mohamed zedan - state of the art in the use of thermal insulation in building

State of the Art in the Use of Thermal Insulation in Building Walls and

Roofs (Part I)

By

Prof. Mohamed Fouad Zedan

Department of Mechanical Engineering King Saud University, Riyadh, KSA

Copyright - Al-Sanea/Zedan ; 2012 1

Objectives and Topics Covered

1. Importance of thermal insulation

2. Best location of insulation layer in building envelopes for different AC operation modes (continuous/intermittent).

3. Optimum thickness of insulation for buildings in the central region of Saudi Arabia (generally applicable to most of the gulf region).

4. Effect of wall orientation and economic parameters on optimum thickness of insulation with emphasis on the effect of future projected electricity tariff.


TOPIC-1 Importance of Thermal Insulation

a. Energy Conservation in Buildings

Energy consumed by AC is about 2/3 of energy consumed in buildings in KSA.

Transmission load through walls and roofs of residential buildings is about 2/3 of AC load.

Accordingly, substantial energy savings can be achieved by increasing the R-value of building envelope by applying thermal insulation.


Importance of Thermal Insulation- cont.

b. Improved Thermal Comfort

Lower indoor air temperature

Lower indoor surface temperature (less radiation effects)

Lower indoor surface temperature fluctuations


Importance of Thermal Insulation- cont. c. Reduces size and maintenance cost of AC equipment

d. Increases time lag and improve load leveling on the electric grid (smaller peak load and higher valley)

e. Reduces global warming , protects the environment, etc.

f. Reduces dependence on operating AC equipment in moderate climates

g. Protects building envelope, preserves furniture, and reduces condensation risk.

h. Reduces transmission of sound


Drawbacks of Using Thermal Insulation

a. Installing insulation adds to overall cost

What is the pay-back period?

b. Insulation layer makes walls thicker


TOPIC-2

Effect of Insulation Location in Walls

a. Under Steady Periodic Conditions b. Under Initial transient Conditions

These conditions are related to the AC operation

mode


Modes of Operation of AC Systems

Modes of operation of AC systems:

Continuously operating mode.

Intermittently operating mode.

Former would generally give rise to steady periodic conditions, whereas latter is associated with initial transient behavior.

Literature reveals lack of detailed and systematic studies that investigate effect of insulation location within building envelope with regard to AC operating mode.


Effect of Insulation Location in Walls under steady periodic conditions (continuously operating AC)

Study is made under the assumptions:

Insulation layer thickness is fixed.

Representative days for July and January.

Riyadh climatic conditions.

Fixed indoor air temperature: 25 C in July and 21C in January

Study is made using a validated computer model


Model: Definition sketch of composite wall


Thermal properties of wall materials.


Material k (W/m.K) ρ (kg.m3) c (J/kg.K)

HWHCB (200mm) 1.05 1105 840

HWHCB (150mm) 0.96 1362 840

HWHCB (100mm) 0.81 1618 840

Molded polystyrene 0.036 20 1215

Cement plaster 0.72 1865 840


• Outside surface is exposed to periodic

variation in boundary conditions.

• Indoor air temperature is kept constant at 25oC

with hi = 8.23 W/m2.K.

15-cm

HWHCB

x

Validation: Periodic heat conduction in three-

layered wall

10-cm

HWHCB

5-cm

MP

Indoor Outdoor

Validation: Comparison of temperature distribution across wall at various times as obtained from finite-volume and semi-analytic solutions; July, west face.


Validation: Comparison of variation of transmission load to space obtained from numerical model and semi-analytic solutions; July, west face.


Wall Configurations used in the investigation: Wall I with inside insulation

Copyright - Al-Sanea/Zedan ; 2012

Wall II with outside insulation


Temperature Distribution Across Wall I (inner insulation)

Representative day in July.

Wall is facing west.

Variations are shown in next figure at different times of day.


Temperature distribution across wall I at different times; July, facing west.


Results indicate:

Temperature variation is smooth across each layer with discontinuities in gradients at interfaces because of different conductivities.

