membrane below solid wood floor

[Halo Jones]

Hi All,

I'm going to try and keep myself busy by laying some solid maple flooring. This part of the house is an old cottage with suspended floor and underfloor ventilation. While the subfloor is solid (tongue and groove floorboards) it is very draughty. Is it ok to put down a some kind of dpm over the floorboards to try and stop the draughts?

Cheers,

H.

 

[Phil Pascoe]

In my old house I covered all the T&G floors first with hardboard, whether carpeting or putting wooden floors on top. The downstairs ones had a huge draughty space under them so I put down hardboard then 6mm insulation boards as well.

 

[That would work]

So are you saying put membrane over the original floorboards and put the new over that??
Also if it's t&g how comes it's drafty?
What is it you want the membrane to function as?

 

[Halo Jones]

Yep. I want to put something over the original tongue and groove. It is old and has the scars of many a renovation project - holes for old central heating pipes, gaps where boards have split when lifted etc. I have found that expanding foam only works so well at blocking things up so wondered if somethings else like dpm sheet could be put down to help.

 

[That would work]

OK so if it's to even things up then go with Phills idea of covering the old boards with hardboard well nailed down maybe with annular ring nails. Put the new floor over that. (Check the integrity of those old boards carefully though to check for worm or rot)
No need for a membrane at all in this instance. There's the normal underfloor ventilation under the original boards which needs to be maintained.
OR...
For a better job all round, (easier to fix the new floor down etc)
Consider pulling up the old boards.
You say you are putting a hardwood floor down which suggests that it would be good enough on its own. Putting down hardboard and the new floor will raise the level a fair bit which could be an issue as well.

 

[Phil Pascoe]

A membrane doesn't hurt so long as the underside is ventilated, it only creates problems if the space under it is enclosed.

 

[Geoff_S]

The first T&G floor that I put down was oak, and I did not use a membrane. It seems that this was a mistake as all the boards cupped slightly. Not too bad, I call it "character" and to be fair it has settled down over the years. Still, I can see it. All other floors I've done I have put a membrane over the joists before pinning the T&G, all OK.

Now, there may be a difference there, as I have done my ground and first floors. Ground floor has a cold void under it, and as I understand it is the difference in the moisture content beneath the floor and above it that causes cupping, hence the membrane. This is not so pronounced on the first floor, but I used membrane none the less.

My experience, not a statement of fact :wink:

 

[MikeG.]

A membrane doesn't hurt so long as the underside is ventilated, it only creates problems if the space under it is enclosed.

That isn't strictly so.

A vapour impermeable membrane acts as a barrier to the passage of vapour, and indeed to liquid water. If this is a bathroom or kitchen, then a spill or leak could just sit on the plastic for years, rotting the timber above it and causing mould. Further, if there is heating in the room then the sheet of plastic will act as as a place for vapour to condense due to the temperature differential between the room and the void below it. This would pool on the top of the membrane.

I can't immediately think of any circumstance in this country where I would advocate putting a vapour impermeable membrane under or within a suspended timber floor. Even a vapour-open membrane would cause the pooling issues I talked of. Everyone thinks damp comes in from outside, whereas the inside of a heated building is damper than the outside for much of the year, and fitting membranes of any description needs careful thought.

-

Just on the design side........an old cottage and a maple floor. Really? That's not at all traditional. If this building is listed then you'll need approval, and you almost certainly won't get it.

 

[Halo Jones]

Water pooling from any spills was my concern. If membrane is a no-no how would you advocate minimising draughts coming up from the void? Will filling any gaps with expanding foam and laying the new floor nice and tight be enough?

btw. although this part of the house is about 150 years old it has been so subsumed by additions and renovations over the years there is no restrictions! Originally the floor would have been a pounded earthen floor with linseed oil if it was fancy!

 

[MikeG.]

Ideally I would tackle the issue from below by insulating between the joists. If access is an issue, then I would lay a sheet material over the top, but I would use a "breathable" one rather than hardboard. Panelvent is one such, but these type of products aren't readily available in small quantities. Would lifting the floor and relaying be possible? If you repaired and tightened up as you went that really should solve all the issues.

 

[Halo Jones]

Thanks all for your advice. I will think it over - plenty of time at the moment!

 

[MikeG.]

......although this part of the house is about 150 years old it has been so subsumed by additions and renovations over the years there is no restrictions! .........

Is your building listed?

 

[Halo Jones]

No. It isn't listed.