Insullating work shop concrete floor.

[trousers]

Errr, not quite.
I think in your first post you said that there was polythene (ie a damp proof course) under the concrete slab. Did you do the slab? If so was it the blue stuff (ie 1000gauge), and is it tucked into the walls at the same level as the dpc there? Also, is it pretty flat?
If yes to the above, then you should be pretty confident that the concrete slab is dry in respect of no dampness coming up from the ground beneath. It should only be damp if the concrete is newly poured, in which case let it dry (approx 1week per inch as a rule of thumb).
Assuming, therefore, the slab is dry, no further damp proof course should be needed. So if you are going the 50 x 50 joist route plus insulation in between, then you only need the vapour barrier above this, which can be blue polythene. Then lay the ply/osb.
As I said, in a workshop it is belt and braces. Some would also specify
treated (tanalised) 50 x 50's, if only to deter any woodmunching insects. The tanalised timber can often be quite damp due to the treatment, so let that dry as well before covering.
My personal preference would be for a floating floor, 18mm T+G ply over foil faced rigid foam boards, joints taped. This saves on the cost of the joists and having to anchor them, and the polythene. Would be plenty strong enough.
Hth

 

[garywayne]

No, I didn't do it myself.

Yes, there is blue polythene under the slab. The polythene has been cut off level, (it doesn't go up the side of the slab). The timber walls are built on DPC, and the cladding go's lower than the blue polythene.

Yes, the slab is flattish.

The slab is just over a year old now, and I have not seen any damp.

 

[MikeG.]

I would just add to what has already been said that two layers of plastic with timber in between is a dangerous situation. If there is a DPM under the concrete, and the slab is dry, then there is no need whatsoever for any more plastic. In fact, it would be counterproductive.

To test if the existing slab is dry, leave a piece of plastic on the floor for a week. If when you remove it the slab is darker in that area, then you have some moisture getting through the DPM under the slab. The best thing to do then is to lower the ground level all around the slab such that the top of the slab is at least 50mm above the surrounding ground.

So, assuming the slab is dry, please forget about any more plastic in the floor build-up!

Mike

 

[trousers]

Mike
Are you saying then that you don't need a vapour barrier in a floating floor construction?

 

[MikeG.]

There is never one answer!

Over a new concrete floor, then you would need one because of the construction moisture, which can take a long time (many months) to totally evaporate. This is the typical floating floor set-up, and therefore, a sheet of plastic under the insulation is usual. Floor manufacturers are unlikely to consider the whole build-up......they simply want to ensure that they keep their stuff dry and thus to not get sued will always ask for a sheet of plastic.

However, with an old dry slab, what is the point? Plastic would trap any moisture spilled from above, and protect from nothing coming from below. Furthermore, an unheated shed in the garden, used irregularly and poorly sealed, with no moisture source other than sweat, is likely to be a fairly dry environment.

Dry environment, dry concrete (with plastic underneath)........why add any more plastic?

Mike

 

[trousers]

Mmmmm
The gist of your reply seems to be no vapour barrier required.
The situation you describe in your first paragraph implies that the new concrete floor is not fully dry, and that the polythene under the insulation is there to prevent damp from the slab getting to the chipboard. OK, well if you're not going to wait for the slab to dry you've got no alternative, but this is not the same as a vapour barrier.
The chipboard manufactures all recommend a vb in their codes of practice. They describe it as a vb not a dpc (which is under the slab). It is on the warm (room) side of the insulation, even if the insulation is between timber battens. I've laid loads of floating floors, and always included a vb, even when the slab has been as dry as a bone.
I agree that in the op's situation it may be overkill (I did say belt and braces). In principle tho' any unventilated (filled) space where a temperature difference exists across it needs a vb on the warm side of the insulation.
You're right - the plastic would trap any moisture spilled from above. Bonus really cos' it would stop it getting to those battens down below :lol:

 

[MikeG.]

A vapour barrier is a barrier to vapour (!!)........it isn't breathable. Whilst it is called a vapour barrier, it is actually, or in effect, just another sheet of plastic. The reason the manufacturers all require one is as I described previously..........they are worried that their product will be installed over a damp sub-strate.

Over a dry substrate they serve no physical purpose that I can intellectualise........why would they stop water vapour going down through the floor build-up, such that there product is sitting in close proximity to the collected dampness? Answer.......they wouldn't. Therefore, its only purpose is to prevent water rising from below. It is simply a secondary DPM for the protection of the uppermost flooring layer.

Mike

Gary, ignore all this!! If your slab is dry, use no more plastic!

 

[MikeG.]

In principle tho' any unventilated (filled) space where a temperature difference exists across it needs a vb on the warm side of the insulation.

I'm not looking for trouble, but this also isn't so.

I have designed (and built) any number of buildings utilising breathable walls. These work perfectly well. The inside of the wall should be more resistant to the passage of vapour than the outside, but there is no need whatsoever for an absolute barrier to the passage of vapour.

There are any number of hygroscopic building materials which absorb the peaks of vapour, and release the moisture gently when the vapour pressure drops. Sheeps wool insulation, many hemp products, Warmcell (recycled newspaper insulation), Heraklith-type wood-fibre boards, lime plaster, straw bales etc all have this property to a greater or lesser degree, and if you know what you are doing when designing the wall or roof, then sheets of plastic can easily be left out. In fact, in these sorts of permeable constructions, they could be a real liability.

Mike

 

[trousers]

Alright Mike, I didn't expect you to go off on one :shock:

I always bow to superior knowledge and you've obviously got a fair bit.
I'm curious to know, though, why, if the slab is still wet, the polythene is placed over the insulation and not directly on top of the slab?

By the way Gary if I've misled you in any way I apologise, and obviously hope your floor works out fine, whatever methodyou decide to use.

 

[MikeG.]

Alright Mike, I didn't expect you to go off on one :shock:

Sorry!!

Bored, headache, tired.............. No excuse really.

Mike

BTW, the plastic is placed below the flooring mainly because it is safer there than on top of the slab, which can be a bit rough, and can chafe holes in the plastic.

 

[trousers]

No worries :lol:

Its good to talk.

 

[garywayne]

Good discussion guys. Now I have a headache. :lol:

I have had a roll of insulation wrapped in plastic on the floor for a couple of months, and when I moved it the other day there was NO damp patch underneath.

So the conclusion is:- ?

PS. I really appreciate your help and advice.

 

[MikeG.]

Perfect........build your floating floor direct on that with no more plastic. At all. Anywhere!

Mike

 

[garywayne]

Thanks Mike.
Oh! my headaches gone. :wink: