aluminium foil

[Karen]

Hi I have recently moved into a new house (to me anyway) and am going to be doing some improvements. One of which is to insulate the attic as the present insulation material is woefully inadequate. I was told that before I lay the new insulation I should put down a layer of aluminium foil before laying the insulation between the joists, as this will reflect heat back down into the rooms below. Is this correct or his the person giving me this advice talking a load of bollocks. Also the house is a upstairs downstairs type and I have discovered that there is no insulation of any type between the floors. Just a gap. Should I put any insulation in there? It would have to be a type that could be put in without lifting the flooring but there is some access from a cupboard under the stairs. Would it be worth while or would the hassle involved outweigh the advantages? Or is this not normally insulated?

I hope that I can get some correct advice from this group as the person who is giving me advice at the moment seems to make it up as he goes along. I think he may be for the chop.

Thanks Karen

[Grunff]

While it's technically correct, the amount of energy it will reflect will be negligible, so for all intents and purposes they are talking bollocks.

Why would you insulate the floor? Any heat lost from downstairs will simply end up upstairs - so overall you lose nothing.

Good plan. His name isn't IMM/Drivel, is it?

[Brian Reay]

But, the foil will act as a vapour barrier, all be it an imperfect one, which is a worthwhile reason to do it.

[T i m]

Where is the 'vapour' coming from Brian? I would have *thought* you wouldn't want to trap vapour in the room but let it permeate out through the walls / ceilings etc and if there was a vapour trap above the ceiling it would keep the plasterboard damp?

All the best ..

T i m

[Ian Stirling]

Nope. Plasterboard isn't an insulator. It'll be at essentially the same temperature as the room. However, it's essentially porous to water vapour, and what that means is that you get warm damp air on the far side of the insulation, shortly after you start breathing or cooking in a room. It then permeates through insulation, cooling as it goes. If you are unlucky, it cools to below the dew-point, and you get condensation in the insulation, which can cause whatever's touching it to rot rapidly.

The right way to insulate nearly all structures is to have the inside a vapour-tight box, with air entry and exit done through ventilators, not a porous structure. Otherwise, if you try to insulate highly, you're likely to get condensation where you don't want it.

[Gripper]

"Karen" wrote

Did this person also advise you to wrap the tinfoil around your head to stop aliens reading your mind? If so definitely give him the chop. Also run away quite quickly.........

hth Neil

[T i m]

Ok ..

That's what I thought ...

Ah .. not a good thing then.

Understood, so anything non permeable 'above' the ceiling plasterboard and between the joists could cause moisture to collect and via capilary action wet the roof timbers? Even of it didn't get *through* the plasterboard to 'cool to the dew point' where would it go?

And that's what I thought ... something to be said for draughty sash windows then ;-)

All the best

T i m

[Brian Reay]

If it doesn't "cool to the dew point", then it stays as vapour.

Either the "damp air" then gets circulated out of the house or, if the more vapour is added, you reach the point where the air at the ambient temperature can't 'hold' the all vapour and it condenses out.

In fact, if this didn't happen in the wider world, we'd not need umbrellas ;-)

Foil under new insulation isn't as good as a proper vapour barrier (new houses should have foil backed plasterboard for ceilings) but it is better than nothing.

[Aidan]

They're quite right. The warm air absorbs water vapour from breathing, plants, laudry, cooking, etc.. People generally prefer Rh around 50%, so it's an advantage. It's not a problem whilst the warm air stays in the heated space. If it can get into the loft it will condense in the insulation & can waterlog it and reduce it's efficiency. It rarely causes problems in the UK, for some reason. In colder climates you can find the insulation saturated and frozen.

You can often get rot/damp problems above the airing cupboard, where great holes are usually hacked for small pipes and where damp laundry is dried.

[Aidan]

PS Where I have seen problems with no/defective vapour barriers is on chilled water pipes. The chilled water is generally run at 6 degC and, if mineral/glass fibre insulation is used, it WILL get waterlogged unless there is a continuous vapour barrier. The water vapour condenses from the atmosphere. The BS on pipe insulation goes into great detail about having a continuous vapour barrier and the pipe supports/clips outside the insulation & vapour barrier.

This is relevant, in that 6 degC is quite warm compared with the winter temperatures you might expect in most loft spaces and the same mechanism is causing the condensation.

[Andrew Gabriel]

It's not. The permiability of the plasterboard is very much lower than the rate moisture escapes from even a poorly ventilated loft. If the amount of moisture in the loft is an issue, then you need to ventilate the loft, and it's much more likely to be comming from a roof leak or header tanks, or holes punched in the ceiling for recessed lamps, loft hatches, etc. Forget about the tin foil.

[PeTe33]

What if he advised earth bonding the foil?

[fred]

not the right answer, foil will only reflect heat where the transfer mechanism is radiation, when in direct contact with any material, the heat transfer mech will be conduction so it wont help at all. It'll only help it's adjacent to an air gap and even then the efficiency will be reduced as the foil tarnishes.

I agree with the end result tho', bollocks was spake . . .

[Grunff]

I was taking that into account, which is why I said negligibl (honest!). The foil won't be in direct contact with the plasterboard right across - there will be a proportion of the area, maybe 10% or more, where the foil is sitting there with a gap between it and the pb. In tose areas, the heat transfer between the pb and the foil will be convective and radiative.

:-P

[Gripper]

""Pet @

;¬)"" wrote

then he is a "Competent Person" with regards to Part P of the regs: not only must the OP follow his advice to the letter, she must pay him great wodges of cash, and not run away at all.

:->

[fred]

Yeah right, was just bringing you back down to earth after my unbridled praise in the solder thread :-).

[Brian Reay]

So why do building regs now specify foil backed plasterboard?

[T i m]

foil will only reflect heat where the transfer

Like the guy in the camping shop that said 'put some of that foil they use for emergency blankets on top of your airbed and under your sleeping bag to keep you warm ..."

All the best ..

T i m

[Ed Sirett]

There are plenty of regs which have only a marginal impact of the quality of the end result, but make significant cost implications for compliance. This problem is not confined to the BRs.