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We live in a Victorian sandstone semi, where the upper floor is effectively in the eaves with dormer windows (lined & tiled roof) It's a proper pain in the arse to heat so it's wood burner on full tilt from now till March. There is a small loft space (approx 5 ft high at the centre which has some old fibre insulation that could/should probably be topped up. The problem is most of the roof space is difficult to get to & completely uninsulated (effectively the area coinciding with the bedroom ceilings). It is possible to slide some kingspan type sheet down/stuff with rockwool from the loft space but does it need to be ventilated?
You need to leave a gap at the eaves so the roof void can ventilate.
I just did my dormer roofs, nightmare! I'm covered in bruises from having to shuffle in and out on my front as its too small to even kneel, all the centres were odd sizes so everything cut to length and width. Got 270mm in there and had triple glazing fitted the same day.
Now sitting in a T-shirt and sweating.
I reckon the gap between the lath and sarking is about 150mm so could squeeze a 80mm sheet down and still give room for ventilation. Same here though-the distance between the rafters isn't equal. It's going to be a nightmare.
I did what you suggest for similar reasons .
Boiling in summer cold in winter .
Make sure you leave an air gap
Air gap should be min 50mm above the insulation. Cut the boards at a slight angle so the 'thin' bit crushes as you slide it in thus as air tight as it can be and should stay in position. Maybe have some wood to slide in over the insulation and twist to push down the insulation. Good time of year to do it - it's too hot to be stuck up there in the summer!