sub-floor ventilation

laxeylady

We have just membraned and plastered our living room. Giving it time to dry but have noticed a problem. Where our external floor vents are (they are the victorian fancy iron type) we have noticed that we are getting condensation on the face of the membrane that faces into the room (the bit that will be covered with skirting) which is dripping onto the floorboards. We don't want to end up with a mouldy carpet/rotten floorboards so anyone have any advice? Will this problem solve itself once carpet/skirting is fitted stopping warm/cold air meeting? Should we fit different vents that point the cold air downwards?

laxeylady

bumpity bump

teneighty

Some questions.

When you say "membraned" are you talking about a cavity drain membrane?

Was it installed by a competent contractor?

If so why did they cover the air vents?

Why are the air vents above floor level, if not sub-floor ventilation are they vents for a fire?

Where is the bottom of the membrane especially with regard to any suspended timber floors?

It is all very confusing.

laxeylady

We have used oldroyd membrane on the walls and yes it was fitted competently. The vents are under the floor and have not been covered. ABOVE where they are there is condensation on the very edge of the floorboards. The membrane goes behind the floorboards so that any water that drains from behind it can escape to the ground.

teneighty

I'm sorry but I'm struggling to visualise the arrangement.

The vents being just below the floorboards does not sound right. However, the issue of condensation is probably due to the new plaster, a tremendous amount of water vapour will be released as the walls dry out. The area around the vents is probably colder so will be prone to condenstion with the elevated humidity in the room.

If you could possibly leave the skirting boards off until it has dried out some more and see if the condensation cures itself?

I'm curious why they used the cavity drain membrane with a suspended timber floor? Is the wall construction unusual? How were the floor joist ends treated? Are they built into the external wall or do they rest on internal sleeper walls?

laxeylady

The membrane we used was Oldroyd XP -

Oldroyd Xp is a studded polypropylene waterproofing membrane with a special mesh welded to one side. It is fixed to damp, contaminated walls using Oldroyd Plaster Plugs, with the studs facing the wall. This creates an air gap, which allows the wall to breathe whilst blocking the passage of moisture. On the reverse side of the membrane, the special mesh can be plastered onto, allowing a plaster finish to be achieved without risk of contamination from any damp or salts in the wall.
We took this route because the house is solid sandstone and is built into a bank - much of the living room walls at the back are under the ground.

The joists sit on a low wall inside the walls (is that a sleeper wall?) and some were starting to go a bit at the very ends so we have treated them with boron rods. Where the membrane goes down on one wall where the joists are perpendicular to it we have had to use dpc over the joists so that any water from behind the membrane doesn't drip down onto the joists. The vents are draughty and I'm not sure how effective they are because with the back of the house being under ground there are no vents there to provide a cross flow of air. I think it won't matter what we do to combat dampness - the house will just be damp for ever. Unfortunately. We have knocked the heating off in the room now and the condensation does seem to be improving. But how can we dry the floorboards in this weather without the warmth?

teneighty

The logic in using the membrane where the house is built into a bank is perfectly sensible and I would have recommended the same action.

The element I am struggling to grasp is using it in conjunction with a suspended timber floor. As you have rightly pointed out any sub-floor ventilation is compromised by the elevated ground level of the bank, as will any ventilation to the rear of the membrane. You will also get the complicated detailing where the membrane meets vents and floor joists etc.

I would suggest that the best course of action is to discuss your concerns with your contractor. If you are not confident with their response pay for an independent damp proofing expert to give you a second opinion. Another option may be to speak to Oldroyd and see if they have a technical department that would be prepared to inspect the installation.

JustAnotherSaver

I know i'm bumping an old thread but i'm looking at using this stuff too & this was one of my concerns - condensation getting onto the wooden floorboards/joists from the membrane.

Laxeylady:

1) What did you do on the Oldroyd? Straight plaster or D&D plasterboard?
2) Did you ever solve the condensating problem?
3) Did this approach stop your damp coming through showing on the wall surface (i don't mean behind the sheeting i mean the visible wall once plastered)?
There's other questions i'm sure i'd have but with you last being active on your last post date i don't want to type too much. I'll PM too.