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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Just about to plasterboard the loft ceiling and thought I might as well add a bit more insulation.

    A bit windy today and up in the loft rooms it is quite noticeable, although should be better once the plasterboard is fitted.

    The build up is

    Metal roof/140mm Kingspan SIP/30mm PIR/Multifoil/25mm airgap/15mm plasterboard

    I was wondering if the 25mm airgap, which is apparently needed to allow the Multifoil to work properly, could be filled with either PIR or mineral wool to improve the acoustics?

    I suspect mineral wool stuffed between the battens in that 25mm airgap would be best for the acoustics, without detracting too much from the U value of the foil.

    Does that make sense?

    • CommentAuthortony
    • CommentTimeApr 30th 2018
    I am afraid that we are going to have to ask you read the thread on MultiFoils http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=125&page=1#Item_0

    It is crucial that the void is totally hermetically sealed to outside air otherwise all the effectiveness of the insulation will be lost.
    • CommentTimeApr 30th 2018
    Posted By: OracsRevengeA bit windy today and up in the loft rooms it is quite noticeable

    The build up is Metal roof/140mm Kingspan SIP/30mm PIR/ ...

    I don't understand there are drafts in the loft if the walls/roof are SIPs? Shouldn't that be airtight?
    Hi, no sorry, I meant the wind noise is quite noticable.

    I just realised that it istn't proper multi-foil, it's Superfoil SFTV reflective vapour barrier. It was installed by the Timber Frame company as being able to replace 30mm of PIR.

    From Superfoil's website "They provide both the VCL or breather membrane function whilst adding additional thermal performance to your build helping you to save money & space on insulation costs."

    I am trying to decide if negating some of the barrier's properties by installing a 25mm rockwool insulation in to the void between the foil and the plasterboard battens is worth doing.

    As I see it, the rockwool will add back some of the lost U (due to filling the void between the foil and the plasterboard), but will reduce the wind noise, so that small loss of U may be worth it for improved useability of the space.

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