It is a well known fact that the maximum heat ingress in any building is through the roof. And the rate of heat transfer is proportional to the thermal resistively of the roof material. Therefore if one simply uses a roof material which resists heat transfer , coupled with a cost effective insulation technique , the room temperatures can be lowered considerably.

In Rajasthan the ambient air temperatures in summers go very high; right upto 48*C. In traditional buildings the extensive use of stone in the walls and the roof provided the much needed time lag for heat to enter the building. In addition the roof would have thick layers of lime concrete, which would provide the necessary insulation to the variation in temperatures.

It was seen in some cases that inverted earthen pots were used to add an air insulation layer in the roof. The actual usage of these pots was for storing water which after the lifespan would be recycled to provide the solution to insulate roofs and also helped in creating the domes of rajasthani style chattries.

The idea, although very innovative as well as cost effective disappeared down the ages. This was mainly due to the downfall of lime as a building material and the influx of modern technology, which allowed air to be cooled by ‘Desert coolers’ as well as ‘air-conditioners’.

Heat ingress in buildings

The first point to be understood by builders and mainly architects is that the heat ingress in a building is mainly through the roof which amounts to upto 80% of the total heat entry. The rest 20% comes through the walls. But that gets taken care of because if the walls are well shaded by plantation or louvres then the heat ingress is reduced.

The main aspect is that if the roof is insulated properly then the 80% component of heat ingress can be reduced to upto 30%. Although many techniques are available in the market but the cheapest amongst them and the most reliable is that of earthen pots.

Laying Technique

No special technology or labour is required for the laying of these pots. Once the RCC roof is cast, its surface is given a slope by cement screed which forms the base of the earthen pots. Then the pots are laid abutting one another tightly starting from one corner of the slab. Care is taken that no broken pot is laid.

Once the pots are laid, a 2” - 3” layer of PCC is placed gently over the pots in slope. Therefore forming the basic roof layer which in the local language we call as ‘dhar’. Thereafter any floor finish can be made including terrazo laid in situ.

This layer of earthen pots are sealed from both sides and therefore the air trapped inside it forms an insulation preventing heat to travel from the top to the bottom slab. A fraction of the heat does manage to reach the slab using the walls of the pot as a medium. But then this heat is removed easily by proper ventilation in the rooms below.

In multi-floor buildings, heat travels from the roof slab to the next slab through the air (convection cycle) inside and through the walls (conduction cycle). So if the heat is curtailed at the very roof slabs then subsequently the rooms below become relatively cooler.

Case study

A small two floor residence in Jaipur with a roof area of approximately 2000 sqft. used this technique to provide cooling to the interiors.

The family was living on the ground floor initially and this single storeyed house was built by a builder. So they had experienced a lot of discomfort due to the climatic changes in the summers as well as winters.

When they decided to add another floor they had already plans for central air cooling by two large 24” desert coolers and air conditioners for the additional comfort in the bedrooms.

The owner was reluctant to insulate his roof with earthen pots as he had already budgeted for the air conditioners as well as central air-cooling system.

After some persuasion, he tried what he called as ‘experimentation’ at his expense.  The results of this experiment were spectacular and his electricity bills came down by 10%. The difference between the ambient temperature and indoor temperature of the upper floor was upto 10*C.

They shifted in the new upper floors of the house in November 2001. The winters were cold but still the family members could walk bare foot on the granite polished marble floors of the house. The difference of the indoor and outdoor temperatures were upto 15*C making the house very warm inside.

In the summer of 2002 in which there was no rain fall, the outdoor temperatures were soaring upto 45*C. But the heat was not felt inside. Although they had installed the two air cooling units, they hardly had opportunity enough to use them. In the peak of the season they would switch on one air cooling unit at 12 pm and then would have to shut the system down by 4 pm because the house would become cool enough.

On some occasions both the units would be used but then the time range was the same. At night time their cooling system would bring temperatures down to 28*C. They would have to shut down the cooling system after mid night.

Cost Factor

The initial cost factor has always been a deterrent in executing any passive cooling system. In this case the cost was not a negative factor. As the labour contractor was hired on a lump sum basis he did not charge any extra money for the labour component.

So the cost was that of the earthen pots which were of the size 3” ht and 4” girth. One piece costed an average of Rs 2/-. In one square feet of area, there were nine pieces so the cost per square foot worked out to be Rs. 18/- per sqft.

In a roof area of approximately 2000 sqft of area the additional costing worked out to be Rs 36,000/-. This is the cost of one split unit AC, which was never installed. So the owner got lower electricity and water bills with such a small investment.

In terms of the whole civil work costing of the building, this additional feature worked out to be only 3.27% of the cost.

The owner claims that he has already recovered this cost in terms of his saving in cooling and heating bills.

Conclusion

Thus the fear of using the technique of passive cooling was removed from the mind of one user. It is hoped that many prejudices related to passive cooling shall be removed in the due course of time.

In fact this cost-effective method is so affordable that people building their houses with small budgets can also use it.

If this insulation method is made mandatory than it will also increase employment in the pottery cottage industry which is right now in a crisis due to the replacement of earthen ware glasses by plastic ones, specially on the railway platforms.

All in all, this not so new method of insulating method of roof insulation has to be revived so that the buildings being built provide greater comfort and are less dependent on artificial cooling systems such as the Desert coolers and Air conditioners.

Mukul Goyal

Architect

This article was written and presented in a National Conference in Jaipur in 2003. It got wide coverage in all local newspaper in Rajasthan which resulted in the revival of this long lost technology.