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suspended floor air tight insulation

SuperHomes Revamp Forums General Discussion suspended floor air tight insulation

This topic contains 2 replies, has 2 voices, and was last updated by  Roger Harrison 4 years, 8 months ago.

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    Roger Harrison

    Hi, I’ve moved to a 1890’s, brick-built, end-terraced property, no cavity wall, and with a large cellar. I want to insulate the ground floor above the cellar (i.e. suspended timber framed floor with access from below.

    My ideas so far are:
    (1) use expanding foam and/or mortar to fill gaps/defects in wall between the timber joists
    (2) do I block off the air bricks which partly come into the cellar (no cavity, double-brick walls)
    (3) fill joist-void with sheep’s wool
    (4) staple air membrane such as Dupoint Tyvek across the joists in the cellar (i.e. cold side)
    (5)tape all seams and also tape onto the wall joint
    (5) bond three layers of 20mm Gutex, to create a 60mm board and use to create a ceiling in the cellar. Tape all joints and the wall joint.
    (6) at some point would I need to put a membrane and board on-top of the floor boards on the ground floor too?

    The intention at a later date will be to continue with internal wall insulation, and by continuing with the dupoint Tyvex and board, ensuring that the board and membrane continue down between the joists (this is part of a staged-approach for a full house retrofit and MVHR).

    Is this the right way to go about it? I don’t really want to have to lift the carpet on the ground floor at this stage, so I’m trying to do everything from the cellar.

    I’d really appreciate your advice. Different threads and forums don’t seem to fully answer my query.



    Daniel Nestlerode

    Hi Roger,
    Leave the airbricks open unless you’re going to make the basement part of the heated envelope of the house.
    Use periscopic vents to channel the air under the insulation if the insulation would block them.
    Use a breathable membrane (Tyvek will work fine) to hold the insulation up under the floor boards (between the joists).
    Use a breathable insulation as well. Avoid spray foams unless they are ‘open cell’. Sheep’s wool will work.
    The idea is to insulate the floor and make the room above warmer without trapping moisture against the wood. Let it breathe.

    Hope this helps!


    Roger Harrison

    Thanks Daniel. I intend using sheeps wool and putting in two layers. Would you advise this or just one layer?

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