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97mm Joists What thickness of Rigid Foam Insulation?

Discussion in 'Floors, Stairs and Lofts' started by bluewire, 7 Feb 2012.

  1. bluewire

    bluewire

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    I want to put rigid foam foil backed insulation between my rafters and probably on top at a later date + boardout.

    I've measured my joists and there actually 97mm so my question is do i buy 100mm or 95mm board.

    Reason I'm not sure is that if I use 100mm and board I'm worried it might push the plasterboard down. 95mm will obviously sort this but then leave a small gap were condensation might form.

    Hope you can help. Thanks
     
  2. freddiemercurystwin

    freddiemercurystwin

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    So you are insulating a roof, is this as part of a legal conversion or just making the loft a warmer place?. Do you have felt beneath your tiles or nothing or a breathable membrane.

    I suspect you may not like the answers following your replies.
     
  3. bluewire

    bluewire

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    Sorry I ment joists as in the title. No its not a conversion. I just want to replace my glass fibre loft insulation with foam board. So I don't have to have 270mm depth.

    Its a ventiled loft space eaves and ridge tiles with felt.
     
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