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    I am in the final stages of planning my self build project utilising a timber frame construction with a blockwork and render/cladding outer skin. Our timber frame supplier has the following frame construction from outside to in of, blockwork, 50mm cavity, breather membrane, 9mm OSB, 140mm stud filled with 120mm PIR glued and sealed to stud, 20mm air gap, polythene VCL, 20mm PIR applied across the studs to reduce thermal bridging, 25mm battens to provide a service void and 12.5mm plasterboard.

    I am thinking of using Pro Clima Intello Plus to provide the VCL and airtightness layer instead of the Polythene, but question if it is worthwhile using because of the 20mm PIR applied directly over it would I assume prevent it diffusing any humidity within the stud back into the house?

    Can you provide some guidance please?


    Hi Alasdair,

    I can't claim to fully understand the moisture needs of walls, but:

    1) Isn't the inner layer of PIR virtually vapour impermeable anyway, so any moisture diffusing inwards would not make it through the inner PIR even if you allowed it to come in through the Intello Plus membrane?

    2) Wouldn't the movement of moisture be outwards anyway, or do you think it would be driven inwards during hot weather?

    3) If you put the 20mm PIR on the outside of the frame, you'd reduce the thermal bridge at the intermediate floor junction.

    4) If you used n 89mm frame, and used 70mm PIR on the outside, you'd have better U values (approx. 0.15W/m2a) and even better junctions.

    Hope this helps,



    The construction sounds a bit odd to me, but I'm no expert in the general style. Why is there an air gap in the middle of the insulation? Is there a particular reason to use PIR – perhaps you're constrained on space?

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