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Prof. Bijon B. Sarma
2007-09-29 07:29:41 UTC

Prof. Bijon B. Sarma

The uppermost roof of the building needs insulation for thermal and
humidity purposes. In Bangladesh Lime-terracing is used for this
purpose. Mortar made of slaked Lime and powdered Brick (local name :
surki) is applied on the r.c.c. roof in 100 mm layer. It works fairly
well in the situation where the variation of internal and external
temperatures is low. In Bangladesh this variation in the naturally
ventilated rooms is not over 30 Celsius and lime terracing shows a
time lag of about 5 hours at this range.

Some of the problems of the conventional Lime terracing are : (a) It
is costly, (b) It allows heat leakage when atmospheric temperature
rises above 350 Celsius and also when there is wide variation in
internal and external temperatures due to air-conditioning of the
room. The authors of this paper devised an alternate system. In this
system air pockets were provided inside the layer such that it became
lighter, consumed less material and had more insulating properties.
The air pockets were provided by using burnt clay pots available in
the market. The system was named Burnt-Pot Roof Insulation,
abbreviated as BPRI. BPRI was first applied in Khulna, Bangladesh in
2002. The cost of constructed was only 10% cost of the conventional
lime terracing. Now BPRI is being used in various regions of the

The thermal and humidity insulation used over r.c.c. roof in
Bangladesh is known as 'Lime terracing'. In this system mortar of
Lime and Surki is applied on the r.c.c. roof in 100 mm to 150 mm
layer. The material is heavily beaten that renders it water-proof.
However, it increases the density of the material and hence its heat
absorbing capacity. The conventional type of Lime terracing works
fairly well in the situation where the variation of internal and
external temperatures is low. In the naturally ventilated rooms in
Bangladesh this variation is not over 30 Celsius. At this range of
variation a 100 mm thick lime terracing shows a time lag of about 5
hours. However, during the summer season, most of the roofs are
exposed to direct sunray for a period of 6 to 7 hours. In such case
heat is found to transmit inside during the afternoon and evening. A
"user's response survey" was conducted in 28 high-rise apartment
buildings in Dhaka city in 2002. All the buildings under survey had
lime terracing insulation. The respondents residing on the top floors
complained of heat from the roof during hot days (Sarma, 2002) .

It has been observed that 'Heat conduction through the material of the
roof can be lowered by using materials with insulating properties,
increasing thickness, using air cavities etc." (Sarma, 2005).
Following this principle the authors of this paper devised an
alternative method and applied the same on the roof of a house in
2002. In this system burnt clay pots were placed in inverted positions
and covered with mortar of Lime, Surki and Building rubbish. The
thermal and humidity insulating properties of the layer was felt
almost immediately. The system is known as 'Burnt-Pot Roof Insulation'
abbreviated as BPRI. Burnt clay pots are manufactured in the country
for the purpose of marketing yogurt and these pots have been found
suitable for this purpose. Other materials used are : Polythene sheet,
Lime, Surki, Building rubbish and Cement. The clay pots are placed in
inverted positions and covered with mortar of lime, surki and building
rubbish. The air trapped inside the pots act as insulator and their
use considerably reduces the weight of the material and their cost.
Polythene sheet acts against leakage of humidity or water. After
observing its initial success and extremely low cost, this system has
been used in a number of buildings all over Bangladesh.

The traditional Lime terracing has got many disadvantages or
limitations, notable among those are the following :

(i) It is quite costly because of the cost of material, cost of labor
for beating etc. When the owners go for vertical phasing, every time
they go for vertical extension they need to remove the previous lime
terracing and apply anew. Thus the money applied on it earlier turns
to wastage.

(ii) The traditional Lime-terracing is heavy in weight. It needs
professional experts for the mixing of materials and contingent of
laborer for beating. The beating creates hazardous vibration and noise

(iii) The removal of the material after the expiry of life (after 15
to 20 years) is quite hard and quite often it damages the roof.

(iv) Lime-terracing shows thermal leakage in the naturally-ventilated
rooms at temperature above 350 Celsius and at lower temperature in the
air-conditioned buildings.

