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Home made vapour barrier |
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| message from IMM on 14 May 2004 |
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?
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| Christian McArdle replied to IMM on 14 May 2004 |
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.
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| Andrew Barnes replied to Christian McArdle on 14 May 2004 |
I would also make sure the oil based paint is fully dry and cured, otherwise
you may get crazing when you paint over.
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| Mark replied to IMM on 14 May 2004 |
Its been implied on here you were full of hot air, I didn't believe it until
now ;-)
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| IMM replied to Mark on 14 May 2004 |
Is this wit?
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| Pete C replied to IMM on 14 May 2004 |
Hi,
Wouldn't an adequately ventilated loft be better? Where is the water
vapour going to go if it can't escape?
cheers,
Pete.
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| IMM replied to Pete C on 14 May 2004 |
Belt and braces, or reducing ventilation so an Arctic gales does not
circulate around the loft.
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| Pete C replied to IMM on 16 May 2004 |
Hi,
If you want to reduce heat loss through the ceiling, it would probably
be much better to add more loft insulation than reduce ventilation in
the loft. The U value of an uninsulated tiled roof must be a lot less
than 1" of rockwool.
How much roof insulation do you have at present, and do you have any
cavity wall insulation?
Also adding a vapour barrier might be OK for your lifestyle, but not
for other people living in the house after you.
cheers,
Pete.
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| Andy Hall replied to Pete C on 16 May 2004 |
I think you mean more, Pete. High U value equals more heat loss.
.andy
To email, substitute .nospam with .gl
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| Pete C replied to Andy Hall on 16 May 2004 |
Oops, of course!
cheers,
Pete.
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| RichardS replied to IMM on 14 May 2004 |
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.
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| IMM replied to RichardS on 22 May 2004 |
I contacted some painters. They said as you do. Two coats of oil based
undercoat and water based emulsion on top. No probs they said. An instant
vapour barrier, by using a roller.
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| N. Thornton replied to IMM on 14 May 2004 |
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
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| Peter Taylor replied to IMM on 15 May 2004 |
Yes, but remember it has only around 1/100th the vapour resistance of aluminium
foil or about 1/3rd that of polyethylene membrane.
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| IMM replied to Peter Taylor on 15 May 2004 |
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.
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| Peter Taylor replied to IMM on 15 May 2004 |
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.)
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| IMM replied to Peter Taylor on 15 May 2004 |
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.
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| Peter Taylor replied to IMM on 15 May 2004 |
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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| IMM replied to Peter Taylor on 15 May 2004 |
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.
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| Peter Taylor replied to IMM on 15 May 2004 |
IMM wrote
?? I presume you mean "benefits". This is nonsense. Keeping a loft warmer
would *increase* the heat loss, not decrease it!! Or do you mean reducing the
rate of air change in the loft would save energy? That would only be true if
you had inadequate insulation.
Condensation is caused mainly by the lifestyle of the inhabitants, not the
nature of the structure.
It would help slightly, but that's all you can say. But this benefit would be
more than wiped out if you had warmer air in the loft (with higher vapour
content) and less ventilation.
Look, what was the point of you posting this thread? You obviously think you
know all about it and don't want to listen to anyone's advice. So why ask? You
do it your way and good luck to you.
Peter
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| Ian Stirling replied to Peter Taylor on 15 May 2004 |
If, by slowing the rate of air changing down in the loft to that
actually required by any upgrading of vapour barriers,
you will cause the temperature to increase in the loft.
This will cause lower temperature differentials across the insulation to
the inside of the house, hence less heat loss.
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| Peter Taylor replied to Ian Stirling on 15 May 2004 |
Ian Stirling wrote
How? Where does this heat come from if not through the insulation? And let's
discount solar radiation as we are talking about night-time now! :o)
Anyway, what stops the heat escaping through the uninsulated roof structure?
As I said, it is only true if the insulation is inadequate.
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| Ian Stirling replied to Peter Taylor on 16 May 2004 |
Nothing.
If you ventilate the roof more than required, you cool the roof space
(assuming that it's warmer than outside).
Cooling the roof space means that there is increased temperature across
the loft insulation for any given temperature inside.
This will increase heat loss through the loft insulation.
Yes, it increases the temperature of the roof, a little, as more heat
is now going out that way, rather than through the roof vents, but
the temperature of the roof is irrelevant.
The important thing to consider about how much heat is going to go out
of your room is the temperature of the top of the loft insulation.
Raise that, and you decrease heating bills.
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| IMM replied to Ian Stirling on 16 May 2004 |
Exactly. Fundamentally simple.
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| Peter Taylor replied to Ian Stirling on 16 May 2004 |
Ian Stirling wrote
I fully understand what you are saying, but you're not listening. Yes, on a
nice warm day when the loft is warmed for free by the sun's radiation there
would be less heat loss from the interior. But then you do not need the heating
on full. I repeat - at night time or when the weather is cold (ie when you need
the heating on full), the only place the heat can come from to raise the
temperature in the loft is through the insulation. Where else can it come from?
