Put up a barrier to beat the heat
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A roof exposed to solar temperature can warm up the interiors. Insulating it is the answer, says T. Lalith Singh
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Protect the top: If the roof can be protected from heat incidence, the conditions down below can be controlled to a great extent.
The roofs of buildings receive the thrust of heat during the day. The construction practices adopted in the country mainly involve reinforced cement concrete (RCC) as the roofing element which is noted for its high thermal conductivity. If the roof can be protected from heat incidence, the conditions down below can be controlled to a great extent. When buildings are air-conditioned, the purpose of the system is to maintain a lower temperature than ambient inside the building. If the roof is exposed to solar heat, it will input continuous heat inside the building which in turn will add to the A.C. machinery load.
Thermal barrier
In this system, a thermal barrier or insulation is provided over the RCC so that the heat of the sun is not allowed to touch the RCC slab of the roof at all. In this way we can preserve the RCC from getting heated up. Once the RCC is heated up, there is no other way for the heat to escape other than inside the building. So even though the thermal barrier is provided under the RCC, as in under deck insulation, some heat passes through it and heats up the ambience of the room. This decreases the comfort level of the room and if the building is centrally air-conditioned, increases the AC load.
Hence it can safely be concluded that over deck insulation has its own advantages against under deck. The over deck insulation material should have adequate compression resistance, low water absorption, resistance to high ambient temperature and low thermal conductivity. This can be carried out by either preformed insulation material or insitu application.
The preformed insulation material is further classified as expanded polystyrene slabs (EPS), extruded polystyrene slab, polyurethane / polyisocyanurate slabs and Perlite boards.
The EPS is a light weight cellular plastic foam material composed of carbon and hydrogen atoms and is derived from petroleum and natural gas by-products. Polystyrene is highly economical and EPS meets most of the performance.
Extruded polystyrene is an improvement of EPS material and comprises beads/globules compressed to form slabs and pipe sections. The beads are very closely linked to each other so that the material becomes rigid and there are no air gaps between them. It is a close cells material and a skin is formed on the top which stops water absorption. polyisocyanurate / polyurethane foam slabs are urethane foam insulation materials having low thermal conductivity, low smoke emission and low water absorption. The application procedure for over deck insulation featuring preformed insulants include cleaning roof surface free of dirt and loose particles, providing a primer and adhesive coat and adhering of insulation with adhesive, taking care to seal all joints between insulation also with a sealant. Provide a protective plaster layer with reinforcement and also provide an elastomeric membrane or felt type waterproofing treatment on top.
Organic board
Perlite insulation is organic rigid board insulation composed of expanded volcanic glass and wood fibres bonded with asphaltic binders. This makes a rigid board light in weight, dimensionally stable and good in compressive strength. Unlike preformed materials, the spray applied polyurethane is applied directly over the roof by spraying. This eliminates separate fixing procedure. and is formed spontaneously when Isocyanate and Polyol are mixed in the presence of a blowing agent to create close cell homogenous jointless insulation cover of the roof.
It is designed to combine highly efficient thermal insulation with great ease of application. It is ideal for a wide range of insulation applications particularly for roofs and walls of the building. By nature, liquid applied foam polyurethane adheres strongly to almost any surface regardless of form. The seamless and monolithic nature of spray foam provided a method of sealing cracks and rendering any surface moisture resistance and drought proof. The adhesion of sprayed material makes mechanical fastening redundant. Besides external use, sprayed foam can be applied internally as well.
A look at the advantages of over deck roof insulation. It does not interfere with inside ongoing activities, is jointless cover to the roof surface, offers thermal barrier against heat in flux, is protector to the concrete surface from fluctuating external temperature and comes as a very fast application. Now, the disadvantages. It needs to be treated with proper water proofing ensuring no water ingress into the insulation. In case the waterproofing treatment is damaged, the water ingress into the insulation may lead to deterioration and compromising its insulation properties.
(From Guidance Note of ‘Environmental Building Guidelines for Greater Hyderabad’ draft prepared by HUDA).
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