Vapour Barrier under attic insulation

tcs

Am putting down insulation in old house.

I gather that a vapour barrier (polythene or foil) should be placed under (on warm side of) insulation between joists on attic floor.

What I'm wondering is whether to cut it to fit the gap between the joist or to put a continuous sheet wrapping it over each joist and bringing it back down to the ceiling between the joists.

If former (i.e. cut to size) by my reckoning this'd be a break in the vapour barrier.

If latter, will the joists be ok (i.e. risk of rotting) if condensation builds up.

Eurorunner

Is foil-backed insulation really required in this instance? I've been doing some work for a builder recently and last job I did was insulate the attic - but just used rockwool - rolling it out between the joists, cutting to size where necessary and making sure there were no gaps.

As far as I know, felt is breathable - allowing vapour to escape but not letting the rain/wet in.

CJhaughey

Vapour barrier is usually a thick 1200gm/m2 Polyethylene sheet. It does need to be a good seal but unless you have good ventilation I would be wary about putting your joists under it.
It should ideally go up under the plasterboard and below the joists.

tcs

Are you saying in put vapour barrier in place before putting up plasterboard for ceiling? Since it's an old house and plasterboard's very much in place already what are my options? Joists are 13 inches apart. Am I down to cutting 13-15 inch wide strips and placing between joists. If so how do I get a good seal?

CJhaughey

Since it is an old house it may be better not to use vapour barrier at all.
Since the house wasn't designed for it and is not ventilated than retrofitting it may cause all kinds of difficulties as you have already pointed out ,rotting joists etc

rooferPete

Hi tcs,

The only option open to you is as you described cutting the membrane into strips, however at best all you will achieve is a poor vapour check not a barrier.

Rockwool roll bats is a good product for insulating old attics but there may be a more cost efficient option, try calling ATIL if you are in Dublin, they will provide expert up to date advice.

.

kadman

Food for thought,

http://www.weatherization.org/atticinsulation.htm

kadman

Victor

Of course, you will also need to make sure the attic is properly ventilated and that any water tank or pipes are properly insulated.