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So are you allowed to use expanding foam or not?

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Hello, I watched a you tube saying expanding foam in the rafters was a huge no no with mortgage companies. Our mechanical ventilation guy said we should use it to help seal and make things airtight at the ends of the rafters and joists saying it would be easier and quicker than the tescon tape which he suspects will peel. I know you have to prep the surface (wash the dust off, anything else?). Oh and here is the advice for the windows. Whatcha think?

window jam.png

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There's a great difference between using a bit of air-tight foam for specific purposes such as air-tightness round windows and joist ends and foaming up a whole roof. I think it's the latter application that upsets Bldg Soc's rather than the very reasonable use of small amounts for small applications (though I could be wrong and they may be more twitchy than I thought!)

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The problem with spray foam in roofs...


Water vapour from the house that gets into the loft must be ventilated out or you get condensation and rot. 


Some roof membranes, particularly on older houses, are vapour impermeable. This means it can't escape through the membrane so vents are provided at the eaves. They are designed so that air goes in one side, across the loft and out the other side taking water vapour with it.


When conventional sheet insulation (PIR, PUR etc) is installed at rafter level it's normal to leave a 50mm ventilated void between the membrane and the insulation. Air goes in the eaves, up this void and out at vents at the ridge.


If you just spray foam onto the membrane there is no ventilated void. No way for the water vapour to escape. There is a risk of condensation causing the raters to rot.


In some houses there is no membrane and the foam is sprayed onto the underside of the tiles and around tile battens, this can also trap water against the battens causing them to rot.


If spray foam companies were to fit some sort of tunnel or spacer between rafters to preserve a ventilation gap before applying spray foam there might be a way to use it without creating a condensation risk. Unfortunately the appeal of spray foam is the speed of application and fitting these spacers would detract from that. Scaffolding would also be required to provide vents at the ridge, in many cases the ridge would need replacing to provide vents.


Mortgage companies have woken up to the issue spray foam can cause and many now won't lend against properties where spray foam has been used, even if used correctly.


This has nothing to do with cans of spray foam used to seal around doors and windows or cavity wall insulation.





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Spray foam depends on a few factors, vapour open or vapour closed.


Retrofitted to a cold roof rafters to help hold the roof together is bad as pointed out above.


New build no issues, as you can design for it. Interstitial assessment is a must for spray foam same, as with any other means of insulation.


Scottish regs require us to install sarking boards, these are installed tight together and as the wood dries it leaves ventilation gaps.  Above this a suitable breather membrane is installed. The foam used is vapour open. So any moisture can migrate outwards.  The slates attached to the sarking boards, provide a ventilation space. Below this it is ideal to install a vapour closed membrane.

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