Vaulted ceiling insulation

19 Mar 2023
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United Kingdom
Hi all,

We have a small extension with a vaulted ceiling. It’s cold in the winter - sometimes 12 degrees in the morning. It has large bifolds unsure how much heat these will be losing. The builder only allowed for 75 mm of floor insulation under the scree with can’t now be fixed. However I suspect the ceiling is possibly losing a lot of heat but would value any opinion. 12.5mm plasterboard was directly to the rafters so the only insulation is between the rafters - but with large gaps where down lights are installed.

Thinking about removing the downlights and installing the missing insulation there, and then overboarding the existing plasterboard with insulation backed plasterboard. Is this likely to make a significant difference? Can’t see any point of removing the existing plasterboard first.

Also adding another radiator!
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Depends how much insulation you add? You'll still have all the cold bits, rooflights and massive bifolds. Did it get signed off by BC without insulation under the rafters? Do you know where the existing rafters are to enable you to fix into them thought the PB?

What are those weird square bits?
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Thanks the the reply. Yes I guess I’m trying to weigh up whether it’s worth the effort with the bifolds etc. I was hoping perhaps some trapped warmer area near the ceiling would help.

Finding the rafters again and marking them out won’t be a major issue - or at least much easier than tearing down all the old plasterboard.

Yeah BC weren’t great with hindsight. Spent a total of about 2 mins on site and didn’t look at anything - quick chat and signed everything off. My first interaction with them.
Blinds for the roof windows and curtains for the bifolds will help?

I have 100mm PIR between rafters and 40mm over them.
If you overboard the existing ceiling, make sure its the Kingspan insulated plasterboard, not polystyrene-insulated plasterboard. The former has its own 'built-in' vapour control barrier, while polystyrene doesn't.
JohnD, sorry missed the question. Temporary sheets to cover the downlights. Because there is no between rafter insulation around them there is a draft coming through the light units. Intent is to replace them with a light unit attached onto the face of the plasterboard so the only whole would be for the wire.
Do you know where the existing rafters are to enable you to fix into them thought the PB?
Strong magnet will help find the screws holding it up

What are those weird square bits?
Look like repairs to me..

I have 100mm PIR between rafters and 40mm over them.
Have you considered using a thermal camera to find/confirm the problem areas? 100mm between and 40mm over is quite a good whack of PIR already; adding another 50mm to it isn't going to make a night and day difference. If cold world air is reaching the back of the plasterboard (for example because there is a service void and some dumbo has removed enough insulation to allow a breeze through it) I'd look to repair that fail first

Cutting a square of plasterboard out with an oscillating multi tool, to give a working hatch, then screwing it back into place after, using OSB flaps to bridge to surrounding board is relatively easy and leaves minimal damage that filler can easily hide

so the only insulation is between the rafters
So what's the confusion with the "40mm over"?
Last edited:
steve, exactly what insulation do you have - between rafters?

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