Waterproofing our concrete box!

[Green Aura]

Some of you will know we've recently obtained another building in the village for our daughter to live/work in.
http://www.durness.org/Balnakeil%20Craft%20Village.htm - this isn't ours but will give an idea of the building ttype.

We've now got the shop up and running (not in its final format but at least we're earning a little money!) but behind the shop it's now a gutted concrete box. Given the problems we had with water in the ceiling space (roof leaks have now been fixed so it's mainly due to condensation) I've been trying to work out some options to resolve this while we're back to bare block walls.

Can I seal the concrete in some way to stop the problem of condensation/leaks dripping from the roof and walls? I was wondering about something like tanking slurry - expensive given the area that needs covering - but would this cause problems in the concrete blocks themselves, holding water there instead of letting it through?

Any suggestions gratefully received. It would be good to sort this problem while we've got a blank canvas.

[MKG]

You can get stuff like this ...

http://www.nonslipfloors.uk.com/shop/se ... r?vmcchk=1

... or you can get the more common silicone-based surface sealer (usually sold as a concrete floor sealant) from people such as Ronseal.

It's not dirt cheap, but probably the cheapest effective solution.

Mike

[Green Aura]

That looks very interesting. I've forwarded the link to OH (in the next room! )

[grahamhobbs]

You say the damp is due to condensation, in which case you do NOT want to seal the concrete, this will only make it worse. Condensation can only be solved by insulation and vapour barrier and/or ventilation. If it is a concrete box, then Thermalboard Plus is probably the most convenient product to use to insulate, provide a vapour barrier and leave you with a plaster surface.

[gregorach]

Condensation and leakage are very different problems with very different solutions, so you need to know which it is. Sealing the concrete won't address condensation problems - for that you need either insulation, ventilation, or some combination of the two. And if you install insulation (e.g. drylining) on the inside, you'll need a vapour barrier on the inner surface to prevent condensation building up in the insulation itself.

Oh, I see Graham beat me too it...

[Millymollymandy]

Can't help there Maggie but thanks for the link which explains more about the craft village and the history of it.

[Green Aura]

More info required methinks so you can help me work out which it is - I think probably both!

The building was empty for at least five years and was a terrible mess - I posted a couple of pictures in another thread. So we stripped everything back to the breeze block walls, except in the shop and workshop. There were a few leaks in the roofing felt (still the original 50+ years old) which we've patched until we can afford to re-do it. That seems to have resolved that problem. The windows (which will again be replaced when finances allow) are as you see in the link above - small single glazed panes in metal frames. These are a bugger for condensation - even in this fairly clement weather the windows are wet inside!

The biggest problem, however, is in ceiling "space". The received wisdom in the village dictated that everyone built a wooden frame, battened to the ceiling, with thick polystyrene insulation jammed into the frame. A layer of builders plastic sheeting is then tacked under this and plasterboard is then fixed to the frame.

A couple of the boards in the shop area needed replacing so we took them down and could see water pooling in the plastic. We cut a couple of slits and a steady stream of water came out over the next couple of days. It appeared to have stopped and as we'd fixed the leaks and had no rain for many days we thought it was OK. After we'd been away for a few days we came back to find the problem just as bad so we unpinned the plastic, removed the polystyrene and got a deluge It kept running for days despite glorious weather - inside the building was pretty cold even then. So I think it must be condensation forming underneath the roof and collecting in the plastic. It wasn't smelly as you'd expect of stagnant water.

Is that clearer? You know me, never use one word when I've got half a dozen to spare

[MKG]

What's the roof made of, GA?

Mike

[Green Aura]

Concrete!!!!!!

[MKG]

Just checking

Mike

[gregorach]

Sounds like you need a builder. Some things are just impossible to diagnose over the internet - it shouldn't be condensation as you've got a vapour barrier (assuming the plastic is more-or-less intact), and leakage doesn't make much sense if it's not been raining.

[grahamhobbs]

If I understand correctly you have a solid concrete roof slab with a patched roofing felt directly on top. Underneath the ceiling has been battened out with polystyrene between and then polythene and plasterboard under the battens. I guess the battens and polystyrene are 50mm thick.

The construction is basically sound from the prevention of condensation point of view, although the thickness of insulation is absolutely minimal, these days it would be more like 200mm. It is therefore possible you could still get condensation in extreme conditions, really cold outside, hot and humid inside. The fact that you say the water appears to be clean would tend to indicate condensation (condensation will essentially be distilled water), however as you say it was cool inside the building and not raining outside, my feeling is that there were simply large amounts of water left in the concrete slab from when the roof was leaking.

This is a problem, yes because it is dripping inside but when the sun gets on the roof it may cause the felt to bubble up as vapour forms underneath it.

Ideally you would take the ceiling and insulation down, let the slab dry out and then replace the ceiling with more insulation. Preferably, Kingspan type insulation as polystyrene tends to soak up the moisture. Other than this, allow the water to drip out in arranged places and hope that over the summer it dries out.

[MKG]

Done a bit more homework. The problem appears to be the temperature of the roofing material - the concrete gets cold and so the water vapour in the air mass within the building condenses on it. That can happen only if the air mass can get to it. So you need to stop that happening. Merely putting normal insulation between the ceiling and the roof may not work unless you can guarantee that there is no way the air can filter through it. Moist air, believe it or not, exerts a higher pressure than dry air - so if there's a way through, it will certainly find it.

However, you can get spray-on polyurethane DIY kits. These would provide you with a complete, seamless barrier and, if you applied it to a depth of, say, 100mm, should be pretty effective both as a barrier and as insulation. All that, of course, presupposes that the roof covering - the felt - really is watertight.

Mike

[Green Aura]

The research I did on insulating the roof suggested the current method as above is fairly cr4p but better than none. It said that the insulation should go on top of the roof, under the felt. Someone else in the village has just done this so we're going to wait and see how his works before we invest any readies in that direction. Hopefully the patches are sufficient pro tem - there were only three problem areas with leaks and they seem to have stopped despite the most horrendous wind and rain over the last couple of weeks.

I agree we need to increase the thickness of the insulation and I'll try and find out more about this polyurethane stuff. Deep joy

Thanks chaps.

[grahamhobbs]

GreenAura you have got it fairly sussed. The insulation ideally should be on top of the concrete immediately under the felt, although you would need to work out how the edge of the slab could be insulated, otherwise you will get a critical cold bridge around the edge of the room.

In theory if the insulation is under the concrete, there should be a 50mm ventilated space between the top of the insulation and the concrete. The ventilated space being vented from opposite sides by continuous 25mm wide, with insect mesh covered, gaps to the outside. Obviously if you have walls all round you won't be able to do this, but you could have a number of airbricks and a bigger gap over the insulation.