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08) 1153–1160

Energy and Buildings 40 (20

Thermal performance of insulated roof slabs in tropical climates

R.U. Halwatura 1, M.T.R. Jayasinghe *

Department of Civil Engineering, University of Moratuwa, Moratuwa, Sri Lanka

Received 14 March 2007; received in revised form 5 September 2007; accepted 2 October 2007

Abstract

Reinforced concrete roof slabs can be an ideal alternative to traditional roofs considering the better cyclone resistance that can be offered due to

the self weight. However, the concrete slabs do not perform satisfactorily in warm humid tropical climatic conditions and tend to act as heated

bodies for the occupants in free running spaces. As a solution, a robust roof slab insulation system is proposed and its thermal performance was

determined experimentally using small and large-scale models. With comfort models developed for the people acclimatized to tropical climatic

conditions, it is shown that insulated roof slabs could provide acceptable indoor conditions while providing many valuable benefits such as cyclone

resistance, regaining of land lost for the house and the possibility of creation of roof top gardens.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Roof slabs; Resistive insulation; Cyclone resistant construction

1. Introduction

Tropical climatic conditions prevail in many countries

located close to the equator. The main features of tropical

climates are the high humidity throughout the year coupled

with low diurnal temperature variations. Heavy rainfall during

monsoon periods is also possible which promotes vegetation.

Most of the countries with tropical climatic conditions in Asia

are experiencing rapid development. This has led to higher

energy demand for transportation and thermal comfort in recent

years. To curb the thermal discomfort associated with built

environments, the use of air conditioning is gradually become a

fashion due to affordability resulting from improved economic

standards and reduced capital cost of air conditioners [1]. This

is not a desirable situation in long term, since any increase in the

use of electricity generated using fossil fuels can increase green

house gas emissions. This can further aggravate the global

warming potential. Therefore, the promotion of passive

techniques that can allow free running buildings or minimize

the need for air conditioning is becoming important.

Many natural disasters that occurred in the recent past have

created another need for built environments. The tropical

* Corresponding author. Tel.: +94 11 2650567/38 2238545;

fax: +94 11 2650622/2651216.

E-mail addresses: rangika@civil.mrt.ac.lk (R.U. Halwatura),

thishan@civil.mrt.ac.lk (M.T.R. Jayasinghe).1 Tel.: +94 11 2650567/34 2221386; fax: +94 11 2650622/2651216.

0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2007.10.006

cyclones in USA in year 2005 and tsunami in Asia in year 2004

indicated the destructiveness of forces of nature at certain times.

It also indicated the need to have disaster resistant built

environments since collapsed buildings can injure and kill people

while needing rebuilding. Since most of the building materials

have significant amounts of embodied energy associated with

manufacturing and transportation [2], it is useful to ensure that

the built environments can withstand disasters with minimum

damage. Any rebuilding of damaged infrastructure means

repeated usage of embodied energy. Thus, disaster resistant

built environments can indirectly assist in reducing the green

house gas emissions. Therefore, the passive features included in

houses, if possible, should enhance the disaster resistance.

The Asian countries located close to the equator are

susceptible to be hit by tropical cyclones. The cyclonic conditions

can easily affect most of the roofing materials. One of the

exceptions is the reinforced concrete roof slab due to its weight.

However, such roof slabs tend to perform poorly in tropical

climatic conditions as far as thermal comfort is concerned. This

paper describes a detailed study carried out on the thermal

performance of reinforced concrete roof slabs provided with

resistive insulation located in warm humid climatic conditions.

2. Objectives

The main objectives of this research are to develop a robust

insulation system and to assess the effectiveness of roof slab

insulation in warm humid tropical climatic conditions.

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–11601154

3. Methodology

The following methodology was used for achieving the

above objectives:

(a) D

Tabl

Clim

Mon

Janu

Febr

Mar

Apr

May

June

July

Aug

Sept

Octo

Nov

Dec

ifferent resistive insulation thicknesses were tested with

models to determine the desirable insulation arrangements.

(b) A

large-scale model was made and tested to determine the

actual indoor performance in free running situations.

