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Building Enclosure Design and Best Practices for Wood-Framed Buildings COLIN SHANE | M.ENG., P.ENG.
ASSOCIATE, SENIOR PROJECT MANAGER
NOVEMBER 19, 2015
Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board.
“The Wood Products Council” is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.
This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. __________________________________
Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without written
permission of the speaker is prohibited.
© RDH Building Sciences Inc. 2015
Copyright Materials
Course Description
à Through a combination of building science fundamentals and current research, this presentation will explore design considerations associated with wood-frame building enclosures. Discussion will focus on best practices for designing durable, energy-efficient enclosures for mid-rise buildings using traditional light wood-frame construction. Differences in enclosure design associated with taller wood-frame buildings using mass timber products will also be reviewed.
Learning Objectives
à Review building science fundamentals and building enclosure design considerations for light wood-frame buildings.
à Discuss best practices for light wood-frame building enclosure design, detailing, and construction techniques
à Explore the thermal benefits of utilizing wood-frame construction. à Compare the differences in building enclosure design criteria and
systematic approaches between light wood-frame structures and tall, mass timber-frame structures.
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à Building science basics
à Water control
à Air control
à Heat control
à Wall assemblies options
à Roof assemblies options
Outline
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Wood-frame Building Enclosure Design Guides
à 2011 Building Enclosure Design Guide – Wood-frame Multi-Unit Residential Buildings
à Emphasis on best practices, moisture and new energy codes
à 2013 Guide for Designing Energy-Efficient Building Enclosures
à Focus on highly insulated wood-frame assemblies to meet current and upcoming energy codes
à CLT Handbook
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à Separate indoors from outdoors, by controlling:
à Heat flow
à Air flow
à Vapor diffusion
à Water penetration
à Condensation
à Light and solar radiation
à Noise, fire, and smoke
à While at the same time:
à Transferring structural loads
à Being durable and maintainable
à Being economical & constructible
à Looking good!
Building Enclosure Design Fundamentals
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Building Science Basics
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Control Functions of Building Enclosures
Building Enclosure
Control Air
Control Vapor
Control heat
Control Rain
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Trends in Building Enclosure Design
à Trend towards more energy efficiently
building enclosures
à Air barriers now required in 2012 IECC and
2013 CEC
à Washington and Seattle are ahead of the curve
à Higher R-value requirements
à Thicker walls
à More insulation / less air leakage = less
heat flow to dry out moisture
à “Marginal” assemblies that worked in the past
may no longer work
à Amount, type and placement of insulations
matters
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Water Penetration Control Strategies
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Rainscreen Cladding
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à Code requirement
à Moisture
à Air holds moisture that can be transported and deposited within assemblies.
à Energy
à Unintentional airflow through the building enclosure can account for as much as 50% of the space heat loss/gain in buildings.
Air Penetration Control – Why?
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Air Barrier Systems
à Air Barrier Systems Must:
à Be continuous, airtight, durable
à Resist structural loads
à Bridge joints across inter-story and drift joints
à Not negatively affect drying ability
à Air barrier is a system, not just a material
à Details are critical
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Types of Air Barrier Systems
Sealed Gypsum Sheathing – Sealant Filler at Joints
Loose Sheet Applied Membrane – Taped Joints & Strapping
Liquid Applied – Silicone sealants and silicone membrane at Joints
Sealed Plywood Sheathing –Sealant & Membrane at Joints
Sealed Sheathing – Membrane at Joints
Self-Adhered vapor permeable membrane
Plywood sheathing with taped joints (good tape)
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New Materials: Air Barrier/WRB Membranes
à Not all are created equal – each has strengths and weaknesses:
à Need to match compatible sealants/tapes/membranes with each.
à Choice will depend on substrate, field conditions, tie-in details.
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Why test it?
à Requirements of WSEC and SEC C402.4
à Applies everywhere for new construction in the state of
Washington. Seattle has additional requirements of the
test (commercial code, which includes multifamily
residential).
à Construction Documents include Air Barrier, Pressure
Boundary, and Area of Pressure Boundary
à As of 2012, “attempt to repair” and “documentation of
leakage sources” if air leakage rate exceeds 0.40 cfm/ft2 at
75Pa
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How do we measure it?
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What are the results?
à No published data regarding air leakage test result
à General interest in results
à 2014 ASHRAE Annual Conference
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What are the results?
à ASTM E779
à 25 Buildings > 1,000,000 sf
à 2009 SEC
à Excludes roofs and below-grade
19%
10%
23%
39%
10% Liquid Applied
Curtain Wall/Window Wall Sealed Sheathing
Sheet Applied
Storefront
35%
39%
26%
Residential
Commercial
Institutional
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What are the results?
Average = 0.25 cfm/ft2
0.21 0.22
0.19
0.34
0.23
0.00
0.10
0.20
0.30
0.40
0.50
0.60
Liquid Applied Curtain Wall/Window Wall
Sealed Sheathing Sheet Applied Storefront
No
rmali
zed
Air
Leakag
e (
cfm
/ft2
)
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What are the results?
à The code-maximum target of 0.40 cfm/ft2 is not prohibitively difficult to achieve.
