November 6, 2025Nov 6 I have a timber frame that I want insulate which varies in thickness from about 118mm to 155mm I was thinking of using 100mm PIR and loft insulation to fill the gap. Which is best: Outside surface, 100 PIR loft insulation Inside surface. or Outside surface loft insulation, 100 PIR inside surface.
November 6, 2025Nov 6 2 minutes ago, Marvin said: loft insulation It is very likely to slump as it's not rigid enough. Use acoustic mineral or glass wool instead. Or Frametherm or similar.
November 6, 2025Nov 6 Author 57 minutes ago, JohnMo said: It is very likely to slump as it's not rigid enough. Use acoustic mineral or glass wool instead. Or Frametherm or similar. Thanks @JohnMo I was also thinking about the dew point. I didn't know which way was best to instal them PIR to the outside and glass wool inside or vice versa.
November 6, 2025Nov 6 I take it these are walls? Can we know more about the lay-up (inside to out)? In general whatever element it is, it wants a vapour control layer on the inside, immediately behind the plasterboard (or floorboards if it's a floor? I would suggest you get a condensation risk assessment done. WUFI is among the most accurate, and is generally paid-for. The BS (Glaser) method is generally available free, and 'DIY' calculators can be found on many merchants' websites.
November 6, 2025Nov 6 If the PIR is inside you can theoretically tape the foil as the VCL, but I'm guessing there's studs between each sheet, so you have discontinuity of VCL and thermal bridging... You could tape over the studs for VCL, but you still have the thermal bridge.
November 6, 2025Nov 6 Author 11 minutes ago, Redbeard said: If the PIR is inside you can theoretically tape the foil as the VCL, but I'm guessing there's studs between each sheet, so you have discontinuity of VCL and thermal bridging... You could tape over the studs for VCL, but you still have the thermal bridge. Hi @Redbeard thanks for this Yes on the inside I am adding 25mm over the joists, then the VCL then the Fireboard. Forgot to say this is for a garden room walls. Hi @Mr Punter Thanks for the info.
November 7, 2025Nov 7 Just fill the studs with Frametherm 32, miss out the PIR between studs. Why not make all the studs the same depth? Add packets? Add a breather membrane outside, 25mm PIR inside (do you need it)...
November 7, 2025Nov 7 Author Hi @JohnMo Yes, need good insulation in narrow space so using PIR. Studs existing. breather on outside. True still thinking about 25mm PIR on inside.
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