I’m sure there will be a builder or architect or experienced renovator on here who can advise. A friend was browsing through photos of a property for sale; the property is a single storey with an attic that looks right dimensionally for converting. If he were to purchase it, he would want to insulate between the rafters and then finish the underside of the rafters with plasterboard. The photos show that the roof structure comprises a ridge beam, with purlins supporting rafters, then battens across the rafters with the slates on top. There is no roofing felt. He is wondering whether (1) the roof would need to be stripped to have felt installed under the slates before proceeding with insulation etc., or (2) should he just add the insulation and put a vapour barrier between the insulation and the plasterboard, or (3) put in the insulation and then fix foil-backed plasterboard to underside the rafters. Any thoughts from the membership?
Ideally, a moisture barrier/breathable membrane should sit on top if the rafters and held in place by vertical battening, tile battens then fixed etc. Only worth doing if the roof is to be replaced. It’s important to retain an air gap so insulating between rafters and then plasterboarding direct isn’t ideal either (and the spacing is unlikely to be right for the plasterboard joints.
It would be better to use a high tech multilayer insulation stapled to the rafters and further fixed with vertical tile battens. Then build a frame (demi-chevron) between the purlins with 60 cm centres to fix plasterboard to.
It is pretty impossible to get rigid insulation between rafters air tight and that is the objective. Its difficult enough with modern machine cut timber so foaming around the edges is often done but that often shrinks back later causing air leaks due to timber expansion and contraction. You can use a memory foam edging tape but its expensive stuff, Gapotape is one I have used on a friends house.
Then you have to address the cold bridging from only insulating between the rafters and not over them which they should do, maybe using insulated plasterboard.
As already stated you must have a breather membrane on the outside above the insulation and an air gap with vented tiles to allow any condensation to vent out. If they use a multifoil on the outside then counterbattening to create the air gap would also be needed and the multifoil fixed properly to seal any air passages.
Your friend has received some good technical advice.
My advice is for your friend to budget for air conditioning as well if they plan on making the attic into bedrooms unless they can sleep comfortably at 30-35°C.
If the insulation is well fitted and has a multifoil on the outside to reflect the summer sun the room shouldn’t get anywhere near 30-35. We used to get those temps in summer before insulating but top out now about 25c.
Mechanical ventilation on the other hand in a sealed space is needed to keep the air healthy.
My place built in 2000 has no roofing felt. I don’t know why as you can buy it. In my case the loft insulation sits on the ceiling with paper vapour barrier. Yes it is hot up there in summer and cold in winter. But house fine. I topped up to 40cm or whatever the norm was a few years back. No condensation as plenty of gaps in the tiles and some air tiles.
Proposed solutions sound good. Personally if I had the roof off I’d put some felt under it doesn’t seem right to have the loft as outside your house space. The loft door has 10cm foam attached to it and any wall touching a room is insulated but only to height necessary.
Many thanks, one and all, for your replies. I’ve forwarded your advice and I’m sure it will be gratefully received.
About time we stopped using the roofing felt name and calling it what it is in 2022, breather membrane, vapor control layer.
I thought roofing felt and vapour control barriers were different? I thought - in an ideal situation, say, like a new build - both should be used, i.e. the felt under the tiles to keep wind-blown rain out and the vapour barrier on the warm side of the insulation to prevent condensation while maintaining air tightedness? I’m glad I asked about this because I obviously got it wrong.
Thanks for the link 
The French in general don’t use roofing felt, in the Vendée the roofs are planked under the clay tiles, but by no means watertight, if you can batton and staple foil insulation between rafters it will help no end, the foil insulation will keep in heat in winter, and expel it in the summer months, you can then plasterboard to the battons, I’ve used 3x2 inch battons, bit extreme but gives you a good surface area to plasterboard too,
Thanks for that Pip