à Buildings not meeting the target:
à Contractor/developer did not prioritize the air barrier
à Significant discontinuities in the air barrier systems
à Companion buildings with similar designs, teams, or air barrier systems show overall improvement.
à HVAC and parapet detailing
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Conductive Heat Loss Control
à Insulation between studs is most common heat control strategy
à Need to consider effective R-values
à Thicker walls and/or continuous exterior insulation on exterior becoming more common
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Wood Framing Factor Impact
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Insulation Placement
à Consider effective thermal resistance
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Cladding Attachment Through Exterior Insulation
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Cladding Attachment through Exterior Insulation
Longer cladding Fasteners directly through rigid insulation (up to 2” for light claddings)
Long screws through vertical strapping and rigid insulation creates truss (8”+) – short cladding fasteners into vertical strapping Rigid shear block type
connection through insulation, cladding to vertical strapping
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Thermally Improved Performance
Continuous metal Z-girts
Fiberglass Clips & Hat-Tracks
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The ‘Perfect’ Assembly
à Rain penetration control: rainscreen cladding
over water barrier
à Air leakage control: robust air barrier system
à Heat control: continuous insulation layer
à Locate all barriers exterior of structure
à Keep structure warm and dry
à 50+ year old concept!
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Wood-Frame Assemblies – ‘Perfect’ Wall
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Wood-Frame Assemblies – ‘Perfect’ Roof
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Wall-to-Roof Detail
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Wall Assemblies
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Calculating One-Dimensional R- and U-values
à Appendix A
à ASHRAE Fundamentals
à Thermal Modeling Software
2x6 wood-framed wall with R-19 batts
Effective R value =16
Effective U value =0.062
2x6 metal framed wall with R-19 batts
Effective R value = 9.3
Effective U value =0.106
(50% loss)
(16% loss)
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Wood Frame Wall Assemblies – Traditional Method
à Assembly
à ½” gyp
à 2x6 @ 16” o.c.
à R-23 mineral fiber insulation
à ½” sheathing
à WRB/furring/cladding
à Standard framing factor
à 77% cavity, 23% framing
à U-0.058
à Does not meet
à WSEC (U-0.054 max)
à SEC (U-0.051 max)
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Wood Frame Wall Assemblies – Option 1
à Assembly
à ½” gyp
à 2x8 @ 16” o.c.
à R-30 high density mineral fiber insulation
à ½” sheathing
à WRB/furring/cladding
à Standard framing factor
à 77% cavity, 23% framing
à U-0.045 (R-22.0)
à Meets
à WSEC (U-0.054 max)
à SEC (U-0.051 max)
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Wood Frame Wall Assemblies – Option 2
à Assembly
à ½” gyp
à 2x6 @ 16” o.c.
à R-21 batt
à ½” sheathing
à 1” Mineral fiber insulation (R-4.2 continuous)
à Standard framing factor
à U-0.0464 (R-21.5)
à Meets
à WSEC (U-0.054 max)
à SEC (U-0.051 max)
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Option 1 vs. Option 2
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Sorption Isotherm
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PNW Climate – Keeping Things Dry
à Quantity of rain only part of the story
à Very minimal drying capacity for most of the winter
à Cool temperatures
à Minimal solar gains
à Ability for walls to dry to the exterior is limited, even with rainscreen
à Exterior insulation reduces risk
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Roofs
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Refresher: Low Slope Roof Types
à Protected Membrane (Inverted)
à Conventional
à Vented/Unvented (Compact)
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
à Control layers
à Water control - roof membrane
à Air control – roof membrane
à Vapor control – roof membrane
à Thermal control – moisture resistant insulation
à Membrane directly on sloped structure
à Insulation above membrane
à “Finish” – pavers, ballast
à Why is this ‘perfect’?
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
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The ‘Perfect’ Roof – Inverted / PMR / IRMA
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Conventional Roof – Exposed Membrane
à Control layers
à Water control - roof membrane
à Air control
› Roof membrane?
› Or separate air barrier?
à Vapor control
› Roof membrane?
› Insulation?
› Or separate vapor barrier?
à Thermal control
› Rigid insulation
à Why is this less than ‘perfect’?
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Case Study: Conventional Roof
à 2 ply SBS
à 4” Stone wool
à 4” Polyiso
à 0-8” EPS Taper Package for Slope
à Single ply SBS AB/VB & Temporary Roof
à Wood Deck Structure
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Vented Roofs
à Control layers
à Water control - roof membrane
à Air control
› Interior drywall?
› Polyethylene sheet?
à Vapor control
› Roof membrane?
› Polyethylene sheet?
à Thermal control
› Batt insulation
à Why is this less than perfect?
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Vented Roofs
à Why do we vent?
à To control air leakage / vapor diffusion condensation
à To dry out the plywood deck
à How does it work?
à Requires offset height to encourage convective looping in the space
à Warm moist air is replaced with dry exterior air
à How does it work in low sloped roofs?
à Who knows… the wind?
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Vented Roofs
à What increases moisture risk?
à More insulation
à More air leakage
à White membranes
à High interior moisture loads
à ‘Wet’ venting air
à Good air barrier at ceiling and at wall-to-roof connection is critical
à Buyer (and designer) beware!
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Summary
This concludes The American Institute of Architects Continuing Education Systems Course
Colin Shane – [email protected]
