Are you stripping right back to the rafters and then insulating in the cavities from inside? or is some of this going on with the roof off? There are a number of options you can use tbh

I would like to leave the slates on, so we would be adding insulation from the inside between the rafters. Tony's comment is suggesting vapour barrier on the warm side in the room then insulation against the sarking boards.

I believe you should only fill the cavity between the rafters if you have breathable sarking/membrane which I don't think you have. I think you would need to build a ventilated roof and would expect the finished construction to be something like..

1) slates
2) sarking boards
3) 50mm ventilated cavity (vents at eaves and ridge)
4) 50mm Celotex between rafters
5) 100mm Celotex below rafters
6) Vapour barrier
7) Plasterboard.

The depth of 3+4 choosen to fill the 100mm rafters.
Second layer below rafters installed at 90 degrees.
The vapour barrier could be formed by taping the joints on the celotex or fixing up plastic sheet.
Total thickness of Celotex 150mm gives U-Value around 0.16.

The Celotex U-Value calculator recommends "25 x 50mm fixing batten under rafterline (recomended where insulation below rafters in excess of 50mm thick)" but I've omitted that from the above.

http://www.celotex.co.uk/

i believe sarking in scotland is just standard timber with no bitumen but welcome correction on this. (An idea of build date to check relevant b.con spec would be useful)
50mm vent void in rafters, spec'd by b.con is 25mm void and 25mm to allow for sagging of the felt. In the o.p's case sarking is rigid so you may be able to go 3)25mm 4)75mm or maybe safer 3)40mm and 4)60mm.
Then i'd counterbatten 50mm x 50mm and insulate between using pir cavity batts to gauge spacing and save cutting. If screwing through rigid foil-faced insulation to rafters use 50x50 of duck tape at screw location to prevent penetration of screw head and retain vcl quality as best as can be hoped.
Then over that fix a laminated osb/pir hybrid board onto the 50x50 c'batten. Mark (hi-vis) fixing lines on the osb to suit the plasterboard sheet size - place a clear polythene vcl and staple up (only on the fixing line) - apply a silicone mastic over staple holes (full length of fixing line) and fix directly over the mastic a 2x1 or 3x1(d.i.y) batten for fixing plasterboard and to provide a service cavity.

Counterbattening 50 x 50 reduces thermal bridging compared to battening directly over rafter but also gives body for mechanical fixing of what comes after. Spec further up the thread assumes fixing p'bd directly onto vcl and through 100mm insltn then into a seasoned rafter.
fostertom has a spec that includes multifoil in the mix so thats also worth considering.

Hi,
Couple of points from a non roofer.
In Scotland its generally its 25mm gap below the sarking planks (not OSB board unless its a new build) and these are covered with the breatable membrane, but on older properties you will find the black bitumous felt. When the breather is used a 2-3mm gap is left between each plank ie the width of a nail, but on older property the planks are butted up tight. Not entierly sure about the ridge. So a 25mm sq batten can be fixed up under the sarking to push the insulation slab up against.
Cheers
Mike up North

Posted By: ali.gilli believe sarking in scotland is just standard timber with no bitumen but welcome correction on this. (An idea of build date to check relevant b.con spec would be useful)

Ali, build date 1974 (evidence suggests that the builder was in a rush to get to the pub).

Sarking is timber. Between sarking and the rafters (attached to wooden pieces going across the roof above the rafters) there is some stuff which looks like a membrane. Apologies for non-technical description. It's multi-layered, black, looks tearable, with coarse cross-hatching and polythene bag type of edging sticking out of the middle layer. Stuck underneath facing into the attic voids is a 1cm layer of stuff which looks like glass wool insulation, although I cannot imagine its function.

Just to be clear, the sarking board appears to be placed on top of battens (?) across the roof. So there is a small gap between the 'membrane' and the sarking board.

50mm vent void in rafters, spec'd by b.con is 25mm void and 25mm to allow for sagging of the felt. In the o.p's case sarking is rigid so you may be able to go 3)25mm 4)75mm or maybe safer 3)40mm and 4)60mm.

Rafters are 150mm, so I could put in 100mm of insulation above the existing plasterboard and leave a 50mm ventilation gap.

Then i'd counterbatten 50mm x 50mm and insulate between using pir cavity batts to gauge spacing and save cutting. If screwing through rigid foil-faced insulation to rafters use 50x50 of duck tape at screw location to prevent penetration of screw head and retain vcl quality as best as can be hoped.
Then over that fix a laminated osb/pir hybrid board onto the 50x50 c'batten. Mark (hi-vis) fixing lines on the osb to suit the plasterboard sheet size - place a clear polythene vcl and staple up (only on the fixing line) - apply a silicone mastic over staple holes (full length of fixing line) and fix directly over the mastic a 2x1 or 3x1(d.i.y) batten for fixing plasterboard and to provide a service cavity.

Counterbattening 50 x 50 reduces thermal bridging compared to battening directly over rafter but also gives body for mechanical fixing of what comes after. Spec further up the thread assumes fixing p'bd directly onto vcl and through 100mm insltn then into a seasoned rafter.

Thanks for the detailed info! Looks highly relevant for the side attic voids. Decision will probably be based on whether I can afford to lose the extra 50mm. Probably yes, although the big side attic is used for storage and the void on the other side may become inaccessible if too much space lost.

For the coombe ceilings I was really hoping not to remove the existing ceiling plasterboard. The Warmcel idea was to find an easy way of installing and still get effective thermal insulation. The problem with PIR type of boards seems to be that rafters with irregular gaps will be very hard to completely fill. I can't easily get into the coombe gap to apply foam, without removing the plasterboard from the existing ceiling.

Biggest problem I see is putting in a vcl above the plasterboard on the coombe ceilings. Can't think how to do it except to take off the plasterboard.

fostertom has a spec that includes multifoil in the mix so thats also worth considering.

I find the physics of multifoils interesting, but doubt whether they are particularly suitable for my situation. Scotland does tend towards the damp and gloomy - strangely enough excess heat has never been a problem in the 13 years I've lived in the house ;). Anyway, I'm looking into summer solar heat capture and inter-seasonal storage so attic heat will hopefully be syphoned away.

It is still a good idea to have a vb even with a vented roof.

Posted By: Mike GeorgeThe reference details a rate of around 1.5ach@50Pa, which is not very good at all.

What do you mean it's not very good? None of the Stamford Brook properties achieved this level of air tightness. Maybe you're not seeing the @50Pa - at regular pressure this is 0.075ACH. 1.5ACH@50Pa is the R2000 maximum allowed air leakage for certification. Doesn't the current UK Part L suggest something like 5ACH@50?

Paul in Montreal.

Posted By: tonyIt is still a good idea to have a vb even with a vented roof.

I agree it's not a bad idea, but that doesn't make it a good idea.

Posted By: Mike George10m3/m2/yr @50Pa

Are you sure there's a per year term in there?

Posted By: Mike GeorgeIn Cananda you use a different measure again.

We do? I thought we used ACH@50Pa and this can be nominally converted to normal pressure by dividing by 20. The m3/m2 stuff I find very confusing since it is dependent on the size of the building whereas ACH is easier to visualize.

Paul in Montreal.

Posted By: Paul in Montreal

Posted By: Mike George10m3/m2/yr @50Pa

Are you sure there's a per year term in there? Paul in Montreal.

No its m3/m2/hr. A typo, probably too much caffiene, good job someone's awake

We don't use ach@50Pa at least not for compliance with Part L.