(c) T

he comfort models were used to determine the accept-

ability of resulting indoor conditions by the people

acclimatized to the tropical conditions.

4. The use of reinforced concrete slabs as roof

The scarcity of buildable lands in suburbs of large cities is

forcing the use of small plots for housing. For example, blocks

of extent 150 m2 and above are allowed in Sri Lanka for

residential buildings [3]. Even in Australia, small blocks of

200 m2 are increasingly used for detached houses despite the

availability of vast extents land [4]. This trend has promoted the

use of multi-story (two or three story) houses on small blocks of

land. The land saved can ideally be used for vegetation thus

creating a desirable microclimate in high density residential

developments [5].

In countries with tropical climatic conditions, the use of

reinforced concrete for floor slabs is much more popular than in

countries like USA, United Kingdom and Australia. One of the

main reasons is the existence of a termite-infested belt in the

Asian region that can affect timber in climatic conditions of

high humidity. Another is the availability and cost effectiveness

of the natural resources needed for concrete. This offers an ideal

opportunity to use reinforced concrete slabs for roof as well

instead of light weight roofing materials. Such slabs will be able

to resist cyclone induced forces very well since the weight of

the slab will be much more than the uplifting forces generated

by the cyclones.

However, the exposed roof slabs can transmit a considerable

amount of heat inwards thus making the indoors unpleasant

during the daytime. The prevalence of high soffit temperature

will allow it to act as a heated body thus emitting long wave

e 1

atic data for Colombo, Sri Lanka

th Sunshine

(h/day)

Average rainfall

(mm/month)

Mean daily temperature

Max (around 14:00 h)

ary 7.5 87.9 30.3

uary 8.2 96.0 30.6

ch 8.8 117.6 31.0

il 7.9 259.8 31.1

6.2 352.6 30.6

6.6 211.6 29.6

6.1 139.7 29.3

ust 6.5 123.7 29.4

ember 6.4 153.4 29.6

ber 6.2 354.1 29.4

ember 6.8 324.4 29.6

ember 6.9 174.8 29.8

infrared radiation towards the occupants. This can continue

during the night as well due to the heat capacity of the reinforced

concrete slab. The heat energy absorbed can cause thermal

expansion in the slab that may lead to cracks in the supporting

structure primarily consisting of load bearing masonry materials

such as brick work or block work. This means that roof slabs will

need a suitable insulation system, which can allow unrestricted

access while solving the above problems.

If access can be allowed without restrictions, the roof slabs

can be used to regain the land lost for the house as a roof top

garden. The existence of a flat top will allow vegetation at roof

level thus facilitating the creation of desirable micro climatic

conditions further [6,7].

5. The effect of insulation thickness

The tropical climatic conditions have few special features.

Due to close proximity to the equator, the day remains with

almost equal day and nighttimes throughout the year. The hours

of intense sunlight will be only over about 6–8 h where the

intensity can reach 800–1000 W/m2. The out door maximum

daytime air temperature reaches about 30–32 8C with a

possibility to rise to about 35 8C only for few days in a year.

The nighttime temperature could drop to about 22–25 8C.

These conditions can be considered as somewhat desirable than

that would occur in Middle East countries with hot dry climatic

conditions [8]. These indicate that the development of an

effective insulation system should not be a difficult task for the

tropical climatic conditions. Some average climatic data

pertaining to Colombo, Sri Lanka (latitude of 78N and

longitude of 798E) are presented in Table 1.

5.1. The experimental setup

In order to determine the effect of insulation thickness, four

models were constructed as shown in Fig. 1. One was without

insulation (Case 1). Others had 25 mm (Case 2), 38 mm (Case 3)

and 50 mm (Case 4) thicknesses. In order to ensure access

without restrictions for various activities, the insulation was

covered with a 40 m thick screed concrete as shown in Fig. 2. In

order to support the screed, the insulation is broken into

Minimum and maximum relative humidity (%)

Min (around 6:00 h) Min (around 14:00 h) Max (around 6:00 h)

22.2 58 90

22.3 59 92

23.3 64 94

24.3 68 95

25.3 72 92

25.2 73 93

24.9 70 90

25.0 65 90

24.7 67 91

23.8 70 92

22.9 67 93

22.4 61 91

Fig. 1. Arrangement of small-scale models.