The following methodology was followed for the BPRI :
(a) The reinforced concrete roof was cleaned and all sharp edges,
removed. It was the soaked with water for 6 hours. .
(b) Cement slurry (cement : water = 1: 16 by weight) was evenly spread
on the surface.
(c) When the slurry turned semi-dry polythene sheet (weighing minimum
1 kg per 6.5 Cubic Meter) was spread evenly. An overlap of minimum
half meter was provided at places where two sheets met.
(d) Burnt clay pots (size : diameter 250 mm, height 75 mm) soaked for
6 hours were placed in inverted positions. Those touched one another
at the rim and left minimum space in between.
(e) The voids were filled with mortar made of lime, surki and building
rubbish (proportion 1:4:16 by volume) up to a height of 25 mm. Prior
to use the lime was slaked (soaked in water) for 8 hours. The size of
the coarse aggregate (building rubbish) was less than 15 mm.
(f) Twenty-four hours after the application of the first layer,
another 50 mm thick layer of the same material was applied. This time
bigger aggregates (not exceeding 40 mm) were used.
(g) Twenty-four hours after the above application the final layer of
mortar (thickness : 25 mm to 50 mm) was applied. This time the size of
the coarse aggregate was less than 15 mm.
(h) The job was left for drying for a period of 48 hours. Then cement
slurry (Proportion - cement : water = 1: 16 by weight) was evenly
applied on the surface.
(i) Slope for drainage of water was maintained at the top layer,.

The system has been explained in the Figure 01.


BPRI was first applied in the roof a two-storied house in Khulna city
in April, 2002. Immediately after application, the users reported to
have the similar thermal comfort as they usually feel due to
conventional lime terracing. The comparative efficiency of BPRI in the
following points was evaluated in 2006, i.e. after 4 years,.

a. WEAR AND TEAR : After normal use for four years, the roof did not
show any sign of crack, wear or tear.
b. HUMIDITY LEAKAGE : A part of the BPRI was exposed to find out if
there was any deposit of water or humidity in the void space. There
was absolutely no such deposit and the material was found fairly dry.
c. EFFICIENCY AS THERMAL INSULATOR : Heat transmission through the
material of the roof is given by : Q=AUTdiff , where A = surface area
in m2, U = transmittance value in W/m2 deg. Celsius and Tdiff =
Temperature difference.

Walls : Normally the external walls are constructed with 250 mm thick
brick with plaster on both sides. Such walls have found to have Time
Lag of over 12 hours. In the summer seasons, the western and southern
walls may be exposed to direct sunray for maximum 7 hours (10 AM to 5
PM). Since this period is less than the time lag of the wall-material,
normally there happens no transmission of heat through the walls.

Roof : In the summer season the average sunshine per day in the major
cities of Bangladesh varies from 5.7 to 9.1 hours (Sarma, 1997).
During this time the roof gets direct solar radiation from 10.00 AM to
5.00 AM, i.e. for about 7 hours. In a condition with atmospheric
temperature exceeding 320 Celsius, 100 mm thick reinforced concrete
roof with no insulating layer starts transmitting heat inside after
only half an hour. In Bangladesh most of the houses are naturally
ventilated through windows, fanlights and ventilators. This makes the
difference between the external and internal temperatures quite low.
Due to this low value of Temperature difference (Tdiff) a 100 mm thick
layer of lime-surki can resist the conducting heat for about 5 hours.
In Bangladesh, the experts recommend a time lag of 3 hours for the
roofing materials (Alam and Ullah, 1994). However, heat conduction
starts much earlier when the atmospheric temperature exceeds 350

In the above context, for comparative evaluation of Lime terracing and
BPRI, it was decided to take comparative Temperatures in two similar
buildings with two types of insulation under two conditions, i.e.
below and above 350 C. Six groups of enumerators @ 3 for each
building were equipped with Electronic thermometers (thermo-hygrometer
with clock and measuring up to 1/10 of a degree) and were advised to
take simultaneous reading at three locations of the building on the
same days exactly at times : 2.30, 2.45, 3.00, 3.15 and 3.30 PM. The
operation was conducted first time on 16th May, 2006, when
temperature was below 350 Celsius and second time on July 20, 2006,
when temperature was above 350 Celsius. The following temperatures
were recorded : (i) Skin temperature of the upper side of the roof,
(ii) Skin temperature of the lower side of the roof and (iii) Room
temperature at 2 Meters below the roof. The findings have been
presented below in Tables 01 and Table 02.

