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Posted to uk.d-i-y
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insulating sloping ceiling-roof space
wrote:
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?
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.
David
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