I am currently doing some tidying up in the loft and I'd like to replace the old insulation.

The current insulation is glass wool, it is missing some chunks and has gaps, it has compacted in places, and in general it seems like it was not installed with care. I would like to remove the old insulation and replace it with higher performance insulation.

I am local to a company which deals with slightly defective (so called "seconds") PIR boards, so I can get a bunch of boards for a good price. The downside is that the PIR boards are more difficult to cut to right size and would need to be friction fitted with products such as Gapotape, or alternatively I could use a bit of spray foam to seal any gaps.

Stone wool is a good alternative and easy to install, has amazing fireproofing properties, but the insulating performance would be around half of PIR boards.

Glass wool is cheap but it degrades with time and I'd like something with more longevity.

The problem is that the joists are only 9cm high, and the loft is quite cramped as it is and I'd like to continue use it for storage. So I would prefer not to add any loft legs, etc. to allow for multiple layers of stone wool insulation.

If I were to go for PIR boards, is there anything to worry about damp/water vapour wise? Please see the diagram below, obviously the insulation would go in each of the spaces between joists. Loft floor diagram

  • Would you be willing to lose about 4cm for loft height. What we call a 2x2(inch) board could be placed on top of your joists and give more height for insulation. Using fiber glass batts will change from R-15 to R-20(very decent insulation).
    – crip659
    Commented Aug 13, 2022 at 0:03

1 Answer 1


We got a wood based insulation - same r value as fibreglass but does not irritate like fibreglass, still needs the dust mask though.

Comes in various thicknesses and is easy to use and cut to size.

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