Loft Conversion - Achieving 0.15 U Value

[Magrah]

Hi All

I did a loft conversion at my old house circa 25 years ago with the help of a mate who was a builder. He helped me with the structural stuff including the steels. I did the rest myself. Back then the regs were 100mm of Celotex / Kingspan, I couldn't get that so went with 2 layers of 60mm bringing it way over what was needed. Suffice to say, we hardly ever needed the heating on in the loft.

Now things have changed, same deal but this time getting a builder to do the construction, first fix plumbing and electrics (getting someone in to do the plastering as well as I am getting too old!).

Ceiling height is not great, we have a cold deck roof with 50mm air gap above the 6 x 2 roof joists, question is can I achieve 0.15 in 150mm (then 12.5mm non insulated plasterboard underneath)? Can't seem to find anything bar SuperQuilt but it's too late to use that now.

Any ideas please?

Thanks

 

[noseall]

question is can I achieve 0.15 in 150mm (then 12.5mm non insulated plasterboard underneath)?

The one important thing you are overlooking is cold bridging through the 150mm roof joists. BC will insist on a thermal break, preventing cold bridge through the joists. So insulated plasterboard (or similar) is inevitable.

If height within the converted space is an issue, why didn't you go for warm deck?

 

[stevie888]

If height is an issue then cold deck is better (the only time I'd ever consider one) because your construction overall is thinner.

There are a few cunning dodges which can reduce the thickness depending on where you are in construction but I read that you're well along?

 

[Magrah]

The one important thing you are overlooking is cold bridging through the 150mm roof joists. BC will insist on a thermal break, preventing cold bridge through the joists. So insulated plasterboard (or similar) is inevitable.

If height within the converted space is an issue, why didn't you go for warm deck?

Thank you for the reply. Head height and not being able to exceed the former roof height was / is the issue. There will be a vapour barrier within regs. Also 150mm of Celotex XR4040? Achieves 0.14 so a bit confused!

 

[Magrah]

If height is an issue then cold deck is better (the only time I'd ever consider one) because your construction overall is thinner.

There are a few cunning dodges which can reduce the thickness depending on where you are in construction but I read that you're well along?

Thans. Yes, she'll is done, now considering insulation but trying maximise head room.

 

[noseall]

Thank you for the reply. Head height and not being able to exceed the former roof height was / is the issue. There will be a vapour barrier within regs. Also 150mm of Celotex XR4040? Achieves 0.14 so a bit confused!

All the insulation in the world BETWEEN the joists wont prevent cold bridging ALONG the joist.

 

[Magrah]

Thanks,who's wasn't an issue before. Is there anyway I can reduce the plasterboard backed insulation to the minimum?

 

[23vc]

Think they’d want 50mm across the underside of the joists. Unless that’s changed again. I’d always use PIR then normal plasterboard rather than insulation backed board, easier to handle and fit separately