And once through the insulation you've lost it anyway - doh!
In simple terms, the way to reduce heating bills is by increasing the
insulation - not using the precious heat to warm up a cold uninsulated roof.
That makes sense, surely?
In theory heat will always be lost through insulation as it cannot be 100%
efficient - even the hot water in a vacuum flask cools down - but in a
well-insulated roof, decreasing the ventilation will have only a very small
effect on heat loss and this is never taken into account in heat loss
calculations. However, it WILL reduce the dispersal of water vapour from the
loft and cause the RH to increase, possibly until it reaches dewpoint and
condenses. In other words it's another of IMM's many daft concepts - don't get
sucked in!
Discussion now closed.
Peter
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| Ian Stirling replied to Peter Taylor on 16 May 2004 |
Agreed.
There is no such thing as an uninsulated roof, or any other structure.
It's all insulating to various degrees.
A millimeter of copper adds a tiny amount of insulation, 20cm of glass
fiber much more, a roof somewhere in between.
I'm not saying it's a big factor, just that it's obviously non-zero.
The ventilation required depends on the amount of water vapour going into
the loft.
Reduce that, and you reduce the required ventilation to keep a given
RH.
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| Peter Taylor replied to Ian Stirling on 16 May 2004 |
Ian Stirling wrote
3rd and last time I'm going to say this! There is virtually nothing to be
gained by reducing the ventilation - you just agreed that yourself. There is an
increased risk of damage due to condensation. Why bother?
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| Ian Stirling replied to Peter Taylor on 16 May 2004 |
If the ventilation is reduced appropriately, then there is no increased
risk of condensation (working this out is moderately hard).
Virtually nothing is not quite nothing.
I suspect it's going to be on the order of 1/100th of a degree, though
have not done the sums.
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| IMM replied to Ian Stirling on 16 May 2004 |
Ian,
He appears not to understand something very simple.
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| Andy Hall replied to Ian Stirling on 16 May 2004 |
If you compare heat loss through a roof not insulated with glass fibre
etc. with one that is, the resultant heatloss is virtually entirely
due to the properties of the insulation at amounts of 100mm or more.
It isn't as easy as that.
Air can enter the roof space from either the house or the outside
through ventilation. As the space cools later in the day, the RH
increases anyway, so given a lack of ventilation, there would be more
tendency to condensation.
.andy
To email, substitute .nospam with .gl
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| N. Thornton replied to Peter Taylor on 16 May 2004 |
Not quite as simple as that. Flow of heat through an insulatinog
barrier is proportional to temp diff across that barrier. So the level
of heat flow between loft and outside does affect the rate of heat
flow through the loft floor insulation. But the insulation is the main
determinant, since it gives the bulk of the insulation effect.
Regards, NT
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| IMM replied to N. Thornton on 16 May 2004 |
news:<ViFpc.3850$wI4.434514@wards.force9.net>...
So if you prevent below zero air from quickly entering the reduce the
difference across the insulation, reducing heat from entering the loft.
If the ceiling has a vapour barrier then less need for excessive
ventilation, which cools the loft excessively.
Very simple.
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| Andy Hall replied to IMM on 16 May 2004 |
You're assuming that the amount of air change in the loft, outside the
insulation envelope makes a big difference to its temperature and the
heat loss. It is a second or possibly third order effect.
.andy
To email, substitute .nospam with .gl
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| stuart noble replied to Peter Taylor on 15 May 2004 |
Where did those figures come from out of interest? Not being sarcastic here
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| Peter Taylor replied to stuart noble on 15 May 2004 |
From a textbook:
Environmental Science in Building
by Randall McMullen
pub The Macmillan Press Ltd
ISBN 0-333-57676-4
Aluminium foil 4,000 GN s/kg
Bitumenised paper 11
Polythene sheet (.06mm) 125
2 coats gloss paint(avg)12 - 40
Vinyl wallpaper 6 - 11
Peter
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| Ian Stirling replied to Peter Taylor on 15 May 2004 |
<snip>
What's a "GN s" when it's at home?
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| Peter Taylor replied to Ian Stirling on 15 May 2004 |
Ian Stirling wrote
SI Units:
G = Giga (1,000,000,000)
N = Newton
s = second
kg = kilogram
in other words the force or pressure needed to push 1 kg of water vapour per
second though a material. This is the unit of measurement specified in BS 3177.
I was quoting from the textbook. It's more usual to quote this value in MNs/g,
but the numbers work out the same.
Peter
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| Ian Stirling replied to Peter Taylor on 15 May 2004 |
Hmm, the obvious answer, which diddn't appear to make sense.
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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| Peter Taylor replied to Ian Stirling on 15 May 2004 |
Ian Stirling wrote
Yes, but that's only one aspect. It's more complicated than that.
http://tinyurl.com/3afhg
Peter
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| Ian Stirling replied to Peter Taylor on 15 May 2004 |
/Thanks!
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