Laying ground plastic under house question

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Eva888

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#284363 16-Apr-2021 11:35

We have arranged for someone to lay black plastic over the ground under the house. Sustainability trust recommend doing this. I mentioned it to a friend and they commented that if there is damp in the ground it would not be able to dry out if covered and thus stay damp for longer.

One of the reasons I wanted it laid is that sometimes after long periods of rain there is a damp smell and some can waft upstairs. The house is in front of but not very close to a hill so naturally in line for some water run off.

I’m now concerned that I may be exacerbating the problem I’m trying to solve. Has anyone had experience of improvement after laying ground plastic? .

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#2693762 16-Apr-2021 11:52

I had this done to my house years ago, it effectively solved the damp smell problem and I have had no repercussions. Make sure the plastic is firmly taped or pegged down.

I had under floor insulation put in at the same time, polyester that just pushed up between the floor joists. I don't know how much difference the insulation made but the plastic made a big difference.

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SirHumphreyAppleby

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#2693771 16-Apr-2021 12:30

The Healthy Homes standards require a moisture barrier for enclosed sub-floors, so I expect this would be recommended regardless of the situation. My instinct tells me this will help.

I had a ridiculous situation recently. Property had a downstairs garage converted to a unit. The people doing the Healthy Homes inspection had no issue with a thin layer of Gib between the downstairs unit and enclosed pile of dirt, but upstairs, with a decent gap above the ground, insulation and solid wood flooring, didn't pass due to their not being a sub-floor moisture barrier. In both cases, the dirt was dry as a bone, but those are the requirements.

Bung

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#2693774 16-Apr-2021 12:45

Shouldn't the garage wall have been regarded as an external wall requiring insulation?

Edit i see only floors and ceilings are included but a wall facing the under floor space should be done.

mdooher

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#2693775 16-Apr-2021 12:46

Put the plastic down, I've done it in the past over ground where the water table is only inches below the surface (South Dunedin), works like a charm

If it is wet now it will never dry out. that's the point of the moisture barrier

Matthew

mulac

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#2693777 16-Apr-2021 12:48

Eva888: if there is damp in the ground it would not be able to dry out if covered and thus stay damp for longer.

As long as the damp is contained below the polythene, and not penetrating into the subfloor space, then that's the polythene serving its purpose; The whole point of putting it down is to keep the damp out.

SirHumphreyAppleby

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#2693778 16-Apr-2021 12:51

Bung: Shouldn't the garage wall have been regarded as an external wall requiring insulation?

Interesting question. It is insulated now, but given it doesn't seem to be considered 'outside' for the purpose of the moisture barrier and ventilation requirements under the same standards, they can't really have it both ways.

duckDecoy

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#2693796 16-Apr-2021 13:18

Question: If there is water than runs under the house when it rains (not sure if this is the case for the OP) is it better to have it go over the black plastic, or underneath (maybe via some trenching)? Or doesn't it matter.

mdooher

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#2693798 16-Apr-2021 13:20

duckDecoy:

Question: If there is water than runs under the house when it rains (not sure if this is the case for the OP) is it better to have it go over the black plastic, or underneath (maybe via some trenching)? Or doesn't it matter.

under

Matthew

neb

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#2693799 16-Apr-2021 13:23

Similar problem here, we're on sloping clay which typically dries itself out from air movement under the house. Neighbours with a similar house have cladding that prevents air movement, and where the clay is almost permanently glistening wet. I've been reluctant to put in a vapour barrier because it seems we'd be sitting on top of waterlogged clay instead of (usually) dry clay...

timmmay

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#2693810 16-Apr-2021 13:38

I wonder if a vapor barrier can be attached to the bottom of the floorboards rather than over the ground if it's really wet down there.

Bung

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#2693815 16-Apr-2021 13:58

If it is that bad the water should be intercepted before it gets under the house.

Batwing

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#2693816 16-Apr-2021 13:59

Remember that your aim is to keep the house dry, allowing the ground to dry out is a lesser concern.

If you try to fix the barrier closer to the floor you reduce the ventilation and may be trapping moisture / creating a condensation issue right under the floor.

Eva888

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#2693838 16-Apr-2021 14:22

Thanks, the consensus seems to be to lay the vapour barrier.

We will also be replacing underfloor insulation after the plastic is laid as the present one is that silver tar paper stuff of the 70s. No idea how that’s going to work out since there is now heating ducting, wires and all sorts under there. The crawl space is very low to the ground in parts to complicate matters.

afe66

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#2693845 16-Apr-2021 14:51

Thats why you pay someone to install it and they in turn
employ a small apprentice to crawl into far corners..

Eva888

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#2693847 16-Apr-2021 14:56

afe66: Thats why you pay someone to install it and they in turn
employ a small apprentice to crawl into far corners..

Can’t help feeling sorry for the poor guy having to do the job. I get claustrophobia just watching anyone under there. He will definitely be getting lunch and goodies to make up for it.

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