Cold flat roof vents

You will need to completely fill with a solid insulation rockwool will still allow air into the roof space
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Thanks for the prompt response Chappers.

When you say rockwool will still allow air flow to the roof space, wouldn't the inclusion of a vapour barrier exclude this? I'd be a little concerned that I'd get the Kingspan tight enough against the rafters or without voids when trying to infill 140mm; the rafters have been in place for 50+ years and some have a slights twist etc.

Could using something along the lines of Actis Triso multi foil help in anyway?
Yes use quilt between the joists, to ensure that no voids are left and its tight up against the deck, then then fit your Kingspan to the underside of the joists and tape all joints and then plasterboard.

Obviously the roof void wont be a vacuum and there will still be air within it, but by eliminating all voids, in particularly up against the deck, and then preventing any moist air from entering the roof, then you remove the possibility of condensation forming within the roof
Thanks Woody,

How much insulation would you recommend? Having just re-measured the joist depth i've found it to be only 135mm. Does say 135mm of Knauf Crown 32/44 and another 50mm of Kingspan/Celotex sound enough or should I try a get more PIR between the joists?
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50mm Kingspan is worth about 100mm of quilt and so at 235mm only 15mm less than was required pre October changes (I'm not sure if this has increased now)

If your joists have furrings on, and so increasing the depth available towards one end, then I would just leave as is as there will on average be more than 135mm of quilt between the joists.

Otherwise, it depends if you want to increase the Kingspan between or below the joists

If building control are involved, then you may need more Kingspan to conform to current regs
Thanks again Woody,

BC aren't involved...........I'm just trying to up-grade/make the most out of what we've already got. The phrase "trying to make a silk purse out of a sow's ear" springs to mind!

I'll probably try and include some Kingspan within and below the joists. I guess the less warmth that can reach the deck the better..........I'll just have to try and cut it well!


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