Expert tips on insulation and building blocks…

Architect Navneet Malhotra is a perpetual student. He loves to break myths and unmask the true cause for bad work by constantly sharing experience… “Path to gaining knowledge is through sharing,” is his motto.




How do buildings heat up? Can you give us some pointers for selecting quality insulation material? Also, how do we pick the right material according to the space where it has to be used?
Pradeep Goenka, Gwalior

Heat is transferred from one surface to another by any of these ways: radiation, when heat is transferred through vacuum (without the use of a medium); convection, when heat is transferred via a medium such as a liquid or gas; and conduction, when it is transferred through contact. While some materials are good conductors of heat and transfer energy very quickly, others are poor conductors and help to insulate the surface. Insulation materials can be natural or manmade. Materials such as vacuum, gas (air), natural fibres (cotton, wool), wood and some minerals (asbestos, vermiculite) are excellent examples of good natural insulation. Manmade materials include polyurethane, polyethylene, rubber foam, rock wool and glass wool, among others.

If you notice, most insulation materials have low density, which translates into larger surface area or volume for an equivalent body mass (or weight). The large surface area helps the material to spread the heat away from the source and release it as quickly as it is absorbed. It is similar to the principle behind a radiator. This is one of the main reasons why an air gap insulates us from heat transfer through conduction.

Every material has a unique inherent property that works best when correctly applied. For example, a highly reflective material will give us best results when it is applied to an exposed surface and not when it is sandwiched between the inner layers of a structure. Similarly, good insulators should be placed as barriers between the outer and the inner layers of the structure. While selecting, we also have to consider fixing details along with the interaction of the material with its surrounding. Some of these materials are prone to fire (styrofoam), while others can harm our skin (glass wool). Some tend to attract and retain moisture (open-cell rock wool), and others have a history of being carcinogenic (asbestos).

Understanding these properties is essential while selecting a material for a location. As your desired location is not mentioned, it is difficult to describe all the available materials and their ideal combinations with the surrounding surfaces or layers.

I’m building a single-level house without a false ceiling in Hyderabad. How do I keep the rooms on the upper floor cool in the summer? I prefer earthy materials and even considered broken tiles on the terrace surface, but didn’t like that look. I also considered thermolite treatment, but there’s no one here to execute it properly. Also, what is the ideal proportion of ingredients in mud phuska? How does one use it on a terrace?    
Madhu, Hyderabad

You will notice that only one or at most two sides of your house receive direct solar radiation at any time while the rest are in shade. Also, the verticality of the walls and the movement of the sun will further dilute the total heat absorbed by the exposed side. But the roof is always exposed and ends up absorbing nearly 80 per cent of the radiation in single-storey buildings.

A part of this heat is reflected into the atmosphere, while the rest is retained by the structure. The absorbed heat is then transferred to the surrounding surfaces through conduction. Even though you don’t like the broken tile look, it adds value to the insulation of the roof. Human access and dust on the surface limit the effectiveness of these reflective insulation materials. Since you’re not too fond of them, we can focus on non-conductive insulation materials for the terrace.

As you are not planning to install false ceilings, the choice of natural earthy materials that can be fixed to the underside of the roof becomes limited. Top deck (above the roof slab) insulation is slightly better than under deck insulation. It blocks the heat early and limits its impact on the inner layers of the structure.

If you opt for top deck insulation, I suggest the following sequence of layering: apply the waterproofing layer directly on the reinforced cement concrete slab. Cover it with a thin protective coat of plain cement concrete (PCC)  while work continues. Apply the insulating layer on top of this protective one. It need not be secured as it will remain in place by the weight of the layers above. The thicker the layer, the better the insulation. Coat it with a layer of uneven thickness to create a slope. Conclude by applying a layer of your choice.

