Downlights in Extension - No Insulation

6 Dec 2022
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United Kingdom
My builders just completed a single story pitched roof extension on the back of my property.

75mm celotex was cut and jammed between the rafters, then 50mm celotex backed plasterboard was screwed to the rafters, so a total of 125mm insulation. Above that is a membrane, then the roof tiles.

My issue: To fit downlights, circa 200mm gaps were left open in the 75mm celotex. 70mm holes have then been cut through the plasterboard and insulation to accommodate the downlight. End result, I have 8 holes in my ceiling that have zero insulation. The only barrier between the downlight and the outside is roof tile and a membrane. These areas act as chimneys, and IMO absolutely negate any insulation anywhere in the room.

My builder claims this meets building regulations. I can't see how that can be true. My plan is to simply jam a load of rock wool into the void (the downlights are rated for it).

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its a vaulted ceiling then?

or do you mean the 75mm celotex was jammed between ceiling joists

either ways its wrong
it sounds like a path for vapour to me

does 75mm in between and 50mm below reach u value for part L?
I believe it is referred to as a vaulted ceiling, but I didn't want to use names potentially erroneously. The ceiling (inside) follows the same slope as the exterior roof, at a pitch of 12.5 degrees. The ceiling is at it's highest where the extension meets the old rear wall of the house, and slopes down to the extension end.

75mm celotex was jammed between joists, and then directly under it, insulation backed plasterboards was screwed to the joists (apologies if I got rafters and joists the wrong way around).

75mm between and 50mm below does meet regs I think, and went through planning. That said, I do know my builders know nothing about insulation due to the way they installed other bits that I am familiar with, for example - no continuity of insulation where they used rockwool, so maybe I'll stick in a call to the local planning office.
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Planning aren't interested in insulation, Building Control will be. If your builder submitted full plans to BC with that insulation scheme and the holes in it it should have been rejected. I'd be surprised if 125mm was adequate even without the holes.
If you really want spotlights, get some surface mount ones (are tracklights still a thing?)

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