The Natural Philosopher wrote:
Lobster wrote:
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.

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...

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.

David