Part of our kitchen has a sloping ceiling; plasterboard nailed to the
sloping roof rafters.
This section is not insulated.
What is the best, and what is the easiest way to do it? Should I
take off the plaster, lower the ceiling by 3 inches, and pack between
the joists and the extra space with fibre - a relatively big job....
or could I successfully take an easier route; drill holes and inject
something - expanding foam maybe?
Can you get access to the void from either end? If you can, you may be
able to slide in sheet PU insulation (e.g. Kingspan). This needs to be
of a depth to not adversely affect air circulation between the joists.
If not, then battening and plasterboard is probably the answer,
although there's better insulators to use than glasswool where the
depth is limited.
Injection into the void - bad. There needs to be air circulation
between ceiling and roofing to keep the joists dry and remove any
rainwater blown under the tiles/slates as well as moisture penetrating
from inside the kitchen.
I would pull the plasterboard off the slopey ceiling to expose the
rafters - how deep are they, 3"? If so, the next step is to fit slabs
of 2" thick Kingspan or Celotex between them, flush with the inside face
of the rafters. This stuff is *way* more effective than any mineral
wool, inch for inch, so in a confined space like this, it's a
no-brainer. Important to leave the air gap above, as others have said;
so injecting anything as an alternative is really not on. (I should
stick with Rockwool or whetever for any flat section, as it's cheaper
and depth isn't an issue.)
You then need a vapour barrier (specified polythene sheeting) to prevent
condensation; the Celotex will be foil-covered, but the joist faces
will be exposed and there will be cracks etc, so I think you're best off
then covering the whole lot.
If you've got the ceiling height, you could then overboard with large
sheets of Celotex, followed by plasterboard, or you could just fit
polystyrene-backed plasterboard; either of which will give more insulation.
I did a similar project myself not long ago in an attic bedroom, and
asked various questions here - might be useful if you can find the thread.
On Sun, 18 Nov 2007 05:46:29 -0800 (PST), a particular chimpanzee,
firstname.lastname@example.org randomly hit the keyboard and produced:
The best; remove the plasterboard, install 25mm PIR foam boards
(Kingspan, Celotex, etc) between the rafters, leaving a 50mm
ventilated space over, then 120mm PIR under, then a polythene vapour
barrier, then plasterboard.
The easiest; an insulated plasterboard under.
No, you buy the celotex foil sticky tape and tape over the joists and
board and make it comptletely AIRTIGHT, then board up. No need for
foiled board, but belt and braces...
Rockwool is useless in any place that is ventilated. In a wind it loses
most of its insulating properties.
> If you've got the ceiling height, you could then overboard with large
> sheets of Celotex, followed by plasterboard, or you could just fit
> polystyrene-backed plasterboard; either of which will give more insulation. >
> I did a similar project myself not long ago in an attic bedroom, and
> asked various questions here - might be useful if you can find the thread. > > David
Yeah, I was going to do that with mine but ended up covering it with the
vapour membrane - achieved the same airtight result. Can't remember
whether it was due to cost/availability of the tape at the time, or
because the membrane would be quicker.
Have I got rhis about right?
|\\\| |\\\| |\\
\| 25mmventilation space
|\\\| ::::::::::::::::::: |\\\|::::::::::::::::::: |\\
\|::::::::::::::::::: 50mm Kingspan
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 25 mm
PIR foam & dpm
So... if the joists are 3",
I'd leave a 1" ventilation gap at the top,
pack a 2" layer of kingspan between the lower 2 " of the joists.
25 mm PIR accross the underside of the joists,
Did something similar recently in our kitchen.
Put 25 mm celotex between the 3" joists - held in place by friction
(leaves a 50 mm airspace between insulation and roof tiles).
Put 70 mm celotex over the joists - held in place by a few long screws
with large washers.
Cover joints in celotex with aluminium tape.
Covered over with foil backed tapered edge 12.5 mm plasterboard - held
in place by 100 mm plasterboard screws from toolstation.co.uk
(Screwfix don't appear to sell them).
If you want more insulation (I think about 160 mm is recommended?)
then attach batten e.g. 50mm to the rafters and fill the space
between rafters with 100 mm celotex then overboard with 60+ mm of
celotex. I would have done this but the sloping parts of the ceiling
were quite small and I stuffed the large void/attice above the sloping
parts with glass fibre insulation.
I've got the idea and I'll do all of that. I can lower most of the
ceiling very adequately by reducing the gradient, and lower the
lowest point an inch or so, so I think that'll be a good job.