Crawl space under floor insulation

Discussion in 'Floors, Stairs and Lofts' started by svouts, 11 Mar 2021.

  1. svouts

    svouts

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    Hi, my ground floor wooden floor is quite cold. Underneath there is vented crawling space and I have access from a small window. I am thinking to add rock wool insulation under the floor between the joists.
    How thick should be the insulation?
    Except from chicken wire, any other ways to support the insulation?
     
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  3. KenGMac

    KenGMac

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    svouts, good evening.

    How thick? short answer is thicker the better, 50.mm as a minimum if you can afford more then great.

    As for fixing "Boards, or Batts" I have seen used strips of timber nailed on to the sides of the joists, the timber does not have to be full length of the insulation, but it is there to support / restrain it.

    Big thing is ensure that the top of the insulation actually touches the underside of the flooring, any gaps at the sides of the insulation between insulation and joists, pack with slivers of insulation, expanding foam, pack with glass wool, Etc.

    Even nails driven into joist sides will assist to support the insulation.

    Solid insulation boards tend to have and give a netter insulation value than glass wool, also glass wool in an underfloor retro-fit situation is [shall we say] not comfortable?

    Ken.
     
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  4. StephenStephen

    StephenStephen

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    I've used plastic netting successfully, though I had the floor up to do it.
     
  5. JobAndKnock

    JobAndKnock

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    Modern mineral wool batting such as Rockwool slab, Earthwool.slab, etc ate considerably better than the fibreglass stuff of yore. It also cuts with a hand saw. With any insulating materials gloves goggles and masks are advisable. Personally not so keen on polymer resin products such as Kingspan or Cellotex, because whilst they are light weight, they don't recycle well and in the light of Grenfell wwith know that they the are often not flame resistant (something mineral would are)
    ,
     
  6. svouts

    svouts

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    I did a bit more checking today.

    There is a first window to access the crawling space with about 62cm diagonal opening, then a secondary window with 55 diagonal opening. So rigid PIR insulation would not go through, only option is rockwool rol that is flexible enough to squeeze through.

    The original would floor had 22mm thick original boards with filler in between gaps and was painted with no visible gaps. Then added 12mm chipboard and 8mm oak floor planks glued/nailed on top. So about 42 mm thickness.

    Would 100mm rockwool insulation be enough?

    It is easier to fit the insulation at the bottom of the space between the joists leaving an air gap between underfloor and insulation. Then secure with plastic net attached with staples. That will leave air space between the floor and the insulation. Will this gap reduce the insulation? Or shall I try to fit the insulation so as to touch the floor underneath?
     

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  7. Bullet-Proof_Biscuit

    Bullet-Proof_Biscuit

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    What is your joist spacing? Usually around 400mm centres, so you could pre-cut the PIR to 400mm widths and that would fit through your 62/55cm entry way?
     
  8. magicmushroom666

    magicmushroom666

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    As mentioned you need the insulation to touch the floor boards, any air gap can allow condensation to form and get trapped.
     
  9. Bullet-Proof_Biscuit

    Bullet-Proof_Biscuit

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    Ah well there should be a vapour barrier on the warm side to prevent that. So that warm moist air from the occupied space cannot enter the insulation and condense where it contacts the cold front dew point..

    Although several on here have said it’s not needed, but it is..

    fixing it from underneath will be grim though!
     
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  11. Why Not Indeed

    Why Not Indeed

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    I don't understand this, in this scenario, the underside of the floorboards will be the warmest thing in that space, so why would condensation form there?
     
  12. JohnD

    JohnD

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    will naturally rise towards the ceiling of the occupied space.

    it will not naturally sink to the floor, where cool air will lie.
     
  13. Bullet-Proof_Biscuit

    Bullet-Proof_Biscuit

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    The entire space will be full of warm moist air, besides, the radiators wouldn't be very well sized if only the top half of the room was warm. But do what you like I guess
     
  14. Mottie

    Mottie

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    When I insulated my workshop ceiling from underneath, I tapped some clout nails in the rafters about every metre, put my insulation up then zig zagged some string across to hold the insulation up before banging the nails in then plaster boarding over it.
     
  15. svouts

    svouts

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    The crawling space is separated in two areas divided by s supporting wall. The first part has 62cm space between the joists. The second has 54cm between joists. I am thinking to buy rockroof flexi roll which comes in 10x100x500cm. It is quite wide so I can use 62x100 and 54x100 pieces. Cut outside before installing underneath.
     
  16. Tigercubrider

    Tigercubrider

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    You could make the second hole bigger without serious consequences, especially if you added a couple of wooden uprights.
    It wouldn't gain much, but would be less of a squeeze.
    Having a mate to help would be a massive boon. I helped my mate insulate his semi loft bedroom. He was squeezed in and I simply passed him stuff but it was safer, more sociable and meant he could get on without worrying about leaving tools outside.
     
  17. JohnD

    JohnD

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    or getting stuck or injured in there.
     
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