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Extreme heat and why U-values do not tell you how well your building performs

With the extreme heat we are experiencing there are many people who’s homes and offices are getting extremely hot, not giving them anywhere to escape the record high temperatures that are becoming the norm in our summers. Many of those buildings are modern, ‘well insulated’ buildings that are quite comfortable in winter but offer little protection from heat in the summer. This significantly impacts the health of elderly and vulnerable occupants and so is vital to address.

The thermal conductivity of insulation materials is measured by calculating the heat flow through a material once it reaches a continuous rate (steady state). However, our environment never stays at a continuous temperature and so the speed at which materials reach that continuous rate of heat flow has a significant impact on the amount of heat that passes through a material over the course of a day.

Natural fibre materials such as wood fibre insulation only change temperature very slowly, due to their high relative density and very high specific heat capacity (the amount of heat energy required to heat 1 kg of material by 1 degree C). When these materials are built in to building elements they increase what is known as the decrement delay. Decrement delay is a measure of the time taken for heat to reach the interior of a wall or roof after being applied to the exterior, or vice versa.

When designing buildings to pass building control requirements or SAP requirements, there is no requirement when it comes to decrement delay, only the overall U-value, which presumably assumes that it will insulate from both heat and cold. However, lightweight materials such as fibreglass are so lightweight that although they give low U-values for fairly slender constructions, their decrement delay is only a few hours. Consequently, in hot weather these well insulated lightweight buildings can become unbearably hot inside, making them uninhabitable unless air conditioning is installed.

Looking at a roof structure (as this is the area with one of the highest heat gains) with a U-value of 0.13 W/m2K, if made of fibreglass the decrement delay is typically around 4 hours whereas with wood fibre insulation it is as much as 17 hours. This means that by around midday, heat is penetrating the fibreglass insulated roof and heating the building, along with any heat coming in through the windows. Although the U-value of a wood fibre insulated building might be the same, there would be no heat coming through the roof or walls, making it much cooler during the day. This is particularly noticeable in Mansard roofs or loft conversions which are notoriously hot in this type of weather.

For wood fibre insulated buildings, the highest density insulation is used on the exterior so as to increase the decrement delay but also to act as a heat store, preventing the heat from penetrating further. The decrement delay is so long that the sun has gone down before the heat finally starts penetrating the building and mostly begins to radiate outwards, keeping the building very cool.

Using decrement delay in addition to U-value is a much better way to understand how buildings behave in a hot climate and to design buildings for the future to withstand the increasingly hot summers that we will experience as the world warms. Using materials like wood fibre insulation not only improves the comfort and energy performance of your building but also locks up carbon into the structure of the building. This instantly helps reduce carbon dioxide levels and helps to reduce the likely intensity of those future heat waves.

If you’d like to speak to us about how to improve the performance of your building through the summer months, please contact us.

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Back to Earth is a natural building material supplier. We're not your typical builders'​ merchant though and instead work closely with our clients during the planning, designing and building phases of projects.

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