(Temperatures at Time : 2.30, 2.45, 3.00, 3.15 and 3.30 PM.
Locations : a. above the roof, b. below the roof and c. 2 meter below
the roof. Conv. stands for "Conventional type of Lime Terracing" and
BPRI stands for "Burnt Pot Roof Insulation)


Time : 2.30 PM 2.45 PM 3.00 PM 3.15 PM
3.30 PM
Insulation Type: Conv. - BPRI Conv. - BPRI Conv. - BPRI Conv. -
a. Temp above : 31.8 - 31.4 32.0 - 31.9 32.5 - 32.4 32.3 -
32.2 32.1 - 32.8
b. Temp. below : 28.3 - 28.3 28.6 - 28.5 29.0 - 28.8 28.9
- 28.7 28.7 - 29.4
c. 2 M. below : 27.0 - 26.4 26.7 - 26.5 27.1 - 26.9 27.1 -
26.8 26.8 - 27.4

Table 02 : TEMPERATURES MEASURED ON JULY 20, 2006. (Time and
positions : same)

Time : 2.30 PM 2.45 PM 3.00 PM 3.15 PM
3.30 PM
Insulation Type: Conv. - BPRI Conv. - BPRI Conv. - BPRI Conv. -
a. Temp above :37.5 - 37.4 37.8 - 37.6 38.1 - 38.0 38.3 -
38.1 38.2 - 38.0
b. Temp. below :36.2 - 33.8 36.4 - 34.1 36.8 - 34.5 37.1
- 34.7 37.0 - 34.7
c. 2 M. below : 34.1 - 31.6 34.0 - 31.9 34.7 - 32.5 35.0 -
32.5 35.1 - 32.6

The following were deduced from above measurements :

Differences of temperatures above and below the roofs.
(a) At temperature below 350 C. Conventional Lime-terracing 3.44
BPRI 3.40
(b) At Temperature above 350 C. Conventional Lime-terracing 1.28
BPRI 3.46

The above findings reveal that the conventional type of lime terracing
allows thermal leakage when the atmospheric temperature exceeds 350 C.
It may be noted that thermal leakage starts at lower temperature if
Tdiff is high, and it happens when the room is air-conditioned. The
temperature in an air-conditioned room needs to be maintained below
260 Celsius for comfort, where the external temperature may be 330-390
Celsius. Even though it was possible to conduct this experiment with
air-conditioned buildings for lack of logistics, it is expected that
BPRI would show better performance because of insulation provided by
the air-pockets of the burnt-pots.

Burnt Pot Roof Insulation (BPRI) has got a number of advantages over
the conventional Lime-terracing. Some of these are :
(i) BPRI is extremely cheap. The cost of civil works depends upon a
number of factors like the cost of materials, labor, transport,
storage etc., all of which varies from place to place and from time to
time. So, it is not possible to find the comparative costs on global
basis. However, in the Khulna region of Bangladesh the cost for the
two types has been calculated and it was found that BPRI costs only
10% of traditional Lime-terracing.
(ii) The construction of conventional lime-terracing needs expert
masons, skilled workers and experienced beaters. It is possible to
make BPRI by the semi-skilled laborers.
(iii) The weight per unit area of BPRI is about 50% of the
conventional lime-terracing.
(iv) Quite often the house-owners construct buildings by vertical
phasing. In such case, every time the building is extended vertically
the owners need to remove the old layer of insulation. BPRI is
suitable for such purpose because of its extremely low cost.
(v) Lime-terracing needs to be removed and re-applied after every 15
to 20 years. It has been found that the process of its removal
involving heavy beating damages the reinforced concrete roof. BPRI is
free from such hazards.

Burnt Pot Roof Insulation or BPRI is now a popular term in Bangladesh.
It has been used in many buildings and showing excellent results. One
of the causes of its popularity is its cost which is extremely low.
With increased demand the potters have increased their production. The
first BPRI constructed in 2002 is still in excellent condition and has
no sign of crack, wear and tear. Since the age of the oldest BPRI is
only 4 years, it is not possible to predict its life-time. However,
the users seem to have little headache for it because of its extremely
low cost.

FORM DESIGNERS", ISBN 984-8684-02-6, Authors Bijon B. Sarma and
Falguni Malick, Published by Bangladesh Publishers, Dhaka, Bangladesh
P. 104.
Sarma, 2002 : Unpublished M. Arch Thesis "A STUDY OF THE FACTORS FOR
Architecture Department, Bangladesh University of Engineering and
Technology, Dhaka, Bangladesh in 2002.
ENERGY IN BANGLADESH", published in the ARCHI TIMES, November 1997,
Vo. 12, No. 11, Published from Gafoor Chamber, Karachi, Pakistan.
Published in the Report of the Technical Conference on Tropical Urban
Climates. WCASP - 30. WMO/TD-No. 617, World Meteorological
Organization 1994. Page 395- 412.
*Prof. Bijon B. Sarma, Head Architecture Discipline, Khulna
University. Khulna-9208. Bangladesh.
2014-04-21 10:46:02 UTC
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