Good afternoon,
I will shortly be having the garage ceiling being taken back to bare joists, so will need to reinsulate and plasterboard. Half the ceiling is under a warmed bathroom. Half the ceiling is under a small, uninhabited roof space.
It's a 60's build so I appreciate that per building regs I just have to improve on the original construction, which I will. As it's a garage I will however observe fire regs (30 mins?)
At the moment I'm thinking 50mm Celotex or 100mm of rockwool between the joists, with 12.5mm fireboard underneath, leaving at least 50mm gap to the bathroom floor above. I don't particularly understand how a vapour barrier would work (on the warm side so closer to the bathroom?) and whether I'd struggle with cold bridging through the joists with nothing but plasterboard under the joists. I could perhaps stretch to 20mm Celotex under the joists as well, especially if only in the part under the bathroom.
I'd be grateful for any thoughts.
Please do assume I'm an idiot and spell it out like I'm 5 years old - there's a huge amount of often contradictory info out there in regards to condensation etc, and I want to do a decent job.
Many thanks.
Edit: I should add that this is somewhat on a budget (the vast majority being chewed up by AIB removal), so that is a consideration. I might be tempted by the rockwool between the joists at least as it's so much cheaper. Perhaps 100mm rockwool between the joists then 20mm celotex underneath would be a good compromise?
I will shortly be having the garage ceiling being taken back to bare joists, so will need to reinsulate and plasterboard. Half the ceiling is under a warmed bathroom. Half the ceiling is under a small, uninhabited roof space.
It's a 60's build so I appreciate that per building regs I just have to improve on the original construction, which I will. As it's a garage I will however observe fire regs (30 mins?)
At the moment I'm thinking 50mm Celotex or 100mm of rockwool between the joists, with 12.5mm fireboard underneath, leaving at least 50mm gap to the bathroom floor above. I don't particularly understand how a vapour barrier would work (on the warm side so closer to the bathroom?) and whether I'd struggle with cold bridging through the joists with nothing but plasterboard under the joists. I could perhaps stretch to 20mm Celotex under the joists as well, especially if only in the part under the bathroom.
I'd be grateful for any thoughts.
Please do assume I'm an idiot and spell it out like I'm 5 years old - there's a huge amount of often contradictory info out there in regards to condensation etc, and I want to do a decent job.
Many thanks.
Edit: I should add that this is somewhat on a budget (the vast majority being chewed up by AIB removal), so that is a consideration. I might be tempted by the rockwool between the joists at least as it's so much cheaper. Perhaps 100mm rockwool between the joists then 20mm celotex underneath would be a good compromise?