On Saturday, 19 October 2019 16:00:54 UTC+1, Another John wrote:
As the nippy evenings and cold nights become more frequent again, I'm
once again thinking about insulating the garage ceiling.

- The garage is roughly 6m by 4m, and our bedroom is above it.

- The bedrooom often feels rather cooler than the main house
(admittedly it's a big room, therefore "airy").

- Yes. I know that bedrooms are supposed to be cooler than the rest of
the house, but I'd rather achieve that by turning down the radiators.

- I keep thinking about sticking (say) 50mm Celotex boards to the
ceiling [of the garage], using mastic of some kind.

- Would it be _worth_ adding more insulation? When built, 25 years ago,
as an extension to our semi, the joists above the garage were packed
with insulation before the plasterboard was fixed. We also have a
roller door fitted, therefore pretty good insulation at that end.

- The whole house is very well insulated in the roof, and also with
cavity wall insulation.

Any comments would be welcome, to help me with this decision!

Cheers
John

I certainly wouldn't stick insulation up with anything.

I think my first action would be to check the bedroom with an infrared thermometer and, with great care, a candle. Looking for cold spots and draughts.

If, after investigation, you still consider it appropriate to add insulation to the garage ceiling, don't leave it bare. Cover it with plsterboard (or whatever is approved) and fix with the sufficient screws.