I will be helping someone insulate their loft. It's about a 1920's house. He wants to cut Celotex to fit between the joists and I have told him that he may need to install a vapour barrier. I have searched for ages, this site and others but cannot find a definitive answer. Can we just lay a plastic sheet over the joists and fit the Celotex between them? Can I fix the plastic sheet in place or will staples defeat the obect?
Apparently there is minimal eaves ventilation up there.
Rednadnerb coughed up some electrons that declared:
One option is plasterboard with an inbuilt foil vapour barrier - you need to tape the joints too.
I don't see any problem with a suitable grade of plastic sheet. The staple holes will be irrelevant as long as the sheet doesn't tear.
Theres a mention here:
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this is probably suitable:
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'm not an expert - I'll be doing this myself next year. Best wait around and see if anyone adds anything to this.
Another question: will there be some external ventilation to the cold side of the insulation (you need some no matter how much vapour barrier you put in because it's never perfect).
Normally what you would do - as I did - is to wedge the sheets between the joist and then use the foil tape that is sold to go with the board, to tape over the joists lower sides and the slabs to form a complete air and vapour barrier, and then plasterboard.
If you are retrofitting from the top, then this will not be possible. However at the least the Celotex is itself a vapour barrier, so the leakage of moisture into the loft space will be low. The bad news is it will be exactly where you don't want it - at the joists.
This does lead to possibilities of condensation on the joists inside the whole lot. There is no simple way out of that - a vapour barrier over the joists will not work as the moisture comes up from underneath, and unless you insulate *over* the joists to keep them warm, you cant keep out the cold either.
What that suggest as alternative, is to either chuck rockwool over everything after sealing any airgaps with eg expanding foam, or possibly put a DPM over all the joists and lay insulation o top, maybe flooring out with chip to spread the load, afterwards.
It's not needed. Just make sure any large holes in the ceiling are filled (such as where any pipes have been put through), and that the loft hatch is a reasonable seal (foam rubber strip around the edge of the frame if not). Also make sure any loft tanks are covered.
If the loft was suffering condensation, improve the ventilation. Otherwise it didn't need improved ventilation.
I cant entirely agree. The Celotex will create non-ventilated spaces next to cold joists. There will be little chance for any moisture coming through to escape.
It may not be a problem,. but ts more likely to see condensation after the insulation than before.
As it is a non accessible loft, ie no loft hatch and we will have to break in just to do the job, I quite like the idea of covering the lot with fibreglass. But I can give him the option of no dpm needed and introduce ventilation.
We get into this question a lot here. In this part of Canada where it does not get as cold and therefore dry, as elsewhere in the country .
Vapour barriers (and appropriate ventilation) are vital to avoid the insulation becoming wet and therefore useless!
Also with today's extra insulated R2000 homes with mandatory and continuous air (and heat) exchangers.
Vapour barriers always go on the warm side of the insulation; or (rule of thumb for this climate) not more than one third of the way through the total insulation between the warm living space and outside.
Omission of vapour barriers allows warm air to get into the insulation where it condenses, at the dew point temperature point within the insulation. Insulation then sogs and can start mildew/rot.
So in this 38 year old house: The initially installed vapour barrier is within the ceiling. Then in some areas an additional inch of foam was added when some ceiling were refinished (so that's on the warm side of the vapour barrier) and in addition to the original six inch f.glass wool ceiling batts above. Then shortly after under an insulation incentive plan an additional four to six inches of f.glass wool was blown in on top of the batts.
Was also very careful to increase the ventilation through the attic by adding additional soffit vents. Correct building practice here emphasizes strongly that vents must allow cross ventilation and that ceiling insulation must not block off soffit or any other venting.
Again on the subject of vapour barriers. BTW every human breathes out; what is it several litres of moisture each 24 hours?
Anecdote: One acquaintance, now in his 80s, built his conventional wooden frame and boards house back in the mid 1950s and refused to install any vapour barriers. That new fangled plastic etc.
But he could never keep paint on the outside of his wooden clapboard house even though he was using good 'marine' and therefore impermeable paint! It blistered and bubbled and every few years had to be scraped and repainted!
Eventually, some years ago, getting older, he installed vinyl plastic siding. Now has indication of moisture condensing inside the vinyl in certain places and may have rot!
We using similar construction use vapour barriers and have always used permeable redwood stain which can breathe out any moisture that does get in a wall etc.
No bubbling or blistering of stain, although the wind and driving snow has worn it off in a few places, Re-stained it about four times in 38 years; it, and the white trim need doing again in 2009. House is about
60 feet long, 37 foot deep and single storey walls about 8 feet high. So no scaffolding needed.
So why not staple a sheet of DPM over the top of the Joists and Rafters, then put Kingspan or Celotex or Rockwool on top of that? (with a few wooden battens or chipboard)
Exactly - in that case why bother with the much more expensive Celotex-type options? If depth is not an issuem, rockwool is far more cost-effective and easy to fit.
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