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

Postby 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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