I'm finally getting round to sorting my roof out (or at least getting round to thinking about it!).

Basically, the loft has been converted into living space by a previous owner, but the insulation is rubbish - bits of glass wool stuff in underneath the plasterboard, lots of bits missing and huge gaps creating thermal bypasses everywhere.

I think I'm going to have to strip out all the plasterboarding and insulation and start again.
I would intend to carefully insulate with PUR between the rafters then more PUR backed plasterboard on top.
I will leave the standard 50mm air gap behind the insulation, but I'm unsure as to how I should create the air gap around the edges (from memory I need the equivalent of a 10mm slot all around).
The roof is a traditional Scottish type - slate on top of wood sarking boards - and the sarking overlaps the main stone walls very neatly (see attached - hope it works).
The only way I can see to achieve this is to drill lots and lots of angled holes down through the stone. This will be very difficult and time consuming though. Any other ideas??

Do you need the standard 50mm air gap? - If it was a SIPS panel would it still have an airgap? If SIPS doesn't need it why would this? I'm sorry I dont' have answers but will be very interested to what other replies you get as it may have a bearing on something I want to do.

Mike, is that just venting over the sarking board or beneath it, please?
RobinB

I bet there is a bit of a gap there already? -- I recon that you could simply carefully tap in 10mm wood wedges between the stone and sarking to form a ventilation gap.

may be not if it is slated, and with tiles they often use overlapping feather-edge upside down, the general idea with northern UK sarking boards is for them to be a bit windproof even sometimes t&g is used.

Thanks for all the comments...finally have a chance to respond...

"Do you need the standard 50mm air gap?"

...I'm assuming so, going by all the advice I've seen before. Anyone?

"Low level roof tile [or slate] vents on opposite slopes as well as a venting system at the ridge. "

...so this would require installing a slate vent between every set of rafters and at both sides of the house. That's a lot of vents and I'm not sure how it would look from the outside (the house is 130 yr old, stone built so I'd prefer not to damage the character of the building). I think creating a small gap around the edge of the wall still sounds a simpler and lower impact option. Open to comment though.

"Where is the insulation in the wall?"

...At the moment, none. This will be a future project! At the moment there are big gaps that I can look right down in the eaves of the loft space. This must be the gap from the battens between the walls and the plasterboard from the lower level. I guess this is letting some outside air blow right up the sides and into the loft area, so should I at the very least cover up the gap in the loft with rockwool??

"I bet there is a bit of a gap there already? -- I recon that you could simply carefully tap in 10mm wood wedges between the stone and sarking to form a ventilation gap. "

Doesn't seem to be as there is no daylight coming through. Access is difficult but I'll try and have a closer look and try and wedge up the sarking.

The sarking boards are tight to each other and I'm assuming there is roofing felt on top as there is no air blowing through, but not sure.

Noticed today that there is frost on the inside of the sarking in areas where the glass wool is touching. Can't be good! Am I better ripping out all the existing insulation, even if not getting around to doing the rest of the work for 3 or 4 months?

For the gaps from the lower floor, i.e. between the walls and the plasterboard - should I plug the gaps into the loft with insulation to stop cold air flow up through the voids and into the loft? Or do I need some air flow here for wall ventilation?

Note that there is currently no insulation in the walls...something that I will rectify in the next year or two.

I covered these gaps in the loft last summer when insulating the loft. The internal roof (sarking) and is now wet and has some fungus growth this winter, unlike the previous winter in the house when it remained quite dry. I've now pulled the insulation back from the gap to allow the circulation and hopefully rectify the problem. Unfortunately the heat loss through the lath and plaster walls will increase. The cold air enters the gap via external vents in the wall.