Fig. 2. The details of the insulation system.

Fig. 3. Laying of 500 mm � 500 mm insulation panels with a 40 mm gap.

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–1160 1155

500 mm � 500 mm square panels with a 40 mm separation as

shown in Fig. 3. Thus, there will be a narrow strip of concrete of

40 mm width between 500 mm � 500 mm insulation panels.

These narrow concrete strips can reduce the effectiveness of the

insulation system to a certain extent, but would provide an ideal

support for the concrete. The insulation used was expanded

cellular polyethylene. It has a thermal conductivity of 0.034 W/

K m2.

Since the thermal measurements were needed continuously,

Type K thermocouples were used to measure the temperatures.

Type K thermocouples are available in the �200 to +1200 8C

Chart 1. Soffit temperature distribution

range. Sensitivity is approximately 41 mV/8C. During pro-

gramming, each channel of the data logger can be set up for

sensor type and range, conversion factor, logging frequency (1 s

to 24 h) and valid reading range. Logged data was stored in

battery-backed RAM, with capacity for 64 k readings.

5.2. The results

There are two important parameters that can affect the indoor

comfort at the top floor of a free running building. They are the

ceiling temperature (soffit temperature in the case of roof slab)

and the amount of heat transmitted through the ceiling. These two

are interconnected in the case of a roof slab. The heat flow will

depend on the air to air resistivity of the roof slab and the

temperature gradient which depends on the roof top and soffit

temperatures. The soffit temperatures observed for the four cases

in a day with bright cloudless sky are given in Chart 1.

Chart 1 clearly indicates that resistive insulation can be very

effective in reducing the soffit temperature. Without insulation,

the soffit temperature reached about 42 8C. This indicates that

the roof slab soffit can act as a heated body and would not allow

the use of the building as free running with acceptable level of

thermal comfort. The resistive insulation can have a significant

influence in reducing the soffit temperature. The maximum

value recorded was about 33 8C for 25 mm insulation and 32 8C

for different insulation thicknesses.

Chart 2. Heat flow values for different insulation thicknesses.

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–11601156

for 50 mm insulation. This soffit temperature achieved with

insulation may offer better possibilities for free running built

environments at top floor since it differ only slightly from the

outdoor maximum temperature.

The approximate heat flow that can be expected for four

cases are given in Chart 2. These values were calculated using

soffit temperature and air to air resistivity. The air to air

resistivity of the roof slabs have been estimated as 0.99, 1.37,

1.71, 0.25 m2 K/W for 25 mm, 38 mm, 50 mm and without

insulation, respectively. The corresponding ‘‘U’’ values are

1.01, 0.73, 0.58, 4.0 W/m2 K (specimen calculation given in

Appendix B). Chart 2 indicates a significant drop in heat flow

with insulation.

6. The thermal performance with a larger model

6.1. The model

Since the smaller models may have some influence of the

boundaries, a larger model was created. The model was a small

building intended for storage of various laboratory equipment

as shown in Fig. 4. It has plastered one brick thick walls and a

reinforced concrete slab as the roof. The roof slab thickness was

125 mm. It was not provided with any insulation. This allowed

gathering of some useful temperature data prior to the

installation of insulation system.

Fig. 4. Large-scale model.

6.2. The temperature measurements

It was not possible to have a comparison for a given day with

and without insulation since the modifications were carried using

only one model. In order to facilitate some comparison, the

temperature measurements were taken over a number of days

prior to the application of modifications. In a warm sunny day, the

soffit temperature reached a maximum value of 45 8C with the

roof top temperature reaching 55 8C. After the installation of

25 mm of insulation, the soffit temperature remained below

35 8C when the roof top temperature reached 55 8C as shown in

Chart 3. This clearly indicated that roof insulation is working

effectively even though the resistive insulation thickness was

only 25 mm. Fig. 2 indicates the components of the insulation,

water proofing and protective screed system used to facilitate the

access without any restriction.