Steepest change in temperature occurs in insulation layer.


Temperature Variation Across Wall II (outer insulation)


Results indicate:


• Again, most of temperature drop occurs in insulation layer near outside surface.

• This leads to much smaller temperature drop across concrete block and consequently smaller temperature fluctuation at inner surface of this wall compared to case of inside insulation.

• Temperature at outside surface is generally higher in present case due to accumulation of heat in outside plaster layer.

Transmission Load Variation with Time


Results indicate:

Peak transmission load is higher and minimum load is lower (hence amplitude of load fluctuation is bigger) in case of inside insulation.

Difference in peak loads is about 14%, e.g. 14% smaller capacity AC equipment with outside insulation.

Above result is generally valid for all wall orientations and in both summer and winter.

Mean transmission load appears to be essentially the same for walls with inside and outside insulation.


Daily Transmission Loads


Effects of wall orientation

Results indicate:

Daily mean cooling loads in summer for east and west faces are 15% higher than those for north and south faces.

Daily mean heating load in winter for north face is 18% higher than those for east and west faces.

Daily mean heating load in winter for south face is 39% lower than those for east and west faces.


Summary

Insulation layer has minimal effect on mean daily cooling and heating loads, with slight advantage for outside insulation in summer and inside insulation in winter.

Outside insulation gives smaller amplitude of load fluctuation and smaller peak load in both summer and winter for all wall orientations.


Outside insulation slightly increases time lag in summer, compared to inside insulation, and has practically same effect on time lag in winter.

More detailed results can be found in:

S.A. Al-Sanea and M.F. Zedan, “Effect of insulation location on thermal performance of building walls under steady periodic conditions”, International Journal of Ambient Energy., Vol. 22 (2), pp. 59-72, 2001.


Effect of Insulation Location in Walls under Initial Transient

Conditions

(intermittently operating AC)


Initial Transient Thermal Response

Initial transient stage arises when AC system is switched on after relatively long period of shutdown and prior to attaining steady periodic conditions again.

Initial transient stage may last for number of hours or even days depending on initial temperature distribution, thermal mass of wall, and location of insulation layer.


Most important of these applications is use of room air conditioners such as window and split units.

These units are normally switched on when room is occupied and off when it is not.

AC of such rooms is quite problematic because of thermal radiation from walls, especially if AC system has been off for few hours.


Validation: Comparison of temp variation with time at various interfaces in 1st cycle as obtained from finite-volume and semi-analytic solutions; July, west face.


Validation: Comparison of variation of transmission load with time in 1st cycle as obtained from finite-volume and semi-analytic solutions; July, west face.


Temperature Distribution Across Wall I (inner insulation)

West facing wall, July

Initial temperature is uniform at 37.2oC (daily mean outdoor air temperature).

Calculations start at t = 0 (midnight).

Distributions are shown later at different times during 1st cycle, and compared with those under steady periodic conditions.


Temperature distribution across wall I at different times in 1st cycle; July, facing west.


Temperature distribution across wall I at different times in steady periodic state; July, facing west.


Results indicate:

Initial transient effect diminishes rather fast for case of inside insulation and steady periodic state is practically reached after about 5 hours.

Fast change of inner surface temperature to value close to indoor design temperature reduces occupant discomfort (due to radiation exchange) and reduces energy consumption.


Temperature Variation Across Wall II (outer insulation) First Cycle:


Steady periodic state


Results indicate:

Most of temperature drop occurs within insulation layer near outside surface.

Concrete block with its large thermal mass on inside is responsible for slower temperature drop at inside surface.

This leads to thermal discomfort and increased energy consumption.

Initially stored energy in concrete block is essentially trapped and prevented by insulation from dissipating to outside.


This is reflected by positive temperature gradients across whole concrete block at all times during 1st cycle; heat is transferred mainly to inside.

Compared with steady periodic response, present results of outside insulation show that transient effects persist much longer compared to case of inside insulation.

