Home made vapour barrier

[Peter Taylor]

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

[Peter Taylor]

Ian Stirling wrote

Yes, but that's only one aspect. It's more complicated than that.

[Ian Stirling]

[Ian Stirling]

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.

[Peter Taylor]

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.

[Ian Stirling]

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.

[Peter Taylor]

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

[IMM]

Exactly. Fundamentally simple.

[Ian Stirling]

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.

[Andy Hall]

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

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[Pete C]

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.

[Andy Hall]

I think you mean more, Pete. High U value equals more heat loss.

.andy

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[Peter Taylor]

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?

[Ian Stirling]

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.

[N. Thornton]

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

[IMM]

He appears not to understand something very simple.

[Pete C]

cheers, Pete.

[IMM]

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.

[Andy Hall]

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

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[IMM]

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.