6.3. The robustness of screed concrete

It should be noted that the use of strip of concrete between

insulation panels to support the 40 mm protective screed can

improve the load carrying capacity significantly while

minimizing any possibility for developing cracks that can

subsequently lead to leakage of water. In order to determine the

actual load capacity, loads were applied at the centre of a panel

through the screed. It needed a load of 4.0 tonnes to initiate

cracks. This exceeds the point load requirement specified in BS

6399:Part 1 1996 [9] for slabs with access. This includes that

the screed concrete can easily support the maximum load of

5 kN/m2 that can be expected due to crowd loading. Therefore,

there is no need to have any restriction for access.

6.4. The thermal expansion

The thermal expansion is a serious problem for roof slabs

since it can cause serious cracking on walls. However, the

presence of 40 mm thick reinforced concrete screed can confine

the expansion primarily to the screed while minimizing the

expansion of the roof slab. The temperature measurements

taken at the top and the bottom screed and top and bottom of the

reinforced concrete slab are presented in Chart 4. It can be seen

that the roof top temperature reached about 64 8C in this

Chart 3. Temperature variation with 25 mm slab insulations in a day with bright sunlight.

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–1160 1157

particular day with plenty of direct solar radiation. The

temperature at the bottom surface is an indication of the direct

expansion that can be expected. The bottom temperature value

in the screed has risen to about 48 8C. The soffit temperature of

the reinforced concrete slab remained at about 35 8C. The

temperature readings taken prior to the installation of insulation

indicated a maximum soffit temperature of 45 8C when the top

temperature was 55 8C. This indicates that the tendency for

thermal expansion can be shifted from the roof slab to the

screed in the insulation system proposed.

7. The feasibility of free running spaces

In warm humid tropical climactic conditions, the absence of

heated bodies is very important to provide thermal comfort

during the daytime where the outdoor temperatures rise to a

reasonably high level. In this context, the use of roof slab

resistive insulation is very important to ensure that the soffit

temperature will remain at a sufficiently low level. The soffit

temperatures can be compared with the allowable range for a

free running building once the allowable range is established.

According to Szokolay [10], the neutrality temperature can

be calculated using the mean climatic data Tn = 0.31To + 17.6,

Chart 4. Surface temperatures fo

where To is the mean temperature. This relationship, which is

based on many comfort survey results, will take account of

acclimatization of the people to a given climate. It is assumed

that there will not be any heated surface such as roof, ceiling or

walls close to the occupants. Mallik [1] also indicated the

applicability of Tn found using above equation for tropical

climatic conditions prevailing in Bangladesh. The same

equation was validated by Soyigh and Marafia [11] for the

determination of neutrality temperature for summer conditions

which could be similar to tropical conditions.

The neutrality temperature on monthly basis is given in

Table 2. It indicates that there is only a minor variation in

neutrality temperature throughout the year. This is in contrast to

higher latitudes where the neutrality temperature could change

on monthly basis, since people tend to respond to the seasonal

changes of the climate [12]. For example, it was shown by

Laxmore et al. [4] that for Ipswich in Australia, Tn can vary

between 22 and 25.5 8C.

For warm humid climatic conditions prevailing in Sri Lanka,

an average neutrality temperature of about 26.0 8C can be used

as shown in Table 2. It was reported by de Dear and Bragar [13]

that about 80% of the people would be thermally comfortable

within a band of 7 8C about the neutrality temperature in free

r 25 mm insulation system.

Table 2

Neutrality temperature for Colombo on monthly basis

Month Monthly mean

temperature, To

Monthly neutral

temperature, Tn

January 26.3 25.7

February 26.5 25.8

March 27.2 26.0

April 27.7 26.2

May 28.0 26.3

June 27.4 26.1

July 27.1 26.0

August 27.2 26.0

September 27.2 26.0

October 26.6 25.8

November 26.3 25.7

December 26.1 25.7

Annual 26.9 26.0

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–11601158

running buildings. This could be further extended to take

account of physiological effect of cooling if sufficient air

movement is available. The use of equation 6V � V2, where V is

the indoor air velocity, was suggested by Szokolay [10].