Heat dissipated from concrete block is passed mostly to inner space, increasing transmission load.


Inside-Surface Temperature Variation

Inside-surface temperature variation with time for walls I and II are compared in next figure under initial transient and steady periodic conditions.

Inner surface temperature drops much faster in case of inside insulation (curve 4) reaching steady periodic state (curve 5) after about 5 hours.

Temperature drops at much slower rate for case of outside insulation (curve 1); it does not reach steady periodic state until after about two cycles (48 hours).


Inside-surface temp variation with time.


Transmission Load Variation with Time

Space heat gain and its variation with time are compared in next figure for cases with inside and outside insulation under initial transient and steady periodic conditions.

Transmission load variation with time shows similar trend to that of inner surface temperature (presented earlier) because it is proportional to difference between inner surface and indoor air temperatures.


Instantaneous transmission load for outside insulation is more than five-fold higher than that for inside insulation during early hours in 1st cycle.

It is concluded that energy consumption by AC during initial transient stage is much less when placing insulation on inside.

Besides, better comfort level is achieved faster with inside insulation mainly because of reduced radiation effects.


Variation of transmission load with time under transient and steady-periodic conditions for cases of inside and outside insulation; July, west facing wall.


Daily Transmission Loads

Daily transmission loads into space during 1st 24 hours of operation for cases of inside and outside insulation and for various wall orientations in July and January are presented in next figure and are compared with those under steady periodic conditions.

It is seen that daily cooling and heating loads are much smaller for inside insulation and for all orientations during 1st 24 hours.


Energy savings in first 24 hours is about 66% in July and 64% in January by placing insulation on inside; savings would be much bigger for shorter durations.

Effect of wall orientation is relatively smaller for outside insulation since heat gain or loss in 1st cycle comes mainly from energy stored in wall which is independent of wall orientation in present investigation.


Daily transmission loads during 1st cycle.


Daily transmission loads during steady periodic state.


Summary

Under conditions of present study, inner surface temperature drops relatively very fast and conditions reach steady periodic state after very short time (5 hours) for case of inside insulation.

For case of outside insulation, inner surface temperature drops much slower and wall needs more than two full cycles (48 hours) to reach steady periodic conditions.


Placing insulation on inside gives instantaneous load that is 20% of that for outside insulation during first few hours in transient process.

Duration of transient process (which leads to steady periodic state) and thus period of thermal discomfort due to radiation exchange is much shorter for inside insulation.


Average heat transmission over first 24-h period of AC operation with inside insulation is about one-third of that with outside insulation.

It is recommended that for spaces where AC system is switched on and off intermittently, insulation should be placed on inside.

This is usually the case in applications that utilize room air conditioners, such as window and split units.


It is suggested that future studies should be carried out to investigate effects of using different initial temperature distributions and different times of operating AC system.


S.A. Al-Sanea and M.F. Zedan, “Effect of insulation location on initial transient thermal response of building walls”, Journal of Thermal Env. & Bldg. Sci., Vol. 24, pp. 275-300, 2001.


TOPIC-3

Determination of Optimum Insulation Thickness


What is optimum insulation thickness?

Optimum insulation thickness (Lopt) is thickness that gives minimum total cost.

Total cost (ctot) comprises cost of insulation material and its installation, plus present worth of energy consumption cost due to transmission part of AC load over lifetime of building.


Typical cost versus insulation thickness.


Economic Model

Total cost per unit area of wall/roof is:

ctot = cins + cad + cenr

= Lins ci + cad + Ce PWF

Lins is insulation thickness, ci is cost of insulation material per unit volume, Ce is current yearly total cost of energy (SR/m2.year) and PWF is present worth factor accounting for inflation and discount rates.