For effective insulation with earthy materials, you can use terracotta cups inverted and placed on the terrace. They trap a small air pocket within their shell and create a natural vacuum between the roof slab and the upper layer. Place these cups closely on top of the protective layer. Each of them can easily support the weight of a hefty person, so you don’t have to worry about breakage. These inverted cups can now be buried under mud phuska (mud or earth + straw). This material provides additional insulation as well a gentle slope (1:100) towards the drainpipes along the periphery of the terrace.  It can be covered with brick tiles or natural stone which will ensure that the mud does not get washed away with rain.

There are other similar options that use hollow blocks embedded in the roof slab itself or by using any of the manmade materials either above or below the roof slab. Do discuss it with your architect to add value to this dialogue.

The main ingredients of mud phuska are mud (earth) and phuska (dry straw). The former should be of a quality used for the manufacture of bricks and be free from pebbles, foliage, excessive clay and sand. The phuska should be mixed in a ratio no greater than 5 kg to 8 kg of phuska per cubic metre of mud (one cubic metre is approximately equal to about 35.5 cubic feet of earth). Hand-mix the two in dry state and then add water to avoid a clumpy mixture. As these ingredients are organic and not factory-produced, there is bound to be variations in quality, weight, texture, etc. Therefore, the ratio of the mixture will need to be controlled locally based on visual and physical cues. Remember, the length of the phuska should be between 25 mm to 35 mm (1 in to 1.5 in). The quantity of water is also critical. Too much, and the surface becomes slushy. Too little, and the ingredients will not bind well. One simple method to determine whether the right amount of water has been used is to take a fist full of mud and compress it into a ball. If the ball retains its shape without bleeding water onto your palms, then it is good; if it crumbles, then you need more water.

Mud phuska should be applied in uneven thickness to achieve a slope of about 1:60 to 1:100 towards the rainwater pipes along the periphery of the terrace. The former will translate as a 1-cm drop in level for every 60 cm to 100 cm of horizontal distance. Hypothetically, if the thickness of the mud phuska is, say, 2 in (50 mm) near the mouth of the rainwater pipe, then it will increase by about 4 in (100 mm) to 6 in (150 mm) at a distance of around 6 mt (6,000 mm or 20 ft) from the rainwater pipe… giving you an average thickness of 4 in or 100 mm.

I am builder by profession and I want to understand the difference between hollow concrete blocks and ACC blocks. Is it worthwhile incorporating any of these in our constructions?
Vikram Verma, MP

Hollow concrete and ACC (air-entrained cement concrete) blocks are masonry modules used for construction of walls and partitions. ACC blocks are also known as foam concrete blocks as their surface texture and weight-to-size ratio are quite like the perforated light-weight foam slabs we use in upholstery. Hollow concrete blocks, on the other hand, have spaces punched out of their thickness. They look like stubby square tubes stuck along one face to make a figure 8.  These spaces align one above the other when placed in the masonry such that no gaps or holes are seen from either side of the wall. Both the types of blocks are made from cement-sand mortar and are fabricated in a controlled factory environment. These masonry units are cast in moulds and then put into autoclaves that control the temperature and moisture to achieve high strength quickly.

The size of a hollow concrete block is much larger than a traditional brick. Yet, its weight per volume is just about 6 per cent to 21 per cent higher than that of bricks. So masonry that uses hollow concrete blocks adds more load on the structure of the building. Though foam concrete blocks are much larger than bricks, they are less than 1/4th the weight of traditional terracotta bricks. This substantially lowers the weight, translating into large savings in the design of structure. Foam concrete blocks have more even and well-defined surfaces as compared to hollow blocks. They can be chiselled and cut easily with an ordinary saw. A mild reinforcement is recommended when plastering either of these masonry walls as the blocks tend to create shallow shrinkage cracks along the mortar joints. Extra reinforcement is also needed in the plaster along the door-wall junctions as similar cracks are caused by movement of the door shutter.

As the blocks are produced with non-conforming materials and construction conditions, it is difficult to determine their exact thermal resistance. However, I think foam concrete blocks have better insulation properties than both hollow concrete blocks and bricks owing to lower density.