It should be noted that this will be a larger comfort zone than

that indicated by a 4 8C range suggested by Szokolay [10]. On

the basis of comfort surveys involving a large number of

subjects, it was suggested that the upper limit of the comfort

zone can be considered at a humidity ratio of 0.015 instead of

0.012, in the case of warm humid tropical climatic conditions

[14]. It is also suggested using a humidity ratio of 0.020 as an

upper limit when the physiological cooling effects are taken

into account [14], since the consideration of 90% relative

humidity line as the upper boundary [10] can allow very high

humidity ratios that are unlikely to be tolerated by people even

with significant air movement.

When all these suggestions are considered, it is possible to

have following guidelines for the comfort zones for those who

are acclimatized to tropical climatic conditions. When the air

Fig. 5. Psychometric chart wi

movement is low, the comfort zone can have a range of

�3.5 8C. The side boundaries would be the Standard Effective

Temperature (SET) lines which can be approximately

determined using the equation proposed in Szokolay [10] for

baseline intercept which is Tintercept = T + (T � 14) � HR. The

upper boundary will be given by a humidity ratio (HR) of 0.015.

The lower boundary by an HR of 0.004.

When air velocity is high, the comfort zone can be extended

by 6V � V2. For example, for an air velocity of 0.6 m/s, the upper

temperature on the 50% RH line can be increased by 3.24 8C. For

this extended comfort zone, the upper boundary will be 90% RH

line and a humidity ratio of 0.020. The gradient of the SET line is

reduced below a HR of 0.012. However, such low HR values will

not be applicable to tropical climatic conditions where the

atmosphere tends to have a considerable amount of moisture.

When all these suggestions are taken into account, it is

possible to propose the comfort zones as indicated in Fig. 5. It

indicates the comfort zone without much ventilation and the

extended zone with air velocities of 0.6, 0.8 and 1.0 m/s. The

sample calculations for comfort zones without much ventilation

are given in Appendix A. The calculations for the comfort zone

extended to take account of physiological cooling effect of an

air velocity of 0.6 m/s are also given.

These extended comfort zones indicate that temperatures up to

34–35 8C could be tolerated in free running buildings with suffi-

cient ventilation in tropical climatic conditions. This is an encou-

raging finding that has many positive implications as follows:

(1) W

th ex

hen insulated roof slabs are used, the soffit temperature

may rise to about 35 8C although the indoor average

temperature may remain bellow 32 8C in warm sunny days

without much cloud cover. However, 35 8C will not be high

enough to act as a heated body for occupants since it is

lower than the human body temperature. Thus, there is a

good potential to provide indoor thermal comfort with

enhanced ventilation and air movement provided by fans.

tended comfort zone.

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–1160 1159

Therefore, it may be possible to reduce the shift towards

installing air conditioners in warm humid tropical climates,

when the reinforced concrete slabs are provided with an

adequate resistive insulation system.

(2) I

n warm humid tropical climatic conditions, the adoption of

many passive features could provide about 2–3 8C lower

indoor temperatures than the outdoor as described by

Ratnaweera and Hestnes [15]. In addition, provision of

courtyards could also be considered [16]. The favourable

performance of insulated roof slabs in relation to relatively

low soffit temperatures and heat flows could be coupled

with other passive features to assist in eliminating the

presence of heated surfaces around the occupants. This is

essential for the comfort zones presented in Fig. 5.

(3) W

hen the roof can be provided with access without any

restrictions, there will be a potential to regain the land lost

for the construction of the house. This roof top also could be

converted to a roof top garden realizing the cooling effect of

greenery cover [6,7].

8. The cyclone resistance

The insulated roof slabs are heavy. The weight of screed over

the insulation and a 125 mm slab could be about 4 kN/m2. The

maximum recommended 3 s gust velocity for Sri Lanka is

49 m/s. This can give rise to an uplift of only 0.5 kN/m2 in a

typical house [17]. This clearly indicates that a tropical cyclone

is unlikely to cause any significant damage to a house with

insulated roof slabs except any damage possible due to flying

debris. The heavy slab can also enhance the stability of walls

improving the cyclone resistance.

