To all you paint experts,
In order to make a vapour barrier on the bedroom ceilings to prevent water vapour penetrating through the plasterboard into the loft, oil based paint can act as a vapour barrier.
Oil based paint looks naff on a ceiling. What paint could be used with oil an base as an undercoat , and what matt paint could be painted over that to give a nice normal ceiling effect?
Personally, I'd just lay it on thick, then lightly sand for key and then cover with an acrylic primer, such as Glidden Primecoat Acrylic Primer/Undercoat. Any paint at all will stick to that, but I'd probably use a water based eggshell, as even a small amount of condensation could affect the film bonding. That lot will probably have the mechnical resistance of a toad, but on a ceiling that wouldn't matter, as it isn't touched, like a door or frame. I've painted acrylic straight onto unroughened solvent high gloss and been amazed to see it still perfectly attached years later.
Anyway, I can't guarantee it would work, but strongly suspect so. Also, I'd use as matt a solvent paint as I could find, assuming they have the same vapour resistance, and wouldn't expect the final result to be as effective as a sheet of plastic or foil.
Christian.
I would also make sure the oil based paint is fully dry and cured, otherwise you may get crazing when you paint over.
I'd just use an oil based undercoat and emulsion on top of that. That's what our decorator does to combat staining bleed-through on ceilings.
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
Hi,
Wouldn't an adequately ventilated loft be better? Where is the water vapour going to go if it can't escape?
cheers, Pete.
If you want an oil based matt paint, use traditional undercoat intended for gloss paint. You can use a layer of this for vapour barrier or to block most stains: the problem is that emulsion doesnt stick to it easily, and it'll take 3 or 4 coats of emulsion to get a decent result. Never again :)
Regards, NT
Belt and braces, or reducing ventilation so an Arctic gales does not circulate around the loft.
Its been implied on here you were full of hot air, I didn't believe it until now ;-)
Is this wit?
Yes, but remember it has only around 1/100th the vapour resistance of aluminium foil or about 1/3rd that of polyethylene membrane.
It all adds up. If one is not already there, as in most houses, then there is protection. If 1/3 of poly then 2 oil based coats will up that substantially. All by just painting the ceiling.
Certainly, but I was concerned about your idea of reducing the loft ventilation. In the coldest weather, particularly, there will still be some vapour getting through the paint film and condensing on the cold roof timbers, although I agree less than at present. I don't see any value in reducing the loft ventilation, only an increased risk of problems. (Assuming you have adequate insulation at ceiling joist level, that is.)
Where did those figures come from out of interest? Not being sarcastic here :-)
If a poly vapour barrier was installed and all the holes in the ceiling loft caulked up, 250-300mm insulation, then reduced ventilation is certainly possible.
The reason there is a vented loft is to remove water vapour. No other reason. Preventing, or "vastly" reducing water vapour getting into the loft means no ventilation is required at all (assuming no open vented water tanks up there).
As a safety measure you could fit an extract fan with a self closing grill and a humidity sensor in the loft. Then no ventilation is required at all. But some goon of a BCO, or surveyor would look at a reg, say it's not right, because he can't think.
Back to the paint. Two coats of oil based undercoat with all holes in the ceiling/loft caulked, will give over 1/2 the protection of a poly barrier with no caulking. In this instance "full" ventilation is not required at all. It cam be most certainly reduced.
"stuart noble"
Actually, it's about 1/3rd, as I said earlier. Bear in mind that water vapour is a gas, not a liquid. The molecules of a gas are *much* smaller than those of a liquid and just because a material like polythene is highly liquid-proof, it doesn't follow that it is gas-proof too.
But what's the point of wanting to reduce the ventilation?
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liquid-proof, it
That is not the prime point, but a welcome point. Keeping aloft warmer befits the house as a whole. Less heat loss. The prime point is to put up a vapour barrier, merely by rolling it on. I know of many houses that have condensation problems, and yet they have the required ventilation. Oil paint on the bedroom ceilings, sealed hatch door and caulking up holes in the ceiling would most likely eliminate the problem.
I suppose I was expecting area to come into it somewhere. Though thinking about it, if flow is proportional to pressure times area, or F/A * A, then the area drops out to leave a force.
I assume you use the difference in vapour pressure on each side?
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