Insulating new loft space

12 Feb 2015
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United Kingdom
To insulate my loft space I was planning to put kingspan in between the rafters, 150mm rafters, 100mm-120mm kingspan with a gap above to let the membrane breathe. However kingspan is cut by hand, not very well, and it's not a good fit in many places. So I did a small area only and then stopped. I attach a photo. To achieve the necessary U-value I also have to have insulated plasterboard, 37mm (25mm insulation + 12mm plaster). That will close all the gaps too I suppose.

I was then approached by a spray foam loft insulation company for my loft. They tried to explain how the foam is breathable so I do not need a gap between it and the felt. Anyone has experiences with this insulation technique?


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Ask them for a condensation risk assessment and insurance backed guarantee.

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Carry on with the celotex, you'll get better at cutting it tight the more you do and for the loose bits use some expanding foam to fill the gaps before foil taping all the joints.

Yes it's time consuming but it's effective, if it makes you feel better I had to do minimum 150mm over a 48m2 roof in our new extension, it took me awhile as we also had detail pieces such as veluxes and dormer windows but once it's done it's done, it's only a few days/week out of your time then it's done forever
Small gaps shouldn't really matter much if they are sealed against drafts. Insulation works based on square meters

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