9. Conclusions

It is shown that reinforced concrete roof slabs could be an

ideal alternative to conventional light weight roofs as far as

preventing the uplifting during tropical cyclones is concerned.

The poor thermal performance of concrete slabs can be

significantly improved with resistive insulation. It is shown that

an insulation thickness of only 25 mm can provide a noteworthy

improvement in tropical climatic conditions specially with

respect to reducing the slab soffit temperature. The comfort

zones generated by combining various guidelines proposed

previously have indicated that temperatures up to 35 8C could

be tolerated by those acclimatized to warm humid conditions

when plenty of ventilation is available. This indicates that there

is very low possibility for an insulated roof slab to act as a

heated body when located above free running spaces. There is a

potential to improve the performance further by adopting the

roof top gardens concept. Thus, insulated roof top slabs with a

robust insulation system could be a viable and effective

alternative to conventional roofs in tropical climatic conditions.

Acknowledgements

Authors wish to thank McBerton Polymer Ltd. funding the

experimental programme. The technical assistance of Messrs

SP Madanayake, SL Kapuruge, HP Nandaweera and SRD Silva

of University of Moratuwa is very much appreciated.

Appendix A

A.1. The calculation for the comfort zone without much

ventilation

The neutrality temperature, Tn = 26 8CWidth of the comfort zone � 3.5 8CTherefore, T1 = 22.5 8C; T2 = 29.5 8CHumidity ratio at 50% RH, for T1 = 0.0086 and T2 = 0.0128

Therefore,

Tintercept = T + 23(T � 14)HR

T1,intercept = T + 23(T � 14)0.0086 = 24.18 8CT2,intercept = T + 23(T � 14)0.0128 = 34.06 8C

A.2. The calculations for enhanced ventilation

The calculations are given for an internal air velocity of

0.6 m/s

V = 0.6 m/s

Therefore, 6V � V2 = 3.24 8CT3 = 29.5 8C + 3.24 8C = 32.27 8CHR for 32.27 8C at 50% RH, is 0.015

T3,intercept = 38.73

The dry bulb temperature corresponding to a HR of 0.012 on

the set line for air velocity of 0.6 m/s = 33.5 8CThe intercept with half slope is (33.5 + 38.7)/2 = 36.1 8C

Appendix B

Material

Conductivity, l (W/m K)

Concrete

1.7

Polyethylene insulation

0.035a

a A value of 0.035 is used to take account of discontinuities in insulation

panels.

Surface resistances

Ceiling upwards

Rsi 0.10

Ceiling downwards

Rsi 0.14

Roofs

Rso 0.04

Calculation of ‘‘U’’ value for an insulation of 25 mm

covered by 40 mm screed and located on 125 mm thick

reinforced concrete slab:

Rbody

=b1/l1 + b2/l2 + b3/l3

=0.04/1.7 + 0.025/0.035 + 0.125/1.7

=0.81 m2 K/W

Air to air resistance, Ra–a

=Rsi + Rbody + Rso

=0.04 + 0.81 + 0.14

=0.99 m2 K/W

U value

=1.01 W/m2 K

R.U. Halwatura, M.T.R. Jayasinghe / Energy and Buildings 40 (2008) 1153–11601160

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R.U. Halwatura He is a research assistant at the Department of Civil Engineer-

ing of University of Moratuwa reading for PhD by research at University of

Moratuwa, Sri Lanka. He obtained his BSc engineering degree with a first class

honours from the University of Moratuwa in 2004.

M.T.R. Jayasinghe He is a Professor at the Department of Civil Engineering of

University of Moratuwa, Sri Lanka. He obtained his BSc engineering degree

with a first class honours from University of Moratuwa in 1987 and his PhD

from Cambridge University in 1992. Since then he has undertaken extensive

teaching and research in the field of structural engineering and building energy.