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


Validation: Periodic heat conduction in three-
layered wall
Indoor Outdoor
15-cm 5-cm 10-cm
HWHCB MP HWHCB

x

• 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.

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
 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


 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 ,
 m
 1 r    1 r  
PWF   i  1  i
 r  r   1 r  
 d i   d 
 

m
In case rd = ri , PWF 
1 rd
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 = Q g / pc
Q g 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 cad ce pc pf m rd ri
($/m3) ($/m2) ($/kWh) (years)
* * 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  c Mat. c. Inst. c.
(W/m.K) (kg/m3) (J/kg.K) ($/m3) ($/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
 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 Optimum
($/m2) 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
 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.


THANK YOU


Mohamed Zedan - State of The Art in the Use of Thermal Insulation in Building

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