ROOFING WITH VOIDS
The mundane mud pot placed strategically by a skilled mason can cool your house.
If people advise us to build only the way our forefathers built, to completely return to the vernacular ways and shun modernity, better not trust them fully. Returning to the past is, of course, a way of building eco-sensitively, but many not be either the sole way or the best way.
This is not to disapprove the benefits that we can learn from the past, but to forewarn ourselves about the possible traps in simply glorifying the past. Roofing practices provide good demonstrations for this theory. It’s well known how advanced were our seniors in providing long-lasting water-proof roofs with passive cooling and minimal annual maintenance. Comparatively, today we often complain about cracking and leaking concrete roofs which turn our houses into ovens in summer.
A thick layer of mud was among the simplest of solutions; it’s effective but not practical today in urban contexts, however much we may like to return to those age-old practices.
However, we may try to have a thick layer, but reduce the mass and weight by having voids inside. Roof heat transfers through conduction in solid materials and convection in empty space. Convective transfer of heat is very slow and much lesser than through solid mass. So, any hollowness embedded within the roof thickness reduces heat transfer, keeping the indoors slightly cooler.
In many regions of northern India, it was a common practice to place small mud pots, either tea cups or 6 to 7 inch diameter clay trays, 3 to 4 inch deep on the terrace, with the bottom up. So, there is a cavity now above the roof top.
This layer of pots is filled to level using screed, either plain cement concrete 1:2:4 if water-proof terrace is needed or one may use stabilised soil-cement mix. Some lime may be added to provide water-proof qualities. The final top surface may be finished with white heat reflective cool roof coat, for additional thermal comfort inside.
In a recent project in Vikarabad near Hyderabad, this technique was revisited. Of course the availability of the clay pots led to using lesser number of pots than what’s ideal, but as an experiment to check its feasibility today, it was a success. In this case, this layer was laid over tandoor stone roofing, reviving another vernacular practice. It’s sad that though clay pots have multiple uses and benefits, their production has reduced due to the onslaught of produced products.
The void should be completely inside the slab to trap the heat there itself. Of course, once the air in the cavity gets heated up, some heat transfer would happen due to convection, but that would be much lesser than otherwise with no void at all.
The best is to have a ventilated cavity roof, which cannot be achieved by this simple using of clay pots. Only for very hot regions we may try such more advanced modes of passive cooling.
Pots inside thick mud phuska roof is not a new idea, but among those which are being forgotten. Before they are fully lost, we need to check the possibility of their modern applications.