Sealing smell of damp from under floor

Discussion in 'Building' started by Gábor Melis, 4 Oct 2021.

  1. Gábor Melis

    Gábor Melis

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    Hello

    I'm new here seeking advice. First, let me describe the problem. In our typical 30s house with suspended timber floors, there are a few places on the ground floor that smell of dampness. The place with the strongest smell is the understairs cupboard with raw floorboards and large gaps in between. I guess it's the ground below that can be smelled quite strong. See the first attached picture.

    The second place is in the reception, where the laminate floor and a brick wall meet under a built-in bookshelf, that can't be removed without much destruction. Brave excavation work revealed that luckily the shelf does not sit directly on the laminate, but there is an inch high gap, where one can reach in (over the edge of the laminate and down, see second picture) and feel the holes in some crumbling concrete (?). The draught can also be felt and hygrometer says 70%, while it's 55% elsewhere in the house.

    At both places, the plan is to seal off the void under the suspended timber by:
    - in the understairs cupboard, covering the raw floorboards with 6mm plywood and filling the gaps left with caulk (a lot of precise cutting is needed here)
    - under the bookshelf, asking my son (who can reach in there more easily) to put some backing rod into the bigger holes and using some kind of concrete filler/patcher (through a short hose to be able reach the corner?). Alternatively, maybe some foam would be easier?

    Questions:
    • Are the better ways to fix these things?
    • If the plywood solution is okay, how to prevent it from becoming mouldy?
    • What kind of filler/patcher/foam to use that's
      • easy to work with through this one inch gap
      • flexible enough so that the hole remains sealed
      • mould resistant

    Cheers,
    Gabor
     

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  2. JP_

    JP_

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    I am not sure (diyer) but maybe the best solution might be to push some mineral wool insulation under the bookcase, which would keep the inside warmer and reduce cold spots and risk of condensation.

    But, if it smells damp, then the problem might be under the floor ... and need more drastic remedial work. The subfloor should not be wet, so if it is more moist than outside then might suggest lack of ventilation. When the hygrometer says 70% under the bookcase, what is it outdoors at the same time? Could be OK, as on a cloudy/damp day it will be around that mark anyway.
     
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  4. jfsoar

    jfsoar

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    I'm no expert also, but I would have thought that sealing off the damp would be the opposite of what you should do? Once you've identified and solved the cause of the problem (e.g. dirt piled against an external wall, leaking drains, etc), you'd want to get as much airflow to the affected areas as possible to help keep them dry. I'd be looking at making sure air is circulating under the floorboards, that air bricks aren't blocked, etc. I can't see how 6mm plywood would do anything positive to remediate the situation.
     
  5. Gábor Melis

    Gábor Melis

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    The bookcase is only as damp as can be explained by being exposed to the more humid air coming from below. Instead of the MDF of the bookcase acting as the barrier, I'd like to fill the gaps in the floor (not visible in the bookcase picture).
     
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  7. JohnD

    JohnD

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    You must cure the damp, not hide the smell.
     
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