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Movable roof insulation in hot climatesS. P. Jain & K. R. Rao

Published online: 15 Apr 2008.

To cite this article: S. P. Jain & K. R. Rao (1974) Movable roof insulation in hot climates, Building Research and Practice, 2:4, 229-234,

DOI: 10.1080/09613217408550323

To link to this article: http://dx.doi.org/10.1080/09613217408550323

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MOVABLE ROOF INSULATION IN

HOT CLIMATES/continued

Experimental test set-up

For this study two identical full sized (3.5 by 2.9 by

3.2 m) test rooms having similar design and the

following specification were selected:

Ro ofs: 150 mm with 12.7 mm cement plaster both

sides.

Walls: 230 mm solid brick wall with 12.7 mm cement

plaster both sides.

More detailed information about this set up is given

in earlier publications (ref 3).

One of the test rooms was treated with movable

insulation on its roof with and without a roof pond

(figs 1 and 2). Ano ther test room with an untreated

roof served as the standard room for comparison.

The arrangement and removal of insulation was done

manually for these studies; however a simple mechani-

cal system of working by pulling a rope twice a day

could be developed.

Test conditions and the duration of various tests are

listed in table 1. An air cond itioner was also installed

in the untreated test room at the southern wall so as

to compare the effect of a movable system of in-

sulation with a roof pond against the system of

air conditioning.

Temperatures were measured by 24 SWS calibrated

copper Constantan thermocouples. Heat flow rates

were measured by calibrated heat-flow meters

consisting of several differential thermal junction s. It

may be mentioned here that all the measurements

described were made round the clock and at hourly

intervals. Only selected days on which clear weather

remained throughout the day have been considered.

Results and

discussions

A series of tests were conducted to test the thermal

equivalence of the two test rooms in both the seasons

without any treatment. For this the indoor air and

the ceiling temperatures of the two test rooms were

Table Test conditions and period of test

compared. The deviations in the maxima and minima

temperatures of indoor air and ceiling surface from

the relevant temperature conditions in the standard

test rooms have been found to be quite negligible.

Factors affecting thermal condit ions

Roof surface temperatures

Summer diurnal variations of the roof temperatures

with and without a roof pond with movable in-

sulation are presented in figs 3 and 4 and plotted

against the temperature of an untreated roof. The

order of reduction as compared with the untreated

unit was 34° and 30

c

C respectively, which is quite

significant.

Ceiling surface temperatures

On comparing the ceiling maxima temperatures in

figs 5 and 6 of the treated unit with and without a

roof pond, there may be seen a considerable drop of

the ord er of 19° and 16°C respectively. With the

roof pond alone a drop of only 13°C was observed

using a 75 mm deep layer of water. This clearly

shows that movable insulation has a marked influence

on the ceiling temperature, which in turn causes

considerable reduction in the radiant heat load and

also in the temperature gradient inside the building.

With air conditioning of the untreated unit it is

observed that the ceiling remains at a higher tempera-

ture, of the order of 9°C as compared to that of the

treated unit with movable insulation on a water pond.

This is definitely due to a greater heat flow through

the roof by virtue of the large differences in the

outdoor and indoor air temperatures.

One very interesting observation is also to be made,

as shown in table 2 about the measured maxima

temperatures in the summer season of indoor surfaces

of the untreated and treated units with movable

insulation only.

TEST

ROOM

No.

1

2

SUMMER

1

6- 7

May 72

Untreated

Untreated

II

9-10

May 72

Untreated

Roof pond

day and

night

II I

13-14

May 72

Untreated

Movable

insulation

with roof

pond

IV

29-30

May 72

Untreated

Movable

insulation

only

V

3-4

June 72

Air-

conditioned

Movable

insulation

with roof

pond

WINTER

1

15-16

Jan 73

Untreated

Movable

insulation

only

II

29-30

Jan 73

Untreated

Untreated

23 0

Building Research and Practice July/Aug ust 1974



Table 2 Measured indoor surface maxima te m-

peratures of untreated and treated units

Surface

Ceil ing

North

East

South

West

Floor

Untreated

Deg C

45

33.3

33.8

34.2

34.4

30.9

Treated

(movable in-

sulat ion only)

Deg C

28.8

30.0

31.3

32.2

32.5

29.6

These measurements clearly show that the ceiling of

the treated unit has the lowest temperature as com-

pared with other surfaces of the unit, and therefore

acts as a heat sink for other surfaces, whereas the

ceiling of the untreated unit, being at a higher temp-

erature than the other surfaces of the unit, serves

as a heat donor. This observed fact further adds to

the advantage of the movable system of insulation.

