If you have a new account but are having problems posting or verifying your account, please email us on for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact

Insulating Suspended Floors with a 1.7m depth

  • 24-09-2022 10:07am
    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?


    Post edited by Spear on

Best Answer

  • Moderators, Computer Games Moderators, Technology & Internet Moderators, Help & Feedback Category Moderators Posts: 25,207 CMod ✭✭✭✭Spear

    Moved to a forum that's related to the topic instead.


  • Registered Users Posts: 14,889 ✭✭✭✭loyatemu

    we have a similar massive void under our floors and I've looked into it, but it seems a big and disruptive job (you could do it from below, but the proper way to do it is as in that video, lifting the floorboards).

    I wonder would it be simpler to improve the draught-proofing from above by putting down a membrane (as in the video) and then a decent underlay and then laminate or manufactured boards, all on top of the existing floor. AIUI most heat loss is from draughts and air convection. The floor timbers would still be ventilated from below.

  • Registered Users Posts: 2,413 ✭✭✭Citizen  Six

    Wouldn't be that much more expensive to lift the boards really. And you avoid having to deal with rising floor level, which would need to be sorted out at the threshold, and the door.

  • Registered Users Posts: 31,059 ✭✭✭✭Lumen

    Just jigsaw a hole under the sofa and insulate from below. 😀

  • Registered Users Posts: 1,233 ✭✭✭monseiur

    Option 1 Get a person of restricted growth to apply spray foam insulation from underneath🤣

    Option 2 Lift floor boards, then working from underneath, fit membrane under joists using treated 2'' x 1'' laths nailed (using battey nail gun) at opposite direction to joists to keep membrane in place. Do not, under any circumstances, use staples for this. From top fit insulation and floorboards. Seeing it's a 1960's house it may be good time to consider fitting new floor boards & skirting if budget allows.

  • Advertisement
  • Registered Users Posts: 27,140 ✭✭✭✭GreeBo

    Can you get from your current access hole to the entire sub floor area? there should be ventilation gaps that allow you to get anywhere you need to.

    Its actually a much easier job when you have the space that you do. You should be able to fit vapour barrier and rigid insulation across the joists, taping joints and down the foundations and get a great finish.

    If you insulate between the joists you will have bridging, hence would I would do it across them, with 1.7M its not like you need the space :)

    tbh, 1.7M is a basement, so there is probably some external access somewhere you havent found yet, how many steps up to your front door?

  • Registered Users Posts: 14,889 ✭✭✭✭loyatemu

    would that not affect ventilation to the joists? or if your airbricks are level with the joists your still going to have cold air blowing through the channels you've now created.

    & how much of a problem is bridging likely to be with wooden joists anyway?

  • Registered Users Posts: 31,059 ✭✭✭✭Lumen

    If joists are fully on the warm side of the insulation they don't need any ventilation, although I see your point about positioning of the vents. Block them up?

  • Registered Users Posts: 27,140 ✭✭✭✭GreeBo

    The joists only need to be ventilated if they are getting moisture, proper vapour barrier details will prevent this since they will now be in the warm zone of your house.

    I'd doubt the air vents are level with the joists on a 1.7M crawl space, but if they are, they dont need to be, so move them.

    Dont underestimate thermal bridging of timber!

    If you add it up they represent a significant percentage of the floor area and you wouldnt consider leaving and area that size uninsulated.

  • Registered Users Posts: 4,813 ✭✭✭Shoog

    Why was my post removed ?

  • Advertisement
  • Registered Users Posts: 4 alcandara

    The question was posted twice by mistake. Your response is on the other question. Thanks for your advice!

  • Registered Users Posts: 4 alcandara

    Thank you for all of your suggestions!

    Another challenge is that we are situated on a flood plane and there is between 10-30cm of water in the void. Although the joists are dry, I am concerned with future damp.

    We have been advised by a neighbour who had a similar void to fill the void completely with gravel, add a radon barrier and insulate.

    Is there any reason why we should not follow this advice to fill it completely?

    Roughly, how much would a void (~84 cubic meters) like that cost to fill...hardcore/other suitable material...not foam?

    Thanks again!!

  • Registered Users Posts: 31,059 ✭✭✭✭Lumen

    Research foam glass gravel.

  • Registered Users Posts: 6,953 ✭✭✭10-10-20

    With the intent of infilling the 1.7m void? Is the loose-fill nature of this conducive to air movement, or does it get applied in a slurry?

  • Registered Users Posts: 31,059 ✭✭✭✭Lumen


    I think the idea is that the air movement is restricted the same as any air permeable insulation, i.e. with membranes/barriers. Whether there's a risk of thermal cycling or whatever in such a deep fill I don't know.