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Loft hobby room - airtec for rafters?

Discussion in 'Floors, Stairs and Lofts' started by richw, 24 Feb 2021.

  1. richw

    richw

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    Hi all,

    The loft in my 30s semi has a cross-braced chipboard floor. I wanted to take to the next level by insulating the 'ceiling' so I can have clean storage and hobby/office space. I bought some 100mm sheets of PIR which I was going to friction fit between the rafters (which leaves about 50mm to the tile, so a nice air gap). Then do 50mm PIR at right angles over the rafters. I am a bit daunted now I have some boards and have seen the scale of the task!

    I asked a local loft firm to give me a quote, and whilst they said a lot of good things (and the price is ok-ish), they said they don't use PIR at all. They use 'airtec double', which looks like a cross between bubblewrap and foil. Apparently they put this on the rafters, and it is equivalent to 50mm PIR, then they plasterboard. It sounds like witchcraft to me!

    Is this stuff really equivalent to 50mm PIR?!

    Is it a good idea to use only 50mm, then plasterboard, on a roof? Will it be too hot/cold?

    I really don't want something poor, as fixing it would be extremely expensive and destructive!

    I would be happy to hear anyone's thoughts. Is it an insane idea?


    Cheers,
    Richard.
     
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  3. Why Not Indeed

    Why Not Indeed

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    To pick a 50mm PIR at random:
    https://www.insulationshop.co/50mm_celotex_ga4000_pir_insulation.html - 50mm - R value of 2.25m²K/W

    Double Bubble:

    https://ybsinsulation.com/wp-content/uploads/2015/08/Airtec1.pdf - 7mm - R value of 0.173m²K/W

    So not even the same order of magnitude, never mind "equivalent". To be fair though, we might be talking about something a fraction of the thickness of the PIR you're proposing to use. How many layers of this stuff were they going to use to make it equivalent? By my reckoning, they'd need a mere 13 layers of it.
     
  4. richw

    richw

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    Hmm, I didn't think quickly enough on my feet to ask how many layers. I assumed one.

    Good idea on checking the R value (duh!). That makes it look pretty black and white, doesn't it?!

    I suppose I was secretly hoping for some magic reason to explain that it is OK really. A bit of a shame, since everything else seemed really good.
     
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  5. catlad

    catlad

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    They want to use it because its easy, go with your original plan.
     
  6. richw

    richw

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    Yeah, I can see how stapling some bubble wrap will take a couple of hours, but cutting PIR will be way longer. Let alone the price difference in the material!

    Thanks for the comments.
     
  7. foxhole

    foxhole

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    Quicker and easier to fit insulated plasterboard.
     
  8. richw

    richw

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    Hmm, yes, but I wanted something in between the rafters too. So 100mm there, then either insulated plasterboard or 50mm PIR and regular plasterboard.
     
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