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Housing - Renovation: Oversite polythene vapour barrier under suspended floor - good or bad idea?

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    • CommentAuthorrhamdu
    • CommentTimeDec 7th 2011
     
    As we all know, airbricks are used to ventilate the subfloor void and stop the joists rotting.

    But it had never really struck me that in an old house, this is a losing battle because the soil or 'oversite' is normally damp and therefore a massive source of water vapour. Then a friend told me that since he laid polythene on the ground under the floorboards, he had fewer damp problems. Even rising damp in the walls had abated.

    I've now done the same - so please don't tell me it's a bad idea! Actually, do tell me, if that's what you think.

    Is it orthodox practice? Has anyone had a good or bad experience with this?
  2.  
    I've not seen it done before, but if the ground is damp & the void above the polyethylene is ventilated, then it can only help.

    My question would be why is the ground below an old house damp? It should be relatively dry having been sheltered from the rain for years & well ventilated. Are the vents blocked? Is there a problem with drains or downpipes? Has the house been built across a land drain or natural drainage path?

    David
    • CommentAuthorseanie
    • CommentTimeDec 7th 2011
     
    I was crawling around in the dirt under the floor of a Victorian house only yesterday; absolutely bone dry.
    • CommentAuthorDarylP
    • CommentTimeDec 7th 2011
     
    I have to agree with the above, having renewed CH pipework under a 1920s house, the crawlspace under the floor joists was bone dry.
    If it is damp, there is something causing it to be that could be investigated...
    Good luck:smile:
    • CommentAuthortony
    • CommentTimeDec 7th 2011
     
    I would have thought that the damp would be forced up the walls more with the poly covering.

    With a ventilated void (which if the floor is uninsulated is a cooling system) there will never be a damp problem, I have seen water on top of the oversite many times.
    • CommentAuthorskyewright
    • CommentTimeDec 7th 2011 edited
     
    By coincidence this is a question I'd been meaning to ask too...

    Posted By: davidfreeboroughMy question would be why is the ground below an old house damp?


    Our underfloor area is (naturally?) from slightly to over a metre below the surrounding grund level. It's not bone dry down there, but neither is it soaking wet. This is Skye, after all - the year round average RH% of outdoor air is around 85%, according to my weather station.

    The soil state is just pretty much as I'd as expect if I took the turf and top soil off the area outside the house in reasonable weather. If I'm working down there I usually put down some cardboard to sit/kneel/lay on; even if the cardboard is left for months it doesn't start to disintegrate (though once retrieved it's fit only for composting).

    However, since I found out that if the house were built now rather than in 1970 it would have a DPM and then 100mm of oversite concrete down there I've been wondering if providing a layer of oversite polythene might not be a useful nod in that direction...
    • CommentAuthorrhamdu
    • CommentTimeDec 7th 2011
     
    Posted By: davidfreeborough
    My question would be why is the ground below an old house damp? It should be relatively dry having been sheltered from the rain for years & well ventilated.


    Capillary action? If moisture can climb up walls, it won't have a problem seeping sideways through porous soil.

    Here on a chalk hill in Brighton, the water doesn't even have to flow sideways. If it wants to get to the bottom of the hill, the only way is under the terraces of houses, which are built along the contours.
    • CommentAuthorborpin
    • CommentTimeDec 7th 2011
     
    Posted By: davidfreeboroughMy question would be why is the ground below an old house damp?
    High Water table? Dig a hole outside and see where it fills to. My last house the water toby (stopcock) access was almost permanently filled to within 150mm of the top except in the driest part of the summer.
    • CommentAuthorrhamdu
    • CommentTimeDec 8th 2011
     
    Posted By: borpinHigh Water table?


    Not here. There's probably 100m of chalk beneath us, and before main drainage was installed ca 1870, the house had a soakaway.

    However, I think the chalk is damp, even where not directly rained on, because it attracts water into its pores. Is it plausible for moisture to diffuse 3m laterally underneath a house? It's physically possible, the only question is the rate of transfer.
    • CommentAuthorWeeBeastie
    • CommentTimeNov 12th 2019
     
    Resurrecting this thread after a visit from the timber preservation man.

    There is a generous void under my floor and he says there are sufficient vents, but there it decay to several of the joists and some floorboards, and previous floorboard replacement.

    His diagnosis is a wet oversite. No leaks from above, 'just how it is', and recommends sealing the soil. Would others here agree? Any downsides to this? Or should I try more vents first (assuming insulation is to be added below the floorboards)? It is a small property, semi-detached, so I'm surprised that there is so much moisture if the space is already ventilated.

    Could impervious floorcoverings, recently removed, have contributed to the problem???
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2019
     
    Posted By: WeeBeastieHis diagnosis is a wet oversite. No leaks from above, 'just how it is', and recommends sealing the soil.

    If you have somewhere for it to drain to then digging a French drain around the outside of the house might be a better way to dry the soil under the house.
    • CommentAuthorWeeBeastie
    • CommentTimeNov 12th 2019
     
    djh

    That seems like a good idea but I think it may be competing for underground space down one side of the house where the soil and stormwater drainage pipes run.
    •  
      CommentAuthordjh
    • CommentTimeNov 13th 2019
     
    Posted By: WeeBeastieThat seems like a good idea but I think it may be competing for underground space down one side of the house where the soil and stormwater drainage pipes run.

    The soil and stormwater drains should be bedded in gravel anyway, so they could form part of a French drain.
    • CommentAuthorWeeBeastie
    • CommentTimeNov 13th 2019
     
    Thanks, djh. I will ask a builder about it.

    Just had three options for sealing oversite presented in the report on underfloor conditions:

    1. polythene + sand blinding
    2. polythene + concrete
    3. Kontract 8 waterproof membrane.

    If this is the route to go down, which is the best, performance-wise? I only know which is the most expensive.

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