Home Forums General Discussion Underfloor ventilation and moisture barrier Reply To: Underfloor ventilation and moisture barrier

Ian McChesney

Hi Kat
Always tricky with moisture issues. But I’m not sure that your client’s suggestion will deal with the issue though, and may create another one. I’d be very wary about stapling polythene to the joists. The polythene will provide a largely impermeable water barrier that may cause a moisture problem within the flooring components between the polythene and the T&G. Water could get into this zone by spills from above, leaky pipes, and by movement of moisture from below. The result could be creation of a damp, high humidity zone. I think looking at reducing the sources of moisture should come first, then work from there.

If water is ‘running’ down the inside face of the basement wall it suggests the source could be more than just normal infiltration through the bricks – does the pointing need repair, are some bricks damaged etc? Drilling some external drain holes at the junction of the foundation and the bricks to divert water from the cavity to the outside might also be considered, although the base of this cavity can often be filled with rubble and not easily drained.

As for the spring, can any of the flow under the house be diverted by digging a drainage trench on the outside?

Reducing sources may or may not work, but regardless I think that sticking with the conventional polythene on ground would be best, and the best way of creating a dry basement. If there is still a problem with damp foundation walls run the polythene from the ground up and secure the polythene say to the bottom plate to cover the walls.