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Using kingspan/celotex in place of fibreglass insulation in a loft

Discussion in 'Building' started by Jon c, 9 Sep 2021.

  1. Jon c

    Jon c

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    Hi all,

    I'm helping a friend renovate an old house he bought and our next project is to insulate a loft that will be used for storage after replacing the ceiling. There is limited headspace up there, so to maximise usability we were planning on using kingspan/celotox type insulation board in place of normal roll out loft insulation.
    We were planning on using 100mm of insulation board between the ceiling joists and then boarding with chipboard on top. Will this setup be sufficiently insulating?
    Any input would be appreciated!
    Thank you
     
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  3. oldbutnotdead

    oldbutnotdead

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    Current standard for loft insulation is 250-270mm of soft, depending which type. Think you'd need a bit thicker than 100mm kingspan (300mm rockwool gives U of 0.14, 100mm pir only gets to 0.23). Plus it's very expensive at the moment & a horrible dusty job cutting it to fit- you need a snug fit.
    Ask your friend if he really needs to hoard junk in the loft cos it's going to cost...
     
  4. Jon c

    Jon c

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    Yes I'm certainly aware that it's expensive! I would have just gone for soft insulation and loft legs and just cope with a small amount of headspace but he (and the Mrs) are pretty set on wanting to use board insulation. We could go up to 120mm as that's the joist width but it seems harder to find.
     
  5. oldbutnotdead

    oldbutnotdead

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    TBH it's up to them (at the moment) how much they want to spend heating the world.
    One quite good compromise (if you can afford to lose 100mm) might be to put 120mm rockwool between the joists, floor with something cheap and thin (10mm OSB or that sort of ilk) then cover with 100mm PIR and chipboard on top. Saves a lot of cuts on the Kingspan (always assuming you can get 8 x 4 sheets into the loft :) )
     
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  6. phatboy

    phatboy

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    The joists will be a cold bridge a bit, you really want to be covering them in insulation
     
  7. ^woody^

    ^woody^

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    The big problem with rigid board insulation for that use is that it is difficult to get a snug fit between all the joists and against the ceiling, and any gaps may well mean that you may as well have just used cheaper quilt instead.
     
  8. cdbe

    cdbe

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    Use a higher performance cavity batt rather than rigid between the joists - much easier/better fit. Put rigid on top of the joists and then floor directly on top of that. Identify the floored area and swap to standard 300mm mineral wool in those areas that won't be floored (heading into the eaves), pay attention to the eaves - go back to the higher performance stuff, ensure it is fitted across to the cavity while maintaining airflow above it.

    This is a picture of the eaves insulation in my house after "professionally" installed insulation - the insulation stops nearly 18" before the cavity - leaving an uninsulated perimeter all the way round the top of my house for the heat to escape.

    Screenshot_20210910-092613.png

    Also depending on age of house consider strength of old joists to carry even slightly more load without causing ceiling cracking - might be worth considering sistering some deeper joists with a lower ceiling to avoid this and give extra depth for insulation.
     
  9. JohnD

    JohnD

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    If you use kingspan or similar, you can lay it on the joists (no intermediate floor) then lay floor on top.

    I can get 8x2 sheets through the loft hatch.
     
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  11. oldbutnotdead

    oldbutnotdead

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    Wasn't sure whether joists would be sufficient spreaders for PIR- useful knowledge, ta :)
     
  12. bsr

    bsr

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    I did as suggested above. Rockwool between joists, cavity batts (small 4x2 PIR boards) on top then cover with hardboard. Ideal loft floor.
     
  13. JohnD

    JohnD

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    It's the floor on top that takes the weight.
     
  14. oldbutnotdead

    oldbutnotdead

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    Indeed, but that weight goes somewhere- I had a mental pic of 50mm wide grooves appearing in the underside of the PIR :)
     
  15. JohnD

    JohnD

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    You can lay a concrete floor on PIR.
     
  16. oldbutnotdead

    oldbutnotdead

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    Yes you can but the PIR is evenly supported with sand etc (I've done a couple)
     
  17. IT Minion

    IT Minion

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    If you stand on one foot on a sheet of PIR you probably won't dent it, although you might at the toes or heel if you walk on it. In a loft with 2" rafters then there's a fair amount of surface area to spread that force over.
     
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