Reducing overheating in the home - material choices
Updated: Jul 2
There is something a little ironic that this should be my first blog post as I have just started building saunas! I guess the difference between an overheating home and a sauna is that the sauna is something people are choosing to experience for a short time and often used in the colder months whilst an overheating home is not something we choose plus it's not always practical to jump in the sea to cool down!
Though we didn't experience the heat wave in the Highlands like much of the UK did last week summer overheating can still be a problem. Overheating leads to uncomfortable conditions in the evenings with disturbed sleep patterns etc. Overheating can also have serious health implications. High internal temperatures are known to increase the risk of respiratory and cardiovascular health issues for vulnerable people.
The materials we use in both new build and retrofit have a significant impact on summer comfort levels. This is especially true with room in the roof bedrooms and rooms that get lots of direct sun. When selecting insulation materials much of the advice centres on the U-value which is the measure of how much heat is lost through a material. A low U-value means low heat loss.
In the summer other factors come into play. We want to slow the increase in indoor temperatures to maintain comfort levels. This is described as the decrement delay (or sometimes Phase Shift) which is a measure of how long it takes external heat to travel through a building material. A long decrement delay is desirable as this shifts the peak indoor temperature to a point in the day when it's cooler outside, allowing for easy cooling by opening windows and vents. If the peak internal temperature is earlier in the day then cooling is much more difficult as the external temperature will also be higher.
Many of the synthetic materials commonly used for insulating roofs and timber frame buildings are lightweight and commonly have a short decrement delay. In contrast natural insulations with higher density such as wood fibre, hemp and sheeps wool are able to store more heat slowing the increase in internal temperature.
When using natural fibre insulations the rigid dense layer - usually wood fibre goes on the outside of the rafters or timber frame. The rafters or studs are then filled with a flexible fibre insulation batt. Using a flexible batt with a slight interference fit ensures there are no air gaps for unwanted air movement. I have used this approach on most of the new build projects I have worked on over the last 4-5 years. The images below show dense wood fibre being installed on a roof and on the outside of a timber frame wall.
Example comparing lightweight and natural fibre insulations
The example below compares a mineral wool insulation with a wood fibre insulation in a roof build-up. The U-value for each build-up is very similar so heat loss in winter should be similar. However in the summer the performance is markedly different.
The Phase Shift/Decrement delay for the wood fibre is 12 hours whereas the mineral wool is only 7 hours. The graph below shows how the wood fibre insulation maintains a more even temperature through the day. The temperature in the mineral wool scenario increases by around 10°C and with the wood fibre the temperature increases by around 5°C.
Wood fibre insulation | Mineral wool insulation |
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Through considered design it's possible to build and retrofit buildings that are fit for the future without having to resort energy intensive mechanical cooling methods.
There is lots of information out there on this topic and best practice guides for using natural fibre insulations I have added a few links below. Do get in touch if you want to discuss anything on this topic or to chat about how I can help with any projects you have in mind.
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