How to Avoid Rot in Floor Joists Sitting on a Radon-Impermeable Plastic Membrane
Questions answered and a range of solutions on problems and issues encountered with timber framed buildings

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How to Avoid Rot in Floor Joists Sitting on a Radon-Impermeable Plastic Membrane

Post by Chapple » Mon Jan 29, 2018 4:15 pm

I have a problem with floor joists sitting on a plastic sheet and going rotten. What can I do?

This is the situation:
In a 12-year-old timber-frame house, the ground floor is a kind of sandwich construction consisting of (working downwards):
- floor boards
- chip board
- joists
- polystyrene insulation between the joists, to the same depth as the joists
- a plastic sheet acting as a radon-gas-impermeable membrane (a requirement in our area). The joists sit directly on the membrane
- the bottom, horizontal elements of the timber frame of the house
- fibre board
- a second layer of polystyrene insulation fills the space between these timber frame beams
- fibre board fixed to the under-side of the timber frame.

The timber frame (and the floor) is held about 300mm off the ground on concrete pillars.

See the attached images for the levels of this sandwich (the photo is just down to the gas-impermeable layer and the insulation between the bottom beams of the timber frame -- it doesn't show the beams themselves or the underlying layer of fibre board).


My problem is that water from leaks around the ground-floor shower and a leaky boiler has collected on the membrane and rotted some of the joints. I've replaced the worst affected but want to know how I can prevent this happening again. How can I separate the joists from the plastic sheet without undertaking major reconstruction work to the floor, stud walls etc?

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Re: How to Avoid Rot in Floor Joists Sitting on a Radon-Impermeable Plastic Membrane

Post by katoosh525 » Mon Jun 10, 2019 5:28 pm

You appear to be in luck. There is a 'small' difference in the thickness of your polystyrene and the depth of your joists. You can get some air moving through that gap, end to end across the joists, which will dry everything out.
You may well have to arrange for an in and an out hole through the floor (these will be at opposite ends of the room, probably diagonally opposite), and a fan to physically move air through this space. You will have to lift the floor to get holes through the joists, or better yet notches to the depth of the top of the insulation to allow air to travel from one joist gap to the next.
If you find some damage to wood in the areas which have got properly wet, you may find it faster and more economical to treat it rather than replace the entire joist.

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