Insulation for pitched roof

[charliefarleyharleywarley]

I have a small part of my ceiling as a pitched roof, about one meter either side of the house is just plasterboard, ceiling joists then roof tiles.
A couple of years ago I insulated the flat part of the ceiling in the loft with about 12" of rock wool, but left the pitched bits as I understood it necessary to have airflow through the roof.
As you can imagine the insulation I put in is not really doing much because of these cold spots.

Do I really need this air flow over the insulation and under the tiles? Or can I just stuff more rock wool in between the joists, plasterboard and tiles? I suppose the best way to do it would be to use celotex of a thickness just under the joist height allowing some airflow.
What's the usual way of getting over this problem?

 

[noseall]

If you have a raked ceiling then a typical detail would be say 100mm between rafters with another 40mm across.
The insulation between the rafters requires a 25mm minimum air gap on the cold side. So if your rafters are 100mm you would need to fix a 25mm batten to the rafter in order to gain an air gap.

 

[mfarrow]

You probably have a similar setup as me then:

View media item 51866
In which case, I opted to install 50mm celotex between the rafters. I started by sweeping all the rubbish down into the soffits to get a flat surface. Once I had cut the basic squares, I had to cut them up to form two wedge shapes because the ends of the ceiling joists got in the way.

Having done that and got the panels wedged together in position, I drilled about 6-8 holes per panel from underneath, countersunk them, and very carefully screwed some plasterboard screws into the celotex to hold it in position.

I probably could have used 75mm celotex as suggested and left only a 25mm gap, but then I wouldn't have had enough room to put my arm/broom handle in and pat the panels down.

View media item 51865

 

[seasickstevie]

I like your wedge idea mfarrow. Been up against that problem a few times.

Why didn't I think of that?

Keep it in mind for next time