.6 21.89 4.286 [2.307 x 10-5]

0.0608 [0.012] upwards

C 1 0.5

none

35.00 21.32 1.319 [7.101 x 10-6]

0.0187 [3.681 x 10-3] upwards

19.6 21.89 131.674 [7.087 x 10-4]

1.866 [0.367] upwards

D 1, 25/32 0.5

12 in. wide by 1 in. thick extruded polystyrene on sheathing side of gap at bottom of wall 35.00 21.32 792.19

[4.264 x 10-3] 11.23 [2.21] downwards

19.6 21.89 59.062 [3.179 x 10-4]

0.837 [0.165] upwards

E 27/32, 0.5 0.5

12 in. wide by 1 in. thick extruded polystyrene on stucco side of gap at bottom of wall 35.00 21.32 325.621

[1.752 x 10-3] 4.615

[0.909] downwards

Discussion of Table D.1 The unit length volumetric flow rates in all cases are very small as might be expected, especially for the 2.3°C temperature difference. The flow dynamics of the plain drainage gap (that is, configurations A-C) are shown in Figure D.4.

DRAINAGE GAP SIMULATED NATURAL CONVECTION FLOW

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Drainage Gap Width (in.)

0

1

2

3

4

5

Net

Flo

w R

ate

(mm

3 /s.c

m)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Ave

rage

Inle

t/Out

let S

lot V

eloc

ity (m

m/s

)

Net flow rate: Text < Tint Slot velocity: Text < Tint Net flow rate: Text > Tint Slot velocity: Text > Tint

Figure D.4 Plain drainage gap flow dynamics

When the interior temperature is larger than the exterior temperature by the relatively small difference of 2.3°C, the flow and slot velocity increase non-linearly with gap width. However, when the exterior temperature exceeds that of the interior by the larger difference of 13.7°C, essentially the flow appears to choke at a gap width of approximately 0.65 in. Whether this is just a numerical artifact or not cannot be determined at this stage, however, it does show that the optimum design of the plain gap geometry is not intuitively obvious. However, in general, for the 3 configurations without any insulation enhancement, the maximum unit perimeter length flowrate that can be expected even with a large (1 in.) drainage gap is of the order of 10-5 cfm/in. This is unlikely to yield sufficient flow to provide any kind of robust drying, particularly in the case of higher frequency rain events. However, adding insulation to the lower 12” of the wall does offer a substantial (10 to 100 times) improvement in unit perimeter flow rates that could provide a sufficiently robust natural ventilation drying potential. Configuration D (with the insulation on the gap interior) offers at least twice the flow performance of configuration E (exterior insulation

30

31

placement) which is fortuitous since, from a “buildability” perspective, configuration D is easier to implement. What these data show is that further, more detailed analysis of configuration D is warranted, particularly in terms of determining the magnitude of the stucco drying rate when exposed to surface flow speeds of the order of 1 ft/min. Further work on optimizing the system in terms of gap width, slot separation distance, insulation height and thickness, etc, also would be of value. However, as the data stand, they indicate that, at least in principle, using a drainage gap as a stucco drying system is viable. Figures D.5 and D.6 below depict the flooded contour velocity profiles for configuration C (1 in. gap width) while Figures D.7 and D.8 depict the profiles for configuration D (12 in. insulation on bottom interior gap face). The figures are arranged in pairs so that figures D.x.T show the top slot and figures D.x.B show the bottom slot. Both temperature gradient profiles are given for each configuration. Each figure contains three profiles, the left hand profile giving the velocity magnitude (or speed) at each point in the flow field; the center profile shows the X-velocity component field; and the right hand profile gives the Y-velocity component field.

Figure D.5.T Configuration C top slot velocity flooded contours: interior temp = 21.89°C, exterior temp. = 19.6°C

32

Figure D.5.B Configuration C bottom slot velocity flooded contours: interior temp = 21.89°C, exterior temp. = 19.6°C

33

Figure D.6.T Configuration C top slot velocity flooded contours: interior temp = 21.32°C, exterior temp. = 35.00°C

34

Figure D.6.B Configuration C bottom slot velocity flooded contours: interior temp = 21.32°C, exterior temp. = 35.00°C

35

Figure D.7.T Configuration D top slot velocity flooded contours: interior temp = 21.89°C, exterior temp. = 19.6°C

36

Figure D.7.B Configuration D bottom slot velocity flooded contours: interior temp = 21.89°C, exterior temp. = 19.6°C

