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UK based.

Looking to install PIR insulation between ceiling joists in our attic. Cold attic space. The ceiling is that immediately above bedrooms/stairs landing.

Joists are 100mm depth and 600mm centres.

We currently have sub-recommended depth fibreglass wool between the joists (only around 100mm thick as is a 1970's property) that I'd like to completely replace with the likes of Kingpass, Celotex etc. 100mm panels.

The ceiling plasterboard panels are directly attached to joists.

I'm looking at buying foiled PIR as it's got the vapour control built in.

My question:

Do I have to leave any air gap between the ceiling plasterboard and the PIR?

I.e. Can I simply take out the old wool -> cut the PIR -> push between joists so it is flush with top of joist -> aluminium tape where boards join -> done?

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No, you don't need air gap between the ceiling and the insulation. You do need ventilation of your attic space, however, which unless there is some other form of ventilation you need to have an air gap between the PIR boards and your eaves.

That said, unless you need the space, it would be more cost effective to supplement your existing glass fibre with additional rolls laid across the top (preferably across the joists rather than along them, in order to minimize gaps). That would also remove cold bridges that are currently formed by your joists. If you need boards in the space, you can buy plastic supports that hold the board 170mm above the joist to allow space for insulation.

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  • Thanks Jules. That's confirmed my suspicion and put my mind at ease. I have toyed with supplementing the existing insulation but our attic is extremely limited in height (it's still a pitched roof but with little above 1m height) we would like some storage space. I'd ultimately like to board over the insulation at least a couple of metres around the hatch so it's got some decent strength without fear of ripping up/dislodging any wool I'd have installed... would I need to have an air gap between the top of any PIR and any boarding (i.e. batten on top of the joists and then board)? Thank you :)
    – Kinnectus
    Dec 29, 2017 at 13:20
  • I'm not sure about requirements at the top. Perhaps the best way of finding that out it is to talk to the technical helpline from your board manufacturer, at least if they operate out of the UK (e.g. Celotex have a UK office that should be able to help). I know that for completely filling cavity walls you need a special type of board, but I'm not sure if there is any such requirement in a ceiling space.
    – Jules
    Dec 29, 2017 at 13:26

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