Floorboards/Damp membrane

[Rick W]

Good Afternoon Everybody,

We have prepared our living room to lay some larch floorboards. It is our ground level living room. In some generation previous to us, the floor has been rendered with cement, and we are going to batten it and lay the boards on top. My question is, do we need to lay a plastic membrane beneath the battens? There is no existing problem with damp in the property, and some people have suggested we should.... I am not so sure!

Removing all the render is not really an option unfortunately.

Many thanks for your help!

Rick

 

[Lime]

I would prevent contact between the battens and the cement using some sort of damp proof membrane but not a continuous unbroken sheet. I'd use the rolls of pvc or similar used in a damp proof course in brick walls.
You could cut your own from heavy duty polythene too.
I wouldn't use a continuous sheet because it would tend to create condensation on it's underside which could cause problems of dampness where it meets the walls.
Using narrow rolls would allow areas of the cement to dry out if it gets damp.
The same effect as using rolls could be produced using a continuous sheet if, once the battens are laid, the sheet is cut away from the cement where not covered by the battens. Leaving 5-15 cms or so protuding from beneath the battens should be just as effective.

I would also ventilate the area that you will be enclosing if you can (preferably using airbricks through the walls) so that any moisture/dampness is able to dry out and not cause problems of rot/rust/damp.

 

[Rick W]

Air bricks? Unless I am being a bit stupid here, I can't see me fitting an air brick as the walls are 2 feet thick! I doubt there is a cavity!!! ;-)

Any other recommendations for ventilation?

Thanks for your advice...

Rick

 

[Nemesis]

A couple of small grilles in the floor - the bellows effect of walking over the slightly suspended floor should help.

I have no doubt now that Matt Green will post and tell us that it's not necessary though...

 

[middi]

There is little point in in trying to fit air bricks unless you intend to fit a suspended floor, which you are not doing.

If you intend to fit DPC strip under every floor batten, that is fine, nevertheless one has to assume that the reason for fitting this DPC strip in the first place is because there is no DPM below the existing floor and some dampness will creep through and unable to evaporate freely. If this is the case then you would still need a reasonably efficient air flow trough each batten gap and at the end of the floor line for any excess moisture to evaporate within the interior of the room.

If there already exists, as you say, a thick cement floor, you might as well fit a complete DPM sheet over the floor aerea overlaid by your floor battens screwed firmly down, at least you then eliminate any raising misture and the need for ventilation has not got to be as severe, but you would still NEED to leave gaps at the orizontal ending of you floorboards, firstly to allow for any expansion and secondly to dissipate any moisture which may be created within the room.

In many instances laying floorboards just on 2 in x 2in battens over an existing concrete floor, with the necessary end expansion gaps, gives no problem at all.

It all depends on the individual application and rate of moisture present.

 

[Matt Green]

A couple of small grilles in the floor - the bellows effect of walking over the slightly suspended floor should help.

I have no doubt now that Matt Green will post and tell us that it's not necessary though...

It's not necessary.

Do everything in Lime's first paragraph- it's a really good specification. The use of airbricks is crazy though, there's no need to ventilate that floor.

Bear in mind that an expansion gap between the boards might be prudent and the boards are best left in the room for a week or two in a drying rack to equilibrate with the environment. Which might be being overcautious, or might save you lifting the floor should it expand or contract to a degree you weren't expecting.


Matt Green

 

[Nemesis]

See - I do listen Matt!

I agree absolutely about leaving the boards in the room for as long as possible. They can shrink considerably and anything which can be done to minimise this is a good plan.

 

[Rick W]

Thanks for all your help!

I was a little unsure as to whether a full sheet would actually cause problems rather than prevent them. I guess the strips method will protect the battens a little, so will probably go with that.

Many thanks again!

 

[Nemesis]

If you have services running under the flooring - pipes etc may be buried - scew some of the boards rather than nail so access in future is possible without too many problems.

 

[Lime]

The use of airbricks is crazy though, there's no need to ventilate that floor.

I like ventilation!

I realise it is generally not requiired in such a situation as this (dry) but I suppose I like to take into account the daft things in life.
..Like a family who regularly spill drinks all over the floor.
....Like a family pet that regularly wees all over the place.
......Like the fish tank breaking.

etc.

 

[skier-hughes]

Sounds like some nice posts for Graham's Top Tips there Lime :lol:

 

[Rick W]

One (fairly important) thing I forgot to mention...

We are going to lay fibreboard insulation between the battens, this will still leave a 17mm gap between the floorboards and the insulation for air movement. Would this fact change anybody's advice?

Rick

 

[Matt Green]

One (fairly important) thing I forgot to mention...

We are going to lay fibreboard insulation between the battens, this will still leave a 17mm gap between the floorboards and the insulation for air movement. Would this fact change anybody's advice?

Rick

You will not get get any appreciable air movement with a 17mm gap. Not that you particularly need it anyway, just pointing out it's a pointless nod to something pointless.

Check the cement has a DPM under it. If it doesn't, the bottom layer of fibreboard will become a wood-boring weevil snack bar within a year. Even if it does it's not a sparkling specification. Could it be suspended off the floor with some plastic mesh or similar? Also, best not lay fibre board around the edges of the room in contact with the walls- leave a gap to avoid water uptake.


Matt

 

[Lime]

I can't see the point in putting fibreboard insulation between the battens.
It is just going to introduce a problem where one did not exist before.

 

[Rick W]

The only thinking behind the fibreboard is that it will provide some sort of heat insulation.

Is this a(nother) fallacy in my thinking then? Only the current floor is mighty cold!

Rick

 

[Lime]

I can understand your wish to insulate the floor, particularly as the cement is a cold heat sink.
However the wood and the air gap will provide a form of insulation by themselves.
If you are going to insulate further I suggest you use something that can withstand rot and animal activity.
The insulation will work much more effectively if in contact with the boards underside too.

 

[Rick W]

Ok, I get the message!

Fibreboard is a no go.

But now I am confused!!! I need air circulation (according to some), but insulation would work best if it was in contact with the undserside of the boards too. Surely then there would be no air movement?

At the moment, I am going to press ahead with polythene under the battens and no insulation. I just hope the boards will prevent the loss of heat through the floor that we are currently experiencing.

Thanks again for all your time, its great to get some advice, but sometimes I feel myself going round in circles!

Rick