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Insulating suspended floors with a depth of 1.7m

  • 24-09-2022 10:13am
    Registered Users Posts: 4

    Recently bought a 1960s house with suspended floors, with a depth of 1.7m (pretty deep!!). We want to insulate. Ground floor is ~60sqm. We only have access to one compartment under the stairs at the moment, where a section of the floor boards have been cut by previous owners.

    1. What are our options for insulating this space?

    2. Roughly, what would these options cost?



  • Registered Users Posts: 507 ✭✭✭mike_2009

    Some links here to check out (not an expert so can't guess on costs - will depend if they have to take out each floor beam or not):

    I'd be wary of spray foam as you want some breathability to avoid wood rot or at least have an expert explain how this is avoided (I know they use it in roofs but period properties behave in a particular way and any changes should be carefully planned out).

    Edit: Ask your neighbours - see what they have done / are considering?

  • Registered Users Posts: 2,366 ✭✭✭Shoog

    We have suspended wooden floors and the way I have insulated them is to lay Urethane boards over them and then put a floating OSB floor over the top.

    The process is to take off the skirting boards.

    Lay a DCM (I used a foil faced reinforced loft barrier that I ordered from Germany) over the existing floor and tape it to the walls behind where the skirting will be reinstalled.

    Lay down a 2-4cm layer of foil backed Urathane insulation.

    Cover with a T&G OSB floor boards which are glued at the seams. These do not need to be nailed down and should not be run right up to the walls to allow for expansion.

    Cover this with the floor cover of your choice or as we did paint the OSB as is. Floor cover can be added at any point in the future.

    Replace skirting boards.

    This is an inferior option to pulling the floors and laying between the joists - but considerably cheaper and less intrusive. It gives you a warm floor underfoot - but the main benefit is to eliminate drafts (the primary heat loss vector in an old building) and reduce the loss of hot air at the ceiling by reducing the stack effect. We have done this on Suspended floors and over a concrete floor.

    Ensure that all your airbricks are clear and protected because the airflow through the underfloor will be much reduced as it doesn't allow stack effect to ventilate the underfloor.