‘Shade, insulate and ventilate’ should be the mantra for achieving ‘thermally comfortable’ buildings
Multiple Indian cities have already witnessed record-breaking temperatures in the last two months. This March was the hottest-ever recorded in India while it was the third hottest April in the past 122 years.
As the mercury soars, the need for ‘thermal comfort’ is once again a hot subject. Along with ‘sustainable cooling’, ‘thermal comfort for all’ also happens to be one of the goals of the India Cooling Action Plan that came out in 2019.
Human thermal comfort is a condition that expresses satisfaction with the thermal environment and is largely dependent on six key factors. They are: Air temperature, humidity, wind velocity, radiant temperature, metabolic rate and clothing.
A disbalance in any of these factors can make the occupant thermally uncomfortable. This can have health-related repercussions such as heat stress, in turn, leading to loss in productive hours.
Some 295 billion hours of potential work was lost in 2020 due to extreme heat exposure according to The Lancet Countdown on health and climate change’s 2021 report. The loss in work hours is not the only economic penalty associated with the condition.
A tremendous amount of energy might be needed by active systems to keep the interiors of a building cool. The energy penalty is even higher if the building is not designed appropriately in accordance with local climatic conditions, leading to inflated energy bills.
Building designers may have no role to play in a person’s metabolic rate or clothing choices. But the other factors can be affected by the intervention of climate appropriate building design, material choices, etc.
The energy burden can be drastically reduced if simple solar passive techniques and interventions are incorporated in the building’s design. However, even if re-construction is not possible, there are other interventions that may be incorporated in existing structures.
Shade, insulate and ventilate are three basic principles to remember for keeping the interiors of a building cool.
Shade the building from unwanted direct sun. Unshaded glass windows are one of the worst things to have in the building in a hot climate. The window shades should ideally not get any direct sunlight during peak summer months while allowing the winter sun to enter.
This can be achieved by using external shading devices, fixed or movable. Shade structures can range from awnings, horizontal sun shades to eaves, shutters or shade sails. It is worthwhile to remember that the internal curtains or blinds are not useful in this regard and may only be used to cut down on glare.
Horizintal sun shades, eaves and awnings are appropriate to be employed on the southern facing windows. In summer, these will act like an umbrella to exclude the higher altitude summer sun.
In winter, the sun at a lower angle will still be able to get inside. The optimum size of these horizontal shades depends upon the geographic location and sun angle. A thumb rule to remember is that a 0.6-metre deep sun shades will roughly work for most of India, assuming the standard window opening of 1.2 metres.
East and west-facing windows are a little more challenging to shade effectively. At sunrise (east) and sunset (west), the sun hangs low in the sky. The trick to shading the morning and afternoon sun is fixed ‘vertical’ fins.
For a long-term solution, trees that can shade the western / eastern façade of the building, should be planted. This will have the added advantage of not just shading the window but the wall as well. If sun shades are not possible, high-end glazing available in the market may be opted for. However, these might be capital-intensive solutions.
Rising ambient temperatures and heatwaves are going to be a long-term reality. Buildings thus need long-lasting solutions. Insulation is an invaluable tool. This restricts the heat passing in and out of the building.
In winter, once the dwelling has been heated to a comfortable level, it will stay that way with less energy input than an un-insulated home. In summer, the insulated home will take longer to heat up and air conditioners will require less energy for cooling.
Top floor inhabitants are most affected by direct sunlight and heat. Some of this heat is reflected, some absorbed by the roof and the rest is transmitted inside the building. Inhabitants of the top floor should thus opt for ‘cool roof’ solutions which might be a combination of multiple strategies.
Reflecting most of the heat that falls on the rooftop can be one of the easiest ways to reduce the temperature of the roof. Dark colours reflect less heat and light as compared to light colours. So paint the roof white or use light-coloured tiles.
Roofs made of cement-concrete have minimal insulation capacity and transfer a lot of the heat inside. This leads to higher temperatures inside the building and more load on active systems to cool down the space. Adding a layer of insulation to the roof can help in resisting the heat gain.