37

Figure D.8.T Configuration D top slot velocity flooded contours: interior temp = 21.32°C, exterior temp. = 35.00°C

38

Figure D.8.B Configuration D bottom slot velocity flooded contours: interior temp = 21.32°C, exterior temp. = 35.00°C

39

Discussion of Figures D.5 – D.8 Figures D.5 and D.6 show the strong recirculating flows within the plain 1 in. drainage gap (no insulation, configuration C) together with the relatively small velocities at the faces of the top and bottom slots. Hence in this case, the horizontal temperature gradient is largely ineffective in inducing a net vertical flow. In contrast for configuration D, Figures D.7 and D.8 reveal how the bottom insulation succeeds in inducing a much stronger net inflow at the bottom slot (center profile of Figure D.7.B), essentially no recirculating flow in the gap adjacent to the insulation (left hand profile of figure D.7.B) and a much weaker recirculation in the gap adjacent to the sheathing (left hand profile of figure D.7.T) compared with that of configuration C (left hand profile of Figure D.5.T). With the temperature gradient reversed, the velocity fields are reversed as well, but the comparative flow phenomenology remains the same (Figures D.6 and D.8). From a practical perspective, it is understood that any recommendation for a 0.5-1” air gap between the WRB and the stucco is a significant departure from existing practice and surely will engender strong resistance from the stucco community. However, this is not the point. The saliency is that the results show that using a drainage gap to induce sufficient air flow to accomplish meaningful drying of an internally wetted stucco assembly requires an air gap thickness of a 0.5-1” magnitude. Air gaps in the usual 1/8 – ¼” range are not effective for ventilation purposes (even with top and bottom slots) and “drainage gaps” that only have a bottom weep screed are ineffective for inducing interior drying. Thus if the benefits of interior drying are to be realized, it is simply the case that the issues arising from larger air gaps than currently used will have to be addressed and resolved with additional research. E. CONCLUSION The simulations of a stucco wall system subject to a transient interior wetting event show that this moisture loading yields a phenomenology that is different from that which occurs with exterior only wetting events that were addressed in phases I and II of this research. While the performance of the base stucco system nominally is adequate under single event transient internal moisture loading, the data indicate a cause for concern as to what could happen with repeated transient internal moisture loading events. Decreasing the vapor coupling between the wet stucco moisture source and the framing cavity (such as replacing 2 layers of grade D building paper with one layer of no. 15 felt) while not changing the rest of the system seems to improve the performance. Increasing the coupling between the framing cavity (such as replacing a warm-side polyethylene vapor retarder with a PA-6 membrane) improves the hygrothermal performance in some respects while exacerbating it in others. Hence implementing both of these modifications together offers the potential of effectively managing internally wetted stucco systems. Taken as a whole, the results do offer some clues as to possible mechanisms for field-observed failures that often defy conventional forensic analysis (such as a patch of sheathing mold in the center of a windowless wall). Using a vented drainage gap of adequate width offers an alternative approach to effectively decoupling the wetted stucco vapor source from the wall cavity system that could be used with a warm side polyethylene vapor retarder. In particular, a drainage gap with 12 in. of extruded polystyrene insulation installed on its bottom interior face shows promise of yielding flow rates adequate to achieve sufficiently robust drying of internally wetted stucco when driven by horizontal temperature gradients alone. However, implementing such a system is likely to engender strong resistance from the stucco community owing to the complexities of achieving a structurally robust stucco system with a relatively large (0.5- 1”) gap between the WRB and the stucco. Conversely, however, current practice with drainage air gaps in the 1/8-1/4” range with bottom slots only are not effective as ventilation planes and it is erroneous to claim otherwise.

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APPENDIX 1 REFERENCES Goldberg, LF. “A Simulation Investigation of Stucco Cladding Wall System Vapor Transport Performance in a Cold Climate”, Research Report, Building Physics and Foundations Research Programs, www.buildingphysics.umn.edu, University of Minnesota, July 2006. Pedracine, S. E-mail communication with Dr Achilles Karagiozis, Oak Ridge National Laboratory, 2007. Kumaran, K, Lackey, J, Nomandin, N, Tariku, F and van Reenen, D. “A Thermal and Moisture Transport Property Database for Common Building and Insulating Materials”, ASHRAE Research Project 1018-RP Final Report, National Research Council of Canada, 2002.

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a simulation investigation of stucco cladding wall system

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