Easiest is fix battens over the insulation then fix plasterboard to the battens.

Thanks for the quick responses, as sinnerboy has mentioned I do mean below the rafters for the 80mm insulation,

I have a breathable membrane VP400 under the slates and was planning on leaving a 50mm gap between the slates and the insulation boards as the insulation will not continue for the full height of the roof just up to the ceiling and then continue above the flat ceiling rather than going all the way up to the ridge.

There is plenty of ventilation above and below the membrane to remove any potential moisture issues, and I am aiming to create an insulated box upstairs.
Due to the depth of the rafters and the need to keep the 50mm gap I only have room for 40mm of board between the rafters and would then have to have the 80mm under them with the plasterboard over the top. I have already purchased the PIR so would not be able to use the board bonded to platerboard option. :o(

In order to attach battens over the 80mm boards I would need 120mm screws which I have not been able to find?

But the batten idea would mean that any cabling could be run in the void between the insulation and the plasterboard.

Screw fix the 80 pir with long screws and then plasterboard over with slightly longer screws ( from tony :-) )

Foil tape the pir joints first to maximise the vapour barrier / check

If you do use battens between the pir and plasterboard - yes place cable here . Although I do think a battens cavity here is a lot a hassle for little benefit

Would I need to put washers on the screws holding the PIR board in?

I have done this sinnerboy's way several times. As usual Tony has put a sprog in the works by coming up with an alternative which is probably better. I suppose it depends if there is a height restriction.

Thanks for all the help guys I think I am going to go for the Tony approach

less insulation/more bridging -- insufficient air space between felt and insulation -- tape is not best either.

Screw or nail through but fully locate all rafters and joists first. No vb needed if you use tape but a sheet of polythene is cheaper and better as I see it.

Beware of outdoor air blowing arround under the floor. You will almost certainly need to attend to this problem and its not easy so do it first in case you have to go through a wall.

Ahem, my first post so here goes.
Hope you don't mind my butting in to this conversation, but as you're on this topic, I have a question of my own.

I'm doing a similar job to this myself at the moment on my loft conversion. The plan is Celotex between rafters, then Thinsulex below rafters, then foil backed plasterboard sandwiching it all together.

My question is this: We've got the 50mm airgap above the Celotex, but only half the roof (the newly built bit) has breathable felt, the other half is the old non-breathable bituminous stuff. I can add soffit vents to let air in at the bottom, but the builder hasn't fitted any kind of vent in the small triangular ridge void at the top. Am I right in saying that the breathable felt will be sufficient to provide this exhaust ventilation? The tiles are large, wavy concrete ones that I believe are naturally breathable. They interlock but they don't sit tightly together like slate tiles and must be draughty as anything.

Also, I have some Celotex left over. At the moment the small triangular void above the ceiling just has regular loft insulation running over the top of the joists. Is it worth me using up the last of the celotex inbetween those ceiling joists, leaving the wool type insulation above, or would that add another level of condensation complexity?

Thanks for your thoughts Jim.

I know there's been some debate over foil insulations and their merits or otherwise. It's what the architect specc'd and he, the builder and everyone else I've spoken to say it's great, will work brilliantly and will give no condensation problems.

Condensation is my big worry at the moment. I keep getting images of the Panorama Special in the year 2020 when they investigate the scandal in the noughties when people were told to fit all this stuff and then their roofs rotted out within 15 years. A bit like asbestos which people were told wasn't remotely harmful when they were sawing it up without so much as a mask in the forties. Now we know better.

As for vapour barriers, the Thinsulex instructions say that it works well as a vapour barrier in itself (provided you seal the joints). I plan to tape the Celotex right along each rafter so that's sealed, then seal up the Thisulex completely then fit the foiled p/board. (I've already bought the board and it only came to about £50 more than standard so wasn't too bad). Hopefully that should do it.

I am trying to work out how best to insulate an attic room, slate roof with bitumous non breathable under-felt on 70mm rafter. All the guidance I have seen recommends a 50mm gap under the felt to the insulation. Because of the roof construction and velux windows it is difficult to ensure effective cross rafter ventilation. My archetect has specified completly filling in the rafter voids with foam board, no air gap, then under rafter foam board with a continuous vapour membrane, sealing all joints. The logic is that by preventing moist air entering the roof there is no risk of condensation. Any comments appreciated.

Similar question, If I fit under rafter insulation now, then in a couple of years redo the roof and fit between and over rafter insulation & breathable felt am I introducing any condensation risk? To get the proper U values all the way across my attic roof with under rafter insulation I am going to bring the room height down too much. I'd rather do as much as I can now with the option to top up when I get the roof redone (out of budget at the moment)

Oh, and where do I get these 120mm screws for fixing through the plasterboard and 80mm Celotex? I see the batten option but I'd rather have 80mm celotex than 50 mm and a batten.

Very Kind offer Tony. If you were nearer I'd take you up on that.

I've sourced some 120mm screws and washers. Although I take your point about waiting to redo the roof properly, I dont think thats going to be an option. Or at least not now the plasterboard is down.....

I've ordered what may be an unreasonable amount of celotex. I see a week or two of work. (I know it shouldnt take that long but I have small kids and that limits my available time.)

After experimenting with 120mm screws perpendicular to foam board into rafter we have found it mich easier using 150mm screws horizontally through kingspan into rafter. Kingspan might not weigh much, - but still rides down the perpendicular screw, - so use a longer one horizontally.
boxes of 100 in 120mm cost me 7p + vat, in 150mm cost 9p + vat from local screw & fixings specialist.
Brand on box is: TIMco 'Classic' multipurpose 6.0 x 150mm S/C NO TIM-77346
Fixing at an angle through foam board is very positive, - takes longer finding joists and marking with chalk than fitting up boards. sealing edges and vapour proofing is time consuming too.
Warm wishes!
Wellburn

Ironmongery shop or store or try on line.