Condensation under suspended wooden ground floor

[Jimbo]

Hi,

I recently had to take up the laminate flooring on one of my ground floor rooms. The real floor underneath is suspended (about 6 inches) above the ground.

When I put the laminate in, I first covered the floor with 6mm hardboard, panel-pinned at 200mm intervals. On top of this I put the Westco polystyrene/foil vapour barrier stuff, then finally the laminate.

When I took up the floor and took out a couple of floorboards, I noticed that the joists etc. at the front of the house were very damp from condensation. This had caused the hardboard to warp, pushing the laminate up in places. There are two air bricks at the front of the house and the flow under the floor is unrestricted.

Condensation forms when warm air meets a cold surface, so I'm puzzled why there should be condensation under the floorboards, when the air under there comes directly from outside the house. It's bound to be cold compared to the floor itself.

Can anyone shed any light on why this condensation is building up?

Cheers

Jim

 

[WoodYouLike]

Rule of thumb: only use DPM on concrete/screed or plastic tiles (vynil etc) under floor, not on original floorboards or any other sheet material. This might cause stopping ventilation under the floorboards or sweating of the tiles.

 

[Jimbo]

Thanks for the tip! I'm curious though -- surely the laminate itself will stop ventilation through the floor as it's a solid plastic surface?

 

[WoodYouLike]

Not as much as an extra DPM does. The laminated floor is installed floating and will 'move' slightly, hence allowing air movement between floorboards and laminate. DPM is very thin and will lay 'still' on your floorboards.

 

[Jimbo]

Okay - so I want to put down hardboard first, rough side up, to help level the floor. Then non-DPM underlay - the fibre board stuff. Then finally the laminate?

I guess I'm just concerned that with no DPM, moisture can get into the laminate from below, and cause it to warp.

 

[WoodYouLike]

The risk of causing condensation is much higher than the risk of moisture coming in to the melamine laminated flooring.
We use this method for Wood-Engineered and solid flooring also.

 

[Jimbo]

Okay, thanks for your help!

Jim