Energy efficiency in construction is important to everyone i.e. builders, owners and prospective users of public, commercial and residential accommodation. This combined with the fact that the number one challenge in the tropical climates in countries like India is to control heat gain. That makes radiant heat barriers an integral part of building design, improve the overall comfort level in a building and reduce energy bills.
ASHRAE Standard 55 defines Thermal Comfort as:
‘Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment.’
From the definition of thermal comfort it is clear that it refers to state of mind that finds the surrounding environment satisfactory. The environmental factors that impact the feeling of human satisfaction are air temperature, humidity, air flow or wind and radiations from Sun or surrounding hot surfaces. Personal factors like clothing, adaptation to environment and activity level also have a bearing on the perception of comfort. People accustomed to hot climates are comfortable at higher temperatures as compared to people from cooler climates. Similarly in Air-conditioned Buildings, people feel perfectly comfortable in heavy clothing during winter and light clothing during summer although there is no seasonal change in the indoor environmental conditions.
In hotter climates like Indian summers, body shedding heat through sweat evaporation is natural way to maintain the thermal equilibrium. The cooling efficiency is dependent on humidity that reduces the effectiveness of evaporative cooling. Humidity is the reason that renders July/August more uncomfortable as compared to May/Jun in spite of temperatures being higher prior to the onset of rainy season. During winters, body must prevent / reduce heat loss by having additional protective warm clothing, moving into a shelter to avoid wind chill or increase body heat production with increased physical activity. During winters, evaporation and humidity are minor factors while exposed body parts and wind chill are major factors for maintaining thermal balance as far as human comfort is concerned.
Radiation plays the most important role in creating conditions conducive to thermal comfort / thermal stress. Radiation during winters may be soothing as everyone who has enjoyed the winter Sun knows, but the same solar radiation can lead to excessive heat load on the person during summers with attendant uncomforting experience and heat stress.
In order to understand the perception of thermal comfort by human body, we need to know the process of heat exchange between body and surrounding environment. A seated human body produces and dissipates about 100 W of heat energy into the surrounding environment and the energy produced would increase beyond 500 W for a person engaged in cardio-vascular exercises in a Gymnasium. If this transfer of heat for a seated person doesn’t lead to extra physical effort like sweating or shivering to maintain the body temperature, environment is considered to be comfortable.
As per ASHRAE Fundamentals Handbook:
‘Skin temperature greater than 45oC or less than 18oC causes pain. Skin temperatures associated with comfort at sedentary activities are 33 to 34oC and decrease with increasing activity. In contrast, internal temperatures rise with activity. The temperature regulatory centre in the brain is about 36.8oC at rest in comfort and increases to about 37.4oC when walking and 37.9oC when jogging.’
Metabolic activity inside the body produces energy that is used for muscular work or transferred to surrounding environment through respiration and skin. The heat energy exchanged by skin to environment comprises convective heat transfer to/from the surrounding air, evaporative heat transfer through sweat evaporation and radiation to/ from skin to surrounding environment. If the surrounding surfaces like walls/floor/ ceiling have temperatures higher than the body temperature, the radiation exchange would result in net heat gain. If the heat produced by metabolic activity exceeds the heat consumed in muscular work and lost/gained during exchange with surrounding environment, the same would lead to increase of body temperature. This change of body temperature is sensed by the hypothalamus in human brain, the god given thermostat to control body temperature. The body temperature is regulated by increasing blood supply to skin to increase heat loss or reduce blood supply and conserve heat loss through skin. If skin temperature gets lower than desired due to excessive heat loss, the hypothalamus triggers shivering & localised heat generation. If the internal temperature increases beyond a certain point, the hypothalamus triggers sweat generation to cool the body through evaporative cooling. We are quite good at sensing the skin moisture / perspiration with the associated feelings of unpleasantness and discomfort especially if involved in sedentary work.
Asymmetric or non-uniform thermal radiations in a building from cold windows in winter or hot ceiling during summers can lead to occupants feeling uncomfortable. As per ASHRAE Fundamentals Handbook:
‘People are more sensitive to asymmetry caused by an overhead warm surface than by a vertical cold surface. The influence of an overhead cold surface and a vertical warm surface is much less.’
Based on various field studies, Nicol has come up with an empirical equation to compute Comfortable Temperature, when 90 percent of the population would find the environmental conditions satisfactory. The Comfort Temperature (Tcomfort) is calculated from the monthly mean outside temperature (To mean) which is derived from the monthly mean of daily maximum (To max) and minimum (To min) from meteorological records. The comfort temperature is calculated usin g the equation Tcomfort = 0.54xTo mean + 12.9
From the weather data from the meteorological records for Delhi as shown in the table, To comfortable for Delhi has been computed.
The same information is depicted graphically for ease of comprehension. Its clear from the graph the To comfort varies from low comfortable temperature 210oC in DEC JAN to a high comfortable temperature of 300C during Jun Jul.
