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
Can we just lay a plastic sheet over the joists and fit the Celotex
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:
And this is probably suitable:
I'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
If the loft was suffering condensation, improve the ventilation.
Otherwise it didn't need improved ventilation.
[email address is not usable -- followup in the newsgroup]
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
Thank you, I can give him those options.
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
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.
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