Insulating under suspended ground floor.

Discussion in 'Eco Talk' started by Crazyhorse, Jan 9, 2020.

  1. Crazyhorse

    Crazyhorse New Member

    Ok I’m trying my luck here hopefully trying to find answer.

    I have a 1930’s detached house, which has suspended wooden floors With joists that sit on timbers laid across small sleeper walls which are one / two bricks and a piece of slate (acting as DPC I guess). My issue is that all my air bricks run level with the joist height. So if I insulate between the joists with 100mm celotex (or similar). Should I fill the edges with expanding foam and try and make a piece of foam fit that divert’s air down slightly?

    Any help greatly appreciated.
     
  2. Abrickie

    Abrickie Active Member

    Telescopic air vent Screwfix item number 12025 :)
     
  3. Crazyhorse

    Crazyhorse New Member

    I've looked at them, I can’t fit it to the air brick. It will only just fit the hole. How would you seal around it? Is it designed to fit inside the cavity? Or could I just pop it in behind the joist? Although I only have 50mm space?
     
  4. masterdiy

    masterdiy Well-Known Member

    Crazyhorse.
    I have done several floors like you are asking about.
    I used 50mm celotex/or Ecotherm, & where the insulation would meet the air brick, cut away underside of insulation to allow plenty of air through. From air brick approx 100mm.
    You may end up with only 15mm of insulation, but thats ok.
    Cut each section of insulation out individually for each span of the joists, as hardly any two measurements are the same.
    I used roof tile battens screwed to the side of the joists to stop the insulation from dropping through.
    When done & floor boards down go round with foam gun to seal up. Topped off with 22mm floor board.
     

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  5. Joe the Plumber

    Joe the Plumber Well-Known Member

    You could always forget the insulation and just buy a decent carpet....
     
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  6. Crazyhorse

    Crazyhorse New Member

    Thanks Masterdiy. I’ve got slightly deeper sub floor and I thought 100mm as I could just rest it on the sleeper wall beams.
     
  7. KIAB

    KIAB Well-Known Member

    Done a few floors,where insulation meets air brick, I just slice away insulation at 45 degrees, allow plenty of air through.
    And another who uses roofing batten fixed to side of joists.
     
  8. masterdiy

    masterdiy Well-Known Member

    Hi Kiab, is that OK then, or a no no.
     
  9. KIAB

    KIAB Well-Known Member

    Definitely ok, tile batten is cheap to buy.:D
     
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  10. Crazyhorse

    Crazyhorse New Member

    Do I need to avoid bridging the DPC on the external walls?
     
  11. masterdiy

    masterdiy Well-Known Member

    Just butt your insulation up to the wall. As stated, cut away the insulation's underside to allow vent to work.
    You can see the air vent to the left of french doors. Butt up to it mark top of insulation approx 100mm. Then cut that section out up to 30mm of top.
    That will give you plenty of room for the air flow.
    When done & floor boards down go round with foam gun & seal up all gaps & holes. View attachment 37007
    Then refit with new skirting board. Glue on with stixall or grab adhesive.
     

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  12. masterdiy

    masterdiy Well-Known Member

    Cut out like so.
     

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  13. masterdiy

    masterdiy Well-Known Member

    Finished job.
     

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  14. Crazyhorse

    Crazyhorse New Member

    What did you do with mains electrics that pass through the joists?
     
  15. masterdiy

    masterdiy Well-Known Member

    They pass under the joists, so no problems.
    Still plenty of space as insulation doesn't touch them.
     
  16. Crazyhorse

    Crazyhorse New Member

    thanks for all the advice, I was getting near to the point of asking you if you fancy doing it for me!
     
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  17. masterdiy

    masterdiy Well-Known Member

    I'm in Sunny Devon. :D
    Why on earth would I want to venture into Kent ?????;)

    Take a few pictures before you start. (once the room is clear, carpet up)
    That way we can follow you & hopefully advise.
     

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