Module 2: Overheating


Overheating in buildings is not a new issue but it has become more of a problem as the requirement to reduce energy consumption has been implemented. The move towards higher levels of insulation by using very lightweight, high performance insulation materials has created very light-weight buildings which heat up quickly. This is useful in the winter but can contribute to overheating in the summer.

Domestic buildings were less likely to overheat historically because they tended to be poorly insulated and have fewer glazed areas, fewer occupants and lower internal heat gains than public or commercial buildings. They were also usually made of masonry which is more able to buffer heat gains than lightweight structures such as steel or timber frames.

Through the use of high performance insulation and better construction methods, modern buildings are generally very good at keeping heat inside during the winter.

However, without careful design and consideration to all of the properties of the materials specified, the same buildings can continue to accumulate heat through the summer months, making for a less comfortable internal environment.

Overheating impacts comfort, sleep and therefore the physical and mental health of the occupants, so is a very important consideration. This is of significant importance to the very young, the elderly, those in poor health and for anyone who cannot leave windows open during the night to cool the house. The ‘urban heat islands’ that occur in built up areas can also prevent buildings from cooling overnight and so simply opening windows may not be an option as a cooling strategy.

Decrement delay

Southerly glazing is generally beneficial to comfort, heating requirement, occupant well being and with minimal shading, does not contribute to summer overheating. However, east and west glazing allow significant solar gain in the hotter months, which can exacerbate overheating in a house already warm from a sunny day.

External blinds, shutters or shades on easterly or westerly glazing can help reduce solar gains through the windows but the best way to reduce heat gains through the main fabric of the building is to reduce heat movement through it. This can be done by simply increasing insulation thickness but is done more effectively by using insulation materials which can reduce and attenuate heat flow, offering high decrement delay.

The ‘decrement delay’ of a construction is essentially the time taken for heat to reach the inside face of a structure after being applied to the outside (or vice versa). This varies for each material and each build-up but is a combination of thickness, density, specific heat capacity and thermal conductivity of the materials involved.

The higher the thickness, density and heat capacity and the lower the conductivity the longer it takes for heat to pass through the structure.

In addition to this there is also the ‘decrement factor’ which is a measure of the ability of the structure to attenuate heat variations from one side to the other. The lower the decrement factor the less variation in temperature on one side of a construction is experienced on the other. For example, during the warmer months you can have variations in air temperature of 15-20 degrees but by the time the heat reaches the interior you might only experience a variation of 1-2 degrees.

Wood fibre and thermal insulation
This delay is of limited use in the winter months as the heat flow across the wall or roof is relatively steady. However, in the warmer months when peak temperatures externally are typically between 1 and 2pm and the surface temperature of your roof or walls may be 40 degrees celsius or more higher than your internal target, the delay and the attenuation have a huge affect on internal comfort.

Wood fibre insulation has a high specific heat capacity of 2100kJ/kg, compared to 800 - 1000kJ/kg for glass/mineral wools and 1400kJ/kg for PIR boards.

It is also very dense with the flexible wood fibre batts, such as SteicoFlex at a density of 50 - 60kg/m3 and some wood fibre boards reaching densities of up to 270 kg/m3. Mineral wools have a density of 10-20kg/m3 with PIR boards at around 30-35kg/m3.

Whilst wood fibre insulation often appear a little thicker than those using PIR or very high performance glass wools, typical constructions using wood fibre insulation give decrement delays of around 15-18 hours compared to around 2.5-4 hours with mineral wools at the same U-value, although both give similar decrement factors. The denser wood fibre boards are usually used externally, their high density acting as a heat store and hugely reduce heat penetration into the building in the summer.

The huge decrement delay with the wood fibre insulation ensures that at a time when the external environment is cooling, the small amount of heat being released from the wood fibre insulation keeps the inside of the building at a very stable temperature.

In addition to heavyweight insulation, heavy weight internal linings can be used to boost thermal mass and help buffer heat produced internally. Where masonry walling with full-fill cavity or external insulation can be used it provides a very effective heat store. Equally, clay boards used to cover timber partition walling or ceilings are a simple way to boost thermal mass and improve acoustics and fire safety at the same time. They provide around 2/3rds of the effective thermal mass of concrete block walls.

There are also combinations of clay and PCM (phase change materials) available which although they are only a similar density to plasterboard, they are able to absorb as much heat as 50mm of solid concrete at certain temperatures.

Finally, to show how heat moves at very different speeds through insulation materials even though the thickness and U-value is the same, see our video here: