In our temperate climate, and to comply with Building Regs, we focus on heat retention when building new homes and renovating existing houses.
This means that we often overlook the even bigger issue of overheating and how to mitigate it.
The Problem of Overheating
With average ambient temperatures predicted to increase due to climate change, and homes becoming more efficient and built closer together, overheating has become a problem that can lead to health issues.
Many new homes are built using lightweight materials to facilitate better insulation, speedy reheat times and lower energy consumption. When the sun shines in through the windows the internal air will rapidly heat up and the heat will be trapped inside the home, similar to what happens in a conservatory.
What Causes Overheating?
Overheating is not just limited to the hottest days of summer. It occurs whenever the temperature inside a building becomes uncomfortable for the occupants for a long time.
It can be caused by:
- inadequate or absent central heating controls
- poor ventilation
- warm outside air
- it is exacerbated by high humidity, either internally or externally.
How to Solve Overheating
Windows, Shading and Blinds
- Specially designed shading such as roof overhangs or brise soleil can keep the sun out in summer and let it in during the winter
- Deciduous trees can be strategically positioned to block the sun in the hottest months but let the light through in winter
- Internal blinds can reduce the effect of the sun’s heat
- Different types of glass can block out the sun. Photochromic and thermochromic glass will change colour according to either light intensity or heat intensity and stop certain frequencies of heat entering the internal space
Wall and Roof Insulation
- The heat from the sun is defined as radiant heat and when it strikes a solid object it is absorbed and re-emitted at a different frequency, either as conductive or convective heat.
- So we really need to look at the types of wall and roof insulation used in lightweight construction. Much of it is designed to stop the transfer of conductive and convective heat and allows the radiant type heat to pass through and transfer into the home.
- It therefore makes sense to design in the use of different types of insulation, reflective membranes and trapped air products together, to cover the widest spectrum.
Lightweight building systems also need some thermal mass included in the design.
It takes around four times more energy to heat up a solid than it does to heat up air. By including some solid thermal mass in the building (in the floor screed and wall, for example) we create a buffer zone to level out the heat peaks. The thermal mass absorbs the heat during the peak periods during the day and releases it when the air is cooler.
- Adding some thermal mass to studwork walls in the form of thicker (around 25mm) and denser plasterboard, such as Brioboard and Fermacell, can also help
- Polished concrete and granite countertops also add to the thermal mass
Thermal mass must be designed appropriately. Including too much will slow down the reaction time and could cause the home to take too long to heat up (or cool down). It could also be a continuous heat sink.
Cooling the House
If the air inside the property does get too warm, the best way to cool it is to cross ventilate with cooler air. A single window is not enough if the air cannot move from another opening.
Ventilation is good in areas with lots of outside space, but in more suburban areas the air outside could easily be as warm, if not warmer, than the inside air. The temperature in cities can be up to around 9°C warmer than surrounding rural environments.
This effect is known as an ‘urban heat island’ and is caused by hardstandings and solid structures absorbing the energy from the sun during the day and releasing it during the night. This leaves no opportunity for cool air to enter the property to lower the air temperature.
These conditions can be managed by:
- planting foliage and vegetation
- having open water to enhance evaporative cooling
- using lighter colours on hard surfaces.
Air conditioning is a large-scale solution in urban areas and if you have photovoltaic panels on your roof the cooling could be seen as ‘free’. However, the result of cooling inside is that the waste heat is exhausted to the outside. The air temperature then rises and adds to the ‘urban heat island’ effect.
How to Design Out Overheating
Ultimately, many modern heat mitigation methods will only be partly effective. The best way to avoid overheating is to consider it with your designer and relevant consultants at the design stage.
Try to let the heat in that you require by:
- designing in the correct use of thermal mass
- insulation and heating controls
- installing external shading
- maximising natural external spaces
- providing the correct cross ventilation regime.
David is an expert in sustainable building and energy efficiency and is also director of Heat and Energy Ltd.
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