Suspended ground floor condensation problem

[The Natural Philosopher]

Mm. Its the recommended way of keeping radon gas below danger levels too

- suspended floor and contiguous DPM under it.

Not many peple are buildng suspended timber floors at gruond floor level these days, but if I had to do it, I'd celotext the joists, and tape right over before laying to boards. Might even add a DPM in addition, tho the celotex tape has the same effect.

I should say that when I demolihed my old house, there was a small pond under the suspended wooden floor, which led to a lot of uptake of water ino teh (non DPC'ed) brick chimneys. The floor was laid on fired clay plates on IIRC a couple of bricks, and tho there was no DPC, the rot was mainly in the timbers around the fireplace, not in general in the floor.

Central heating had kept the boards dry enough.

[G&M]

Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ?

Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there !

Any thoughts ?

[The Natural Philosopher]

cant see it makes much difference really, I thik I'd put it safely underneath tho, from pure 'in case someone digs a screwdriver into it and pours a botle of wine over the top' ness :-)

[xavier]

Absolutely agree. IME Rockwool is an absolute sod for holding moisture and the wetter it gets, the better it gets at getting wetter

- if you see what I mean!

The object of all this is to stop moisture laden air getting into the sub floor space. If you put your dpm on top of the floor then this will work. The problem is that at some stage in the proceedings someone is going to come along and remove it (perhaps after you're long dead and gone) because they won't realise the purpose of it. The Rockwool still being there will then start the IC ball rolling!

whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there !

Visible condensation won't normally occur in the subfloor space if it's ventilated adequately and the inclusion of a dpm separating it from the living spaces will reduce the risk even further (as indeed would another dpm over the actual ground itself). The Interstitial Condensation ocurring because of previously mentioned circumstances is the greater risk.

xav

[G&M]

My suggestion was that since the Rockwool is pressing the DPM against the underside of the joists and floorboards, these won't be getting any ventilation and in fact are at quite some risk of damage. But this depends on whether moist air would condense on top of the DPM above the Rockwool or not ? Any thoughts ?

[The Natural Philosopher]

No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate.

[The Natural Philosopher]

it won't condense. You need a markedly colder surface and that is the far sie of teh insulation only.

Te rule is always 'warm and moist side, internal surace finish, then DPM, then insulation (these two may be the same as with e.g. celotex and its sealing tape, then cold side with ventilation'

The point is that the thermal and moisture barriers need to be in the same plane or the DPM on the warm side of the insulation.

[G&M]

consideration

Good point.

And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex.

[Cicero]

============== Can you describe your mixture of Rockwool and Celotex? Is it substantially different from my version of insulation (described in an earlier part of this thread)? I installed 2" polystyrene boards between the joists and the difference is already noticeable. An added bonus is that the rooms are much quieter especially so the room fronting the road. I believe that Celotex would be even better than polystyrene but I don't understand why you would use the Rockwool as well.

Cic.

[G&M]

floorboards

Sorry to be confusing - they are separate areas. Celotex is used under the main living room and is stuffed tightly between the joists. Rockwool is used in a similar manner under the conservatory as that has underfloor heating and the Rockwool forms a good bed for the pipes to sit in whereas the usual manner of keeping foam in place by friction would probably not work and fall down every time the joists heated and cooled.

Foam is definitely the right solution in your case.

[Cicero]

substantially

============== Thanks for the explanation - knowledge for future reference.

Cic.

[Alex]

Thank you all for your replies. There are still some points which need to clearify but I'll take care to examine carefully your advice first. In fact I think I'll install a vapour check and monitor the results...thanks god for that trap door! Meanwhile, it looks as the problem is not so nasty as I initially thought. The condensation problems seems to arise only on very hot humid days (e.g. last July). I had to crawl with mop and bucket to clear those poodles of stagnant water. Then I left the trap door open and the rockwool got completely dry in a couple of weeks. I want to see what is going to happen this winter when the central heating is on.

Thank you very much

P.S Those morons at Manchester Town Hall building controls...I contacted them before installing the insulation and they guarantee me there was no need for vapour check?

[G&M]

How would they know. They're just people waiting for jobs in the more enjoyable traffic warden department next door.