Time lag, which is a fundamental thermal characteris-

tic of building elements, is also enhanced by three to

four hour by the movable insulation treatment with

and without a w ater pond in the summer season.

In winter an increase of the order of 6°C in the

minima ceiling temperatures was observed with this

system of insulation during the night but removed

during day time. This increase is also quite appreciable

for radiant heat gain and helps in getting rid of

winter chill.

Discomfort degree hours of cei l ing sur-

face temperatures

Table 3 gives the total degrees of discomfort above a

base temperature of 30°C for summer conditions and

less than 24°C for winter conditions, the duration of

such temperatures, and also the peak degree hours

for the various treatments (ref 4). The table is

self-explanatory and clearly indicates the great

advantages of a movable system of insulation in both

seasons.

Indoor air temp eratu res a t a level of 1.2m

above floor

Figs 7 and 8 show the variation in indoor air tempera-

tures, with and without a roof pond, of the treated

unconditioned test room . These are plotted against

the temperature of the untreated unit. The drop in air

tem pera tureis ofth eord erof 3°an d 4.5°C respectively.

Here also the drop in air temperature has been found

to be slightly more for a movable system of insulation

as compared with the roof pond system alone.

Building Research and Practice July/August 1974

T I M C H O U R S )

I I H I ~H 0 U A S )

FIG 3 (top). Hourly variation of the outside roof surface

temperature.

FIG 4

(above), also shows summer diurnal variations (at

different dates) in both cases comparing results with and

without a roof pond with movable insulation. The order

of reduction of the treated roof was 34°C and 30°C

respectively.

23 1



MOVABLE ROOF INSULATION IN

HOT CLIMATES/cont inued

Table 3 Effect of roof treatments on degree hours for ceiling surface temperatures

S.No.

1

2 :

3

i

i

ROOF TREATMENTS

Roof pond

Movable insulation

only

Movable insulation

with roof pond

Summer

Summer

Winter

Summer

TOTAL DISCOMFORT DEGREES

AND DURATION

Untreated

Degrees

(C )

14 7

15 3

21 3

20 2

Duration

(hrs)

18

18

24 .

24

Treated

Degrees

(C )

3

0

13 5

0

Durat ion '

(hrs) ;

i

0 .

23

0

PEAK-DEGREE HOUR

Untreated :

Degrees

(C )

14.5

15.0

16.0

Time

of |

occur-

:

rence

(hrs)

• 1600

:

: 1800

| 1800

i

Treated

Degrees

(C )

1.3

- 1 . 2

-2 . 7

Time

of

occur-

rence

(hrs)

' 1600

2100

2200

1 I M E (H 0 U I j )

J 30 5 7?

20 00

Hourly variations of ceiling

temperatures of the treated

unit with and without a roof

pond are shown in fig

5

above left) and fig 6 left).

With the roof pond alone a

drop of only 13°C was

observed. With movable

insulation as well, the drop

was of the order of \9C.

232

Building Research and Practice July/Au gust 1974



In the case of an air-conditioned test room, although

the mean level temperature (24 hourly mean of air

temperature), is 4°C less than the unit treated with

movable insulation on the roof pond, on comparing

the differences in the maxima and minima air tem-

peratures a remarkable improvement can be seen in

the treated unit as opposed to the air conditioned one.

In the treated unit it is 2.7°C as compared to 4.7°C

in the air-conditioned unit without any treatment.

This further brings out in clear terms the beneficial

effects of a movable system of insulation with a roof

pond when used for partial air conditioning.

In winter the indoor air temperature is raised from

1° to 1.5°C througho ut the day due to the movable

system of insulation alone.

Heat f low through the cei l ing surface

It can well be observed from figs 9 and 10 that reverse

heat flow takes place throughout the day in the

treated unit with movable insulation in both cases,

with and without a water pond, as compared to the

untreated unit.

The order of reduction is 36.25 and 33.5, Kcal/m

s

at

the peak heat flow hour, whereas it is nearly 22.25

Kcal/m

for a roof pond system alone.

