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19 Apr 2024 < Back

HSE – Spray foam insulation applied to timber sloped roofs in dwellings

As previously noted just before Easter, the Health and Safety Executive (HSE) published a research report on moisture risks from spray foam insulation titled: Spray foam insulation applied to timber sloped roofs in dwellings. It had been long awaited and many expected it wouldn’t see the light of day. But for those who are a little cynical, you could consider the release date and time of the document a little more sceptically. 

What does the study consider? 

The aim of the research was to determine the moisture risk associated with spray foam when installed in a pitched timber roof. 

It essentially took both one- and two-dimensional modelling methods and applied them to two scenarios (one from London and other from Newcastle) at the two ends of a spectrum for England’s climate. It then compared how the installation performed depending on the application i.e. type of foam (open/ closed cell), inclusion of Vapour Control Layers and the substrate it had been applied to, amongst others.

The headlines  

The good news – “the modelling indicates that risks are low when an open cell (moisture permeable) insulant is applied in line with guidance described in British Standard BS 5250:2021.” In simple terms this means you need a vapour control and/or an air gap.

The bad news - “the highest risk assessed is when spray foam insulation is applied directly onto the roof covering. This leads to high risks under all modelled scenarios.” It also identifies that the modelling identified higher levels of risk in some scenarios when a sprayed foam is applied to high resistance underlay such as a bitumen felt. 

Other considerations  

The research also highlights the significance of the ceiling insulation. If present, it is likely to increase the risk of condensation and mould in the void above. Whilst it is not “new news”, it is pleasing to have this confirmed by the study.

If you try to compensate for this by ventilating the void, then you negate any benefits that the spray foam would provide.  

Rainwater ingress 

The report alludes to the fact that consideration was given to the potential concerns about rainwater ingress passed roof coverings but being prevent from drying because of the insulation. Although it is difficult to determine where this had been taken into consideration during the study or how much of a factor this could potentially be. 


It should be borne in mind that this study is based solely on modelling and is by its very nature limited. It infers that when installed in an appropriate manner such as that detailed in BS5250, the risks are quite low. However, it is evident that in certain applications of spray foam insulation in pitched roofs, there is an increased risk of condensation and decay.

Next steps?  

To discuss the full implications of this document, we have set up an online meeting for any members who are operating in or have an interest in this sector. This meeting will be held virtually on Tuesday 15th May and if you would like to take part please contact [email protected]

Want to learn more?  

If you are looking to learn more about assessing spray foam, there are spaces available in July's Evaluating Sprayed Polyurethane Foam in Domestic Roofs workshop. To find out more, click the button below.

Sprayed foam workshop



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