This is what counted as insulation to a pitched roof in 1975.
I suppose at least there was something, there was nothing in the stud walls that partitioned off the eaves storage space. Also nothing to stop the external air from the eaves blowing all the way across the room, under the floor boards, between the floor joists to the other side. Lets just say it is well ventilated!

By 1981, in my own house where no one would stop me (Atlas) I went completely crazy with 75 glass fibre between the rafters (25 air gap above carefully vented eave to eave - not at ridge) plus plasterboard/EPS laminate like that beneath the rafters. "Why both?" they said - to kill the conduction thro uninsulated rafters. "Unh?"

which I think would give me a U-value of 0.2

If you have some 25mm PIR, it might be better deployed to fill the cavity directly behind the plasterboard? Helps to exclude any chance possibility of drafts/mice/fires finding their way into the cavity in future, while leaving plenty ventilation space for the tile felt to sag into. (Obvs run any wiring to lights first).

My childhood 1970s dormer bungalow had the dwarf walls of the dormer bedroom 'insulated' with some 1970s orange carpet, which someone had crawled into the triangle void to tack over the outside of the dwarf wall studs. Was as effective as you would expect. Many years later we bought an old farmhouse where the same thing had been done with fertilizer sacks.

Dominic - whilst you don't say I presume that it is a skeiling ceiling.

In the first example you quote 12.5 plaster board, in the second you use 9 mm PB. Over here we don't use 9 mm PB much because it has a tendency to sag especially if it is not vertical. Was the change intentional or a typo? Also no one here skims PB its all tape and fill, much cheaper and quicker.

I'd just leave 25-30mm ventilation space - that's plenty. And lose the battens - just glue the plasterboard to the PIR. It works fine. My skeilings are done that way (100mm PUR+12mm PB)

Wookey, that's interesting, did you also rely on glue to fix the PIR to the rafters, or also screw through something? Did the pb need any support while the glue dried?

Ive been gluing pb onto PIR but only for vertical wall - not tried on ceilings yet! I used instant grab polymer adhesive, got through quite a few tubes trying to run continuous beads round each board edge and criss cross across the middle.

Dominic, the 1200x900 plasterboards suit me better for working on ceilings alone. The 9mm stuff doesn't seem to be sold so much round here, maybe there are some fire or noise regs or something. The 1970s plasterboard ceiling has sagged a little round each fixing, but not really noticeable because of the swirly artex and the woodchip wallpaper..

Could you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

I'm thinking of doing this unless someboy has experienced a failure of such a scheme ?

Posted By: DavycrocketCould you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

I'm thinking of doing this unless someboy has experienced a failure of such a scheme ?

I put PIR insulation boards directly onto the existing plasterboards on my dwarf walls/skeilings. However I would not trust adhesives so I used 25 x 50mm battens to secure the first layer of 50mm PIR (screwed through) and then put 25mm boards between the battens (friction fit with Alu tape over the joins) and then screwed the new PB to the battens. Worked very well, no problems AFAIK.

Note: I am aware that purists would be horrified at the thought of all the thermal bridges caused by the screws!

<blockquote><cite>Posted By: tony</cite>I would advocate mechanical fixing too</blockquote>

Hi Tony did you read my reply re loaning of the cavity wall gun mate ?

Posted By: DavycrocketCould you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

In this particular case I removed all the old plasterboard because I have also removed the dwarf wall on each side of the room, as well as the flat ceiling in the middle. So the original skeiling part was less than one third of the plasterboarded area of this room.
This has gained us 800mm each side of the room, so 1600mm in total, less 100 each side when I put the PIR on the block walls plus the plasterboard so more like 1.37 metres total extra on this room dimension. It will look and feel bigger and more spacious, it will bring the previously cold ventilated-to-outdoors eaves storage inside the warm envelope. The whole job will prevnt cold outdoor air from blowing all across the room underneath the floorbards (Kitchen is below).

We will likely still use the eaves area for storage but in furniture (blanket boxes etc.) and also form a rearranged airing cupboard that will now be properly insulated!

Helpfully the 1200 x 2400 boards I cut lengthwise to 400 wide and they fit between with a very small gap, which was then foamed in (held in place temporarily by nails). In one place I had to cut 390mm / 410mm but I'm glad I noticed it before I just cut them all to 400!
I cut both sides of the board with a sharp knife and snapped the board, so almost no dust created.

I still have the top bit to do above the chords & purlins, but helpfully this is approx 800mm long so my pieces cut 400 x 2400 will then get cut into 3 lengths of 800 x 400

The 100mm layer will go over these held in place with 25mm battens.

The radiator under the window will probably go somewhere else, as it will need moving inwards anyway to make room for 100mm PIR on the end wall.

In my recent exerience with the pink foam adhesive for PIR and Plasterboard, it sticks very very well.
It is almost impossible to remove without wrecking either the PIR or the P/B or both. I did manage to carefully peel a 25mm sheet of misplaced PIR off of a very dusty stone wall so that I could re-do it (it was sticking out proud of the one next to it - I was just using it as gap filler in this location on an internal wall).
We used it on the walls of the barn conversion we did recently, but we also fixed the PIR mechanically with plastic insulation fixings, then pink foam glued the P/B to the surface.
I can see the appeal in using it for skeilings, but it would need to be held in place while it goes off, for which screws are an easy solution anyway. You could use just a few screws and take them out again after, but this would seem a bit pointless.
My concern with adhesive-only on existing skeilings or ceilings is that any paint layer could act as a release agent?
Also longevity, although it stuck very very well when we used it last year, would this still be the case in 10, 20 or even 47 years time?

Thanks for the replies Dominic Tom and Jeff, very informative.

These ones are even worse. The one at the top was pressed hard against the roofing felt.
I have managed to pull it away a small amount and squeeze 12mm of Aerogel board behind it.
The location of these pipes are the reason that during the "Beast from the East" in 2018 we had no hot water.
(this part of the roof is on the east side)