You are here
De-coding Thermal Mass and R-Value for Optimizing Building Efficiency
Residential and commercial buildings account for about 30% of Indian’s electricity which was approximately 294 TWh in 2012 and would increase to 11948 TWh by 2047, if present trends continue.1 The Ministry of Environment and Forests estimated that the combined energy used for heating and cooling in residential buildings alone would amount to 450,000 GWh/ year by 2030.2 While it may not be possible to completely do away with air conditioning and heating given India’s location in the tropical and sub-tropical belt, it is possible to reduce energy consumption on heating and cooling through proper building design.
The only way to do this is to reduce the heat transfer between the outdoors and indoors. This can be achieved in two ways- by increasing insulation or by increasing thermal mass of walls.
Used correctly, both these mechanisms are instrumental in stabilizing internal temperatures, isolating internal temperatures from outside and decreasing heating and cooling requirements. Used incorrectly, both can at best be ineffective and at worst reduce comfort.
Buildings in India have traditionally had a high amount of thermal mass because conventional building materials include bricks and concrete. However, due to greater availability of heating and cooling technology as well as changing lifestyles, indoor temperature control has become much more prevalent than it was twenty years ago. While earlier, the use of thermal mass was enough to regulate internal temperatures to an extent, now it becomes necessary to use insulation in order to improve building energy efficiency.
How Insulation and Thermal Mass work
To better understand how Insulation and Thermal mass can be used, it is useful to first understand the basics of building heat transfer.
- The heat gain of a building is from two major sources- the sun, and heating appliances (heat generation from human beings is much smaller)
- In order to maintain comfortable conditions indoors, it is necessary to prevent heat gain from the sun during summer in order to maintain cooler temperatures indoors. In winter, the need is exactly the opposite- to prevent heat loss from the interiors and keep the heat generated by heating appliances indoors during winters.
Insulation forms a resistive layer through which heat can pass slowly. Insulation effectiveness is measured by the resistance (R Value) of the concerned material: how much resistance it presents to heat transfer (the more resistance, the higher the R Value, the better the insulator). An analogy can be drawn between insulation material and a resistor in an electrical circuit. Like a resistor reduces flow of current in a circuit, insulation reduces flow of heat between two surfaces.
Insulation can reduce the cooling and heating load on a building by preventing solar gain and heat loss respectively. In the former case, because less heat passes into the living space, less energy is required to provide thermal comfort for occupants. In winter, insulation keeps a room warm, by not allowing the heat generated by appliances escape.
Bulk insulation of walls is most useful where buildings are air conditioned – in this case, insulation helps maintain interior temperatures and reduces the demand for cooling or heating energy. It is also very important to reduce all sorts of leakage (from doors and windows) for insulation to work effectively. Good insulators include foam, wool and air; poor insulators include glass, metal, earth and concrete.
Thermal mass stores heat. As heat moves from the hot side to the cold side, it gets absorbed and stored into the mass. This causes a lag between internal and external temperatures and thus the internal temperatures are stabilised. As the surrounding air cools down, the heat stored within the mass is released. Thermal mass’s effectiveness is measured by its heat capacity (Cp): fundamentally how much heat energy it takes to change the material’s temperature (more the energy, higher the Cp and better the thermal mass).
One of the best examples of substances with high thermal mass is water. Since the specific heat capacity of water is high, it tends to store heat. This is the reason that swimming pools are warmer than the surroundings in the evening and cooler than the surroundings at noon. The concept of thermal mass is very useful in maintaining comfortable temperatures in places that have a high diurnal range of temperature. Because of a 6-10 hour time lag between the indoor and outdoor temperatures, when the days get uncomfortably hot, the indoors remain cool. Similarly, at night, the indoors are warmer than outdoors.
The use of building walls with high thermal mass is usually recommended in places where either the day gets uncomfortably hot or the nights get uncomfortably cold or a combination of both. However, there is a pitfall associated with buildings having high thermal mass. Thermal mass is used primarily for passive heating and cooling utilizing the natural temperature cycle of the day to maintain comfortable conditions. If external heating or cooling is used, buildings having high thermal mass will take much longer to reach the desired temperature. However, once that desired temperature is reached, it will also tend to maintain that temperature for a longer duration.