In almost all types of construction, roof has the maximum exposure to solar radiation and contributes the most towards heat gain due to large surface area getting exposed to the sun leading to extreme temperatures of roof surfaces. Roof temperatures reach upwards of 650oC leading to uncomfortable inside environment in unconditioned buildings. At peak times, more than 40% of the energy that enters the conditioned airspace through the ceiling is the direct result of radiant energy being transferred from the deck to the top of the false ceiling. Radiant Heat Barriers like Double sided Aluminium Foil like Flamestop 138, are the most economical, efficient and long lasting solution for minimizing heat gain from the roofs.
It is important to understand the simplicity of principle on which the radiant barriers work and their application in construction. Heat always travels from hot surface to cold surface by the three methods of heat transfer i.e. radiation, convection and conduction. The principle on which the radiant barrier functions is the same as that of Thermos Flask i.e. preventing heat transfer through radiation. Thermos Flask takes care of the other two means of heat transfer i.e. convection and conduction by having vacuum in between the two reflective surfaces. However, one doesn’t have to conduct an experiment with two thermos flasks, one without reflective insulation and another with reflective insulation to find out that convection and conduction play a minor role in heat transfer.
During hot summer days, the roof of a building absorbs solar radiation and then the heat is dissipated outside into the atmosphere through all the three means of heat transfer i.e. through conduction, convection and radiation and a significant portion to the inner roof surface through conduction resulting in increased inner roof surface temperature. The roof temperature keeps on increasing after Sunrise as the amount of heat energy gained is more than heat energy lost through radiation, convection and conduction. The Solar heat gained keeps on increasing with the increase in sun angle till a balance/equilibrium is reached with roof temperature reaching maxima for the day sometime in the afternoon with heat energy gained being equal to the heat energy lost. Subsequently, with the Sun going down sun angle reducing, the heat loss becomes more than the heat gain leading to reduction of roof temperature and the process continues till the next day morning till a minima is reached some-time after Sunrise.
As much as 93% of the total heat gain from the roof is through radiation with conduction contributing the remaining heat transfer through roof and convection playing no role as a convective loop is not possible for heat gain through the roof. A properly installed radiant barrier under roof will stop as much as 97% of radiant heat by reflecting 95% of the incident radiations while transmitting just 5% of the 5% absorbed heat energy. On the other hand, mass/bulk insulation acts as a barrier for heat transfer by convection & can only slow down the process of heat transfer through conduction.
Roofing materials (RCC Slabs or ACC/Metal Roofs) are often poor reflectors & good emitters of heat. The traditional roofing materials absorb as much as 90% of the incoming solar energy resulting in roof temperatures rising to levels as high as 650oC to 700oC during summers. The hot roofs start transferring the absorbed heat to the cooler surfaces inside the building through conduction & radiation with radiation being responsible for more than 90% of the heat gain. The inner surface of the heated up ceiling transfers the absorbed solar heat to all objects inside. The radiant surface temperatures of the walls and ceilings have a direct influence on the comfort level of the occupants and increased energy costs in conditioned buildings. Most energy conservation consultants recognize effectiveness of radiant barriers, in controlling heat flow from roof to the building’s interior.
A polished film of aluminium is the primary component of a radiant heat barrier system. Compared to ACC/Metal /RCC roofs with high emittance value 0.7 to 0.9, aluminium radiant heat barrier emits only 3% to 5% energy as heat radiant energy from its surface when compared to non reflective ceiling surfaces. Thus, radiant barriers can effectively block 95% to 97% of radiant heat transfer from roof to inside.
There are several ways to install radiant barrier under the roof. It can be laid over the purlin in case of new buildings or under the purlin in existing buildings with ACC/Metal sheet roofs. In case of RCC slab roofs, the process is similar to installation gypsum boards and it is attached with the help channels with an air cavity in between RCC slab and radiant barrier. A properly installed radiant barrier under the roof is the best performing application and gives the best results with radiant barrier surface temperature getting reduced to just about 20C above the ambient outside air temperature and relief from thermal stress associated with high ceiling temperatures.
During a typical summer afternoon in the Sunbelt, a properly installed under roof radiant barrier system will:
– Reduce room temperatures by
as much as 060oC to 080oC in unconditioned buildings.
– Increase the human thermal comfort level for the occupants of the building.
– Extend the life of the air-conditioning unit.
In today’s market, with rising energy costs and increased consumer awareness in all areas of industry, it is important to market a product that is not only smart economics for the builder but also for the consumer. The builders that have adapted radiant barriers in their programs are clearly setting the benchmark in the industry for energy efficient and environment friendly green buildings with competitive advantage.
– ASHRAE Standard 55
– Chapter 08, ASHRAE Fundamentals Handbook 2001
– Climate of Delhi from Wikipedia