In case rd ri ,

In case rd = ri ,

r1

r11

rr

r1PWF

d

i

id

i

m

r1

mPWF

d

ri inflation rate in energy cost,

rd discount rate

m expected lifetime of building (years)

Current Yearly Total Cost of Energy (Ce)

Ce = Etot ce

ce is current electric charge ($/kWh)

Yearly total electric energy consumption is:

Etot = Ec + Eh

For vapor-compression cooling, electric energy consumption Ec is: Ec = Qg / pc

Qg heat gain per unit area per year, pc coef. of performance


For heat-pump heating, electric energy consumption

Eh = Ql / pf ; pf is performance factor

Economic Parameters are:

Cost of insulation material, ci

Cost of installation of insulation, cad

Cost of electricity, ce

Lifetime of building, m (years)

AC performance factors,

Discount and inflation rates, rd and ri


TOPIC-4

Effect of Wall Orientation and Economic Parameters

on

Optimum Insulation Thickness


Nominal values of parameters used in economic model.


ci

($/m3)

cad

($/m2)

ce

($/kWh)

pc pf m

(years)

rd ri

* * 0.0317 3 4 30 0.07 0.04

* Cost depends on insulation material; details are given later.

Properties of materials and costs of insulation materials and their installation.


Material k (W/m.K)

(kg/m3)

c (J/kg.K)

Mat. c.

($/m3)

Inst. c.

($/m2)

HWHCB (200 mm) 1.05 1105 840 - -

Plaster board 0.17 800 1090 - -

Cement plaster 0.72 1865 840 - -

Polystyrene (molded) 0.036 20 1215 42.67 1.60

Polystyrene (extruded) 0.032 26 1215 69.33 1.60

Polystyrene (injected) 0.032 20 1215 50.67 1.60

Rock wool 0.042 30 837 48.00 1.60

Glass fiber 0.038 24 837 45.33 1.60

Polyurethane (board) 0.024 30 1590 138.67 1.60

Schematic of wall structure used in this investigation


Plaster board (12.5 mm)

Cement plaster (25 mm)

Molded polystyrene

Insulation (optimized)

HWHCB (200 mm)

Inside Outside

Effect of Wall Orientation on Total Cost and Optimum Insulation Thickness

Total cost is shown versus Lins in next figure using molded polystyrene.

Total cost comprises cost of insulation material and its installation plus present value of cost of energy spent to remove transmission loads over lifetime of building.

Total cost curve shows minimum value that corresponds to Lopt.


Total cost versus insulation thickness for molded polystyrene showing effect of wall orientation.


Effect of wall orientation on total cost and optimum insulation thickness using molded polystyrene.


Wall orientation Min. total cost

($/m2)

Optimum

thickness (cm)

South 9.74 8.75

North 9.88 8.88

East 10.14 9.20

West 10.19 9.25

Effect of Economic Parameters on Total Cost and Optimum Insulation Thickness

Parametric study is carried out to investigate effect of varying values of economic parameters (from their nominal settings) on total cost and Lopt.

Costs of insulations, electricity, etc. can vary appreciably with time; therefore, this sensitivity study is warranted.


Study is done by using molded polystyrene and for west facing wall.

Only one factor is changed at a time while keeping rest at nominal values.

Changes investigated cover rather wide, though still practical, range of economic parameters.

It is found that total cost and Lopt are quite sensitive to these changes; however, trends obtained are as expected.


Effect of insulation cost


Effect of electricity cost


Effect of AC equipment performance: pc and pf


Effect of building lifetime


Effect of discount rate


Effect of inflation rate


Summary

Wall orientation has significant effect on thermal behavior but relatively smaller effect on total cost and Lopt.

South facing wall is most favorite and gives about 12% lower yearly transmission load and 5% lower total cost compared to least favorite orientation which is west wall.

Total cost and Lopt are sensitive to changes in economic parameters.


Lopt is found to increase with cost of electricity, building lifetime and inflation rate; and decrease with cost of insulation material, coefficient of performance of AC equipment and discount rate.


S.A. Al-Sanea and M.F. Zedan, “Optimum insulation thickness for building walls in a hot-dry climate”, International Journal of Ambient Energy, Vol. 23, No. 3, pp. 115-126, 2002.


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