On 22/12/2020 21:49, Theo wrote:
John Rumm wrote:
On 20/12/2020 23:30, Theo wrote:
I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.
That's not a huge change in temperature, which suggests that those bays
may not be insulated as well as the others, but it unlikely they were
"forgotten" as such.
It's about comparable with the patches of ceiling I see elsewhere with no
loft insulation behind it - ie the loft is warm because of heat flux from
other rooms, which means the temperature differentials aren't so obvious.
For exmaple, on the loft side of one of the (uninsulated) hatches the
thermometer reads 15C, when the outside temperature is a few degrees.
Being a cold loft, if it was a perfectly insulated ceiling I'd expect the
temperature to match outside.
Only way to be sure will be to look...
What you can do will depend a bit on the type of roof covering. In
places where there is a breathable membrane (aka "sarking") under the
tiles, then you can full fill the space between tiles and plasterboard.
However if you have a traditional roof sarking, then you need to leave a
ventilated air gap above the insulation so that no moisture can get
trapped between the insulation and the tiles.
There is indeed sarking. It looks shiny, so I'm not sure how breathable it
is (I don't know if the roof has been retiled since building in the 60s but
it looks in good condition).
2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.
That's doable. Unlikely to be asbestos in most parts of a roof - the
exception being the soffit boards on some '50s onwards places that have
asbestolux sofits.
(asbestos cement boards - low risk if left alone)
I thought some ceilings were asbestolux in 60s/70s?
Possibly, but not much advantage under rafters (between floors might
make more sense to suppress the travel of fire).
You normally need a vapour control layer that can stop warm moist air
reaching cold "interstitial" spaces. The foil cover on PIR foam boards
like celotex does this. Then foil tape over any joints etc.
Ah. So assuming it's sealed that would suffice? And if the board isn't
butted right up against the tiles there's some airflow for the rafters?
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.
4. Treat it like a wall cavity and inject cavity fill (spray foam? beads?).
Is there something that can be used that won't ooze out under the tiles?
How does this work in terms of preventing moisture, especially collecting
near the rafters or running down the inside? Is there a risk of the fill
absorbing water? I'm guessing this isn't DIY-able?
Probably not a good choice (see comment above about maintaining an air
gap between insulation and tiles). You also don't want to bond the tiles
to the roof - which injected foam would tend to do.
Yes, I saw the wiki article warning about spray foam insulation on roofs.
(and lucky I don't have a canoe)
You probably have two realistic options. Underboard the lot with an
insulated plasterboard. It need not be particularly thick - say 30mm of
insulation - but that would make a reasonable difference and also reduce
any thermal bridging at each stud.
or, strip the existing plasterboard and then do as above - taking the
opportunity to inject expanding foam into any board gaps, and tape up
joins if required.
You could always chop out a bit of PB and investigate the construction
before committing to a course of action.
I should probably invest in one of those USB endoscope cameras and take a
look...
Even chopping out a 4" square of PB for a better look leaves an easy to
patch hole:
http://wiki.diyfaq.org.uk/index.php/...g_plasterboard
All these things are DIYable if you are prepared to have a go, but it
does depend on how much time/effort/expense you want to go to. There is
a bit of detail on roof insulation on my loft conversion page:
Thanks... some useful food for thought. I should do some more
investigations - after I've fixed this week's task of a flat roof leak 
Yup once you know what you are dealing with you can decide what the best
option will be.
--
Cheers,
John.
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