Materials that readily absorb, store and release heat – such as concrete, bricks, stone and masonry have a high thermal mass. In temperate zones, these materials absorb heat during the day and radiate it at night. In tropical areas, such materials heat up during the daytime, but warm night time temperatures prevent heat loss. This has the potential to increase the requirement for cooling energy.
Many good insulators have poor thermal mass properties and vice versa, so it is important to use a combination of both thermal mass and insulation to effectively reduce heating and cooling loads of buildings.
Use of Thermal Mass and Insulation for Various Climatic Zones
The use of thermal mass and insulation is very specific to the location of the building since the climate conditions of a place will affect it significantly.
Arid and Semi-arid Regions
In arid regions where there is a high diurnal range of temperature, like Rajasthan, it is best to use thermal mass in isolation. This ensures that the indoor temperature stays cool during the day and becomes warm at night. Arid regions can use thermal mass effectively to do away with the need for external heating or cooling. In case cooling or heating is used, a thin layer of insulation on the internal surfaces can reduce load.
In contrast, many semi-arid regions of India, like Punjab, Delhi, require both heating and cooling. In a city like Delhi, where temperatures are high throughout the day in summer and low throughout the day in winter (often with little sunlight), it is detrimental if there is no insulation on the inner surface since the building will take very long to cool down in summer and to heat up in winter.
For equatorial and tropical regions that do not have a high diurnal temperature range, thermal mass should be used sparingly. A high thermal mass wall will prevent heat loss at night. On the other hand, insulation will reduce cooling load to a large extent by preventing heat flow from the outdoor surfaces to the indoor ones.
In a city like Mumbai, which has a hot summer, but pleasant winter, with plenty of sunlight, the heat gain from the sun should be reduced and hence, the insulation should be placed on the external surface.
In the northern parts of India which experience very cold winters and cool summers, it is important to heat the room from inside, while preventing the heat from escaping to the cooler temperatures outdoors. The way of ensuring this is to have insulation on the external surfaces, and moderate thermal mass on the inner surfaces. The thermal mass absorbs heat and keeps the interiors perpetually warm while the insulation ensures that the heat stored in the thermal mass does not escape. For effective heating, the thickness and quality of the insulation is much more important than the thickness of the thermal mass. Studies have shown that it is preferable to have walls with thermal mass of medium thickness for optimum heating and cooling. This is because if the thermal mass of walls is very large, the walls themselves take so long to reach the desired temperature that the energy consumption of the building actually increases.
This climate is characterized by hot, usually humid summers and mild to cool winters. There is a greater need for air-conditioning in these areas even though winters also require some amount of heating to maintain thermally comfortable conditions indoors. These regions are also characterized by a low diurnal range of temperature, making thermal mass less important than insulation for maintaining low temperatures indoors. In this case, thermal mass of moderate thickness should be used on inner surfaces, with external surfaces being insulated.
The table below presents an outline of the combination of thermal mass and insulation that should be used in various climatic zones. In study of thermal mass and insulation, the yearly heating and cooling load is taken into account, rather than seasonal heating and cooling load.
Thermal Mass and Insulation can Reduce Cooling Energy Requirement by 55%
In summer, the use of thermal mass, insulation and night ventilation can typically reduce cooling loads by 50% or more and in some cases the need for mechanical cooling may be eliminated altogether. An ASHRAE study3 conducted in India found that just the use of insulation can reduce energy consumption by as much as 55% in summer and 35% in winter in a perfectly sealed room. While perfectly sealed rooms are not possible to realize in practice, the average annual savings due to use of thermal mass and insulation tend to be closer to 15-25%.4 A 15% reduction in HVAC energy consumption translates into potential reduction of carbon emissions by 46 million tons and annual electricity savings of 67,500 GWh based on projected use in 2030. To put that into perspective, 67500 GWh of electricity can power 1.85 Billion 100W bulbs for 10 hours a day for a whole year.
There is no doubt that Green Buildings and Building Energy Efficiency is a growing area of interest in India. India has plans for providing 24x7 power to its 1.25 billion citizens by 2019. Yet today, one-third of Indians don’t have access to electricity. To realize that vision, energy savings from all sectors, particularly buildings (which are set to consume 76% energy by 2040) will not just be desirable, but necessary. The effective use of thermal mass and insulation can be some of the simplest, yet extremely effective means to improve a building’s energy efficiency. It is high time that these concepts become the norm, rather than the exception to meet India’s ambitions for Power-for-all by 2019.