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Q&As
How should I insulate my roof?
Hi…based on the inspiration of the Bicester show home…we are building our ICF home on an insulated raft.
I need to choose an insulation method for our attic trusses and I think a spray foam solution might be the go to purely based on speed and help in air-tightness…can I get your thoughts as there is open/closed and wood sarking might not be required as we are planning a slate roof.
Any tips would be great…my preferred would have been wood fibre but the cost and labour extensive detailing is what puts me off as our roof is not that simple.
Thanks.
3 Answers
Thank you for your question. We’ve passed this on to one of our experts here at Build It and we’ll get back to you as soon as possible with an answer.
Best wishes,
Georgina Crothers (Editorial Assistant)
I’m not quite sure of the construction method, although attic trusses are mentioned, but if there is an attic above a flat ceiling, then mineral wool is best. I would never use any form of spray foam insulant other than in very small inaccessible areas, as some plastics pollute the environment indefinitely. Spray foam insulation also adheres to the substrate, making other materials difficult or impossible to recycle. It also requires a lot of energy to produce, and you need to check which chemicals are being used and thus released into the environment.
We built and lived in houses for millennia without the use of such materials, so they should be viewed as a 20th century aberration. Where spray foam is used as a retrofit measure on the underside of rafters on slate or tiled roofs, all old buildings experts will tell you that this is the quickest way to kill a roof. Any water that gets in will get trapped and the rafters rot.
Nigel Griffiths (sustainability and heating expert)
Hi 105,
As Nigel suggests, there are plenty of alternative insulation approaches to consider and you may want to think carefully about the sustainability credentials of the different options.
Just wanted to add a couple of extra comments in specific response to your questions about spray foam. Firstly, open-cell spray foam insulation is a breathable product so my understanding is that it shouldn’t lock-in moisture (unlike old-school closed-cell versions). Hence, of the two, open-cell is preferred for the kind of application you’re considering.
From an in-situ performance perspective, as you suggest, using spray foam can be a helpful back-up for airtightness, as it’s relatively straightforward to get it into nooks and crannies. That said, good initial design and site practice (accurate construction, correct spec & application of airtightness tape etc) should be your primary route to airtightness on a new build.
I’m not convinced by the idea of direct-applying any spray materials beneath slate tiles, as it strikes me you could be asking for trouble with any future patch repairs etc. So, if you want to pursue spray foam, I would have thought you’d want a vapour-permeable sarking of some kind. Your slate supplier may be able to advise.
Chris Batesmith (editor, Build It)