On comparing the unit with the movable system of

insulation on a water pond with that of the air-

conditioned unit, a tremendous difference in heat

flow is observed, as shown in fig 11. The heat flow in

the treated unit is in the reverse direction throughout

the day as compared to the air conditioning unit, in

which the range of heat flow per hour is 10 to 35

Kcal/m

2

/hr. Peak heat flow in the treated unit is

— 6.5 Kcal/irr/hr as against 35 Kcal/m

2

/hr in the air-

conditioned unit.

In winter evidently the heat flow through the treated

roof with movable insulation only has always been

positive, as compared with the untreated unit in which

negative heat flow has been observed in the morning

hours. A difference ranging from 0 to 10 Kcal/m

2

/hr

has been observed between the treated and untreated

units.

Integrated heat f low at cei l ing surface

As integrated heat flow at the ceiling surface should

give a better comparison of the roof treatment in the

whole day this has also been computed with the

hourly variation of heat flow.

For unconditioned test rooms with and without

water the drop is of the order of 489 and 379 Kcal/nr/

day as compared to that of the untreated unit. For

•he roof pond alone this drop is of the order of

365 Kcal/mVday.

Building Research and Practice July/August 1974

T

I H

E

(M 0 U ft S")

IG

7

(top) and IG

8

show recorded hourly variations in

indoor air temperatures with and without a roof pond,

of the treated room no a ir conditioning) plotted against

the temperature o f the untreated unit. Here also the air

temperature drop was greater with a movable system o f

insulation than w ith a roof pond system alone.

23 3



10

f

z

2 20

MOVABLE ROOF INSULATION

IN

HOT CLIMATES/continued

00 04

T I N

(HOURS )

The integrated heat flow

in

the air-conditioned room

is 443 Kcal/m

s

/day as compared to 155 Kcal/m'/day

in the treated unit with a roof pond. The higher

value for the air conditioned test unit is again due to

the large difference between

the

outdoor

and

indoor

air temperatures.

On

comparing

the

integrated heat

flow

for

winter conditions

a

difference

of

98

Kcal/m

2

/

day has been observed between the treated and un-

treated units.

on lusions

A movable system of insulation for thick and heavy

re roofs is the most effective way of controlling the

indoor thermal environment

as

compared with

any

other design treatments developed so far. Hence there

is

a

considerable potential and scope

to put the

idea

into a wider practical use.

Besides

its

usefulness

for

unconditioned buildings

in

both seasons, this system on a roof pond can serve to

provide partial air conditioning in the summer season.

Finally there

is a

need

to

develop

a

mechanical

system

of

providing external

and

internal movable

insulation for roofs of buildings.

References

1 HAROLD R. HAY and j . i. YELLOT, Natural Air-

Conditioning with Roof Ponds

and

Movable-Insulation.

ASHRAE Transations,

Vol. 75,

Part I. 1969.

2 ii. R.

HAY. Improved Natural Air-Conditioning

for the

Tropics. Paper presented at the Symposium on Environ-

mental Physics as applied to Buildings in the Tropics

at

the

Central Building Research Institute, Ro orkee

UP,

India, Feb. 25-27,

1969.

3

s.

P . JAIN

and

K. R. RAO. Effect of Roof Spray Cooling on

Conditioned

and

Unconditioned Buildings.

Paper accepted

for publication

by the

Journal

of

Building Science,

U.K.

4 K. R. RAO, s. P.

JAIN

a nd K. N.

AGARWAL. Degree Hour

Rating of Thermal Performance of Enclosures.

Paper

presented

at the

Symposium

on

Environmental Physics

as applied to Buildings in

the

Tropics Feb 25-27 1969 at

Central Building Research Institute, Roorkee , UP , India.

234

H 0 U S)

FIGS 9

top

left) amllO (above left), showing hourly'

variations of heat flow at the ceiling surface, illustrate

that reverse heat flow takes place throughout the day in

the treated unit with movable insulation, with

or

without

the roof

pond.

Fig

11 (left) shows rev erse heat flow

throughout the

day

for the unit w ith movable insulation

on a roof pond compared with the variable results for

the unit with air conditioning.

Building Research and Practice July/A ugus t

1974

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Sistema Pasivo de Enfriamiento