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Insulating first floor joists above garage

Discussion in 'Building' started by mattu27, 19 Aug 2021.

  1. mattu27

    mattu27

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    Afternoon all,

    I'm at the stage of insulating between the floor joists above the garage to the bedroom above. I've got quite a lot of spare PIR board that I'd like to use for the area at the very front of the garage where it seems coldest.

    Does the celotex go tight up against the chipboard flooring above to leave a void below? Or does it sit flush with the bottom of the rafters? Thinking it should sit tight to the floor above but would like a nod from someone in the know.

    The rest of the joists with cables and pipes running through will have the itchy insulation placed between.

    Can someone advise?

    Thanks
     
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  3. motorbiking

    motorbiking

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    Generally it goes tight up against the warm side, to prevent condensation. However, there can be reasons such as damp/rot to not do that if there is a need to ventilate the void and the insulation would block that.
     
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  4. mattu27

    mattu27

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    Thanks for the reply. It's a new extension above the existing garage so new joists. One end of the joist is in a pocket in the wall and the other is on joist hangers between a steel (if that makes sense) would that make any difference?

    The ceiling will be double boarded following the insulation so no air will be circulating.

    Thanks
     
  5. motorbiking

    motorbiking

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    Pipes need lagging too.
     
  6. mattu27

    mattu27

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    Yep sorted. Just need a confirmation that tight to the floor above is the way to go.

    Thanks.
     
  7. sotal

    sotal

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    We've just done very similar, planning wouldn't allow celotex type board in the void. The initial plans were rejected which had it in place. They insisted on the wool type. I can't remember now if it was 200mm or 300mm. Same with the void above the extension (that was 300mm though). Only the walls were allowed celotex type insulation.

    I'm not 100% sure why but I think it was something to do with the R value actually being higher with the wool and noise insulation being better too.
     
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  8. ^woody^

    ^woody^

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    You need sound and heat insulation so the best option is PIR under the deck to keep the heat in, and Rockwool above the ceiling to keep the noise out. 100mm each, or half and half for thinner joists.

    Equally important is to seal all holes and gaps around the floor void.
     
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  9. JohnD

    JohnD

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    I only use the brown mineral wool treated with Ecose, which does not shed irritant dust and fibres.

    It is made by Knauf but widely sold as an own-brand.

    You will see the word flashed on the packaging.

    BTW very little heat is lost downwards through a floor. It's mostly draughts, especially round the edges of the room. Mineral wool is very good for muffling draughts and noise because it is easily stuffed into irregular spaces to fill them completely, with no precision cutting required. It is also non-flammable and does not emit poisonous fumes in a fire, which I think is an advantage.
     
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  11. mattu27

    mattu27

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    Interesting, our plans stated the wool type insulation between but we did our own plans and copied off another set. Not spoke to BCO about it but it may be worth a ring. So was that 300mm compressed into the void space?

    I'd have thought that the R value of pir would be well above the wool equivalent, I've got offcuts at 150mm and 100 to use up.
     
    Last edited: 20 Aug 2021
  12. mattu27

    mattu27

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    Was hoping you'd show up Woody!

    So 100mm PIR tight to the floor above and then rockwool flush with the bottom of the joists in the garage?

    So shall I PIR what I can below (tight to the floor above?) And wool type the rest when I run out? I've got 195mm to play with. Any tips on getting the wool to stay insitue? Staples and fishing line?
     
    Last edited: 20 Aug 2021
  13. mattu27

    mattu27

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    Yep, the stuff I've got is the ecosse but it still makes me itch when I use it. Was a B&Q 3 for 2 offer last time but probably quadrupled in price now.
     
  14. sotal

    sotal

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    Just dug out the correspondence from planning, the reason given for insisting on rockwool type insulation was for noise with it being a garage under a bedroom.

    We double boarded the garage ceiling first, then put the insulation in so it had nowhere to fall, then put the floorboards down.

    I had specifically chosen celotex type insulation as I wanted the best thermal efficiency in the space, so was a little disappointed when it was turned down, but I was told that over a certain thickness the difference is minimal. I don't know how accurate that is though.
     
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  15. mattu27

    mattu27

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    Thank, appreciate your time. I'm in the reverse situation and attacking from beneath, just to confirm, was yours 300mm comoressed into the void then? Or did you have a 300mm void?
     
  16. sotal

    sotal

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    Checking on the plans it was "200mm fibreglass quilt". Joists are 200*50 according to the plans.

    The new insulation was brown and lovely to use. No itching at all with it.
     
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