Vegetative green roofs have a thermal mass layer in form of earth and vegetation, which helps reduce the flow of heat into a building. The absorbed heat in the green roofs is dissipated by evapo-transpiration carried out by the plants, hence, cooling the roof.
Insulation should be applied on external walls, especially if they are thin. Care should be taken so that the insulation covers all potential points in the building envelope from where thermal leakages can take place.
In a hot climate, the insulation should ideally be placed on the exterior surface of the walls. These solutions however, could be capital-intensive and as an immediate solution, the external walls which get direct sunlight (especially east and west facades) can be made more reflective by being painted in light colours.
Ventilation as a strategy needs to be used with precaution. Cross ventilation needs to be limited when the peak outdoor temperatures are high during the hot and dry months (April-June).
This can be done by closing the windows. When the outdoor night time temperatures fall within comfort levels, purge the buildings by keeping the windows open at night time. This can store the coolth and delay the indoor peak temperatures.
This may be done by opening the windows or forcing the air to flush through the spaces by switching on exhaust fans. During the months when humidity is high, it is advisable to ventilate the buildings whenever the air conditioner is not being used.
Apart from these solutions, there are other solutions that can be taken into account.
Temperature, water vapour, air movement and clothing, all play a role in one’s thermal comfort perception. Fans, desert coolers, window shades and clothing can be manipulated to achieve an acceptable, thermally comfortable environment.
Different solutions and strategy combinations might be used during the different phases of the day or the summer. It is important to remember that thermal comfort cannot be defined by a single temperature, humidity or wind value but by wider ranges and combinations of these values.
There are several devices that may be employed to achieve thermal comfort, all in their unique ways, so that they play around with thermal comfort factors.
Indoor fans are the least energy-intensive device for achieving thermal comfort once all design options have been taken care of. Fans increase the air velocity and help in taking away the heat emitted by our body.
As air moves past our skin, the wind also helps in evaporation of perspiration from its surface, hence, having a cooling effect.
Desert cooler is a simple device in which external air passes through wet pads. This cools and humidifies the air with help of evaporation. The device achieves this without recirculation of air (as happens in an air conditioner) while using up significantly less amount of energy than an air conditioner would.
This is an especially useful device during the hot and dry phase of the summer.
Air conditioners are energy guzzlers and while they may cool the interiors, they do so by taking the interior heat and throwing it outside, heating up the ambient air in the process and hence should be avoied.
However, if the indoor temperature reaches beyond 32°C in high humidity conditions and ACs become unavoidable, energy can still be saved by keeping some points in mind.
Air conditioning temperature can be kept as high as 28-30°C and may be combined with fans to give comfort to the users. Every 1°C rise in temperature results in energy savings to the tune of six per cent.
Keep the area that you air condition to a minimum. Only frequently-used habitable spaces should be air conditioned. Airtight the conditioned spaces to avoid leakages.
Keep in mind the efficiency rating of the air conditioner. A high usage space should have an efficient air conditioner so as to keep the electricity bill in control. Size your AC according to your need. An AC that is too big is inefficient and wastes money.
A well-shaded condenser will use up less electricity than those in direct sunlight. Cleaning the filter of your AC regularly can help reduce the energy consumption. A clean filter requires lesser energy to pump air through it.
Offices can take some managerial decisions to achieve thermal comfort in their buildings.
Start and end early. Ensure that your office timings end before the temperature reaches its peak inside the building. Occupy comfortable parts of the building like the north side when not running on full capacity.
Encourage colleagues to wear climate-appropriate clothing to better adapt to the heat. In offices where electrical equipments are a major source of internal heat, switch off unnecessary lighting and other avoidable equipments.
Zone the work stations in a way that physical distancing is maintained among people and equipments. This will increase the effectiveness of ventilation.
An office policy ensuring that air conditioners are not switched on until the interior temperatures cross a certain threshold temperatures should be put in place.
With inputs from Avikal Somvanshi
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