Garage wall insulation - thinnest I can get away with?

[Stu666]

I am converting my attached garage into a gym space. The three external walls are single leaf brick. with no render or plaster on either side. Garage door has been removed/bricked up and double glazing being put in.

In order to save space, I have decided to go down the treated batten route rather than stud it out. I've been looking at foil-backed insulated PIR board to affix to the battens, as I understand this gives a superior U-value and also has the benefit of acting as a vapour barrier. The question is, how thick does it need to be? Looking at some online calculators, it's suggesting around 75mm would put me into the desirable U-value range, but it's a lot of space to lose. I'm wondering if this would be overkill, and whether I'd get away with 50mm or even 37mm?

Also, I don't mind paying a bit extra for better performance. Any recommendations or advice on newer products would be appreciated, thank you.

 

[Wayners]

superquilt insulation maybe

 

[freddiemercurystwin]

Superquilts and the like are a misnomer, they typically require a 25mm air cavity each side to perform properly, thus negating any benefits, they are not super.

 

[^woody^]

50 on the walls.
If you can, highly insulate the ceiling.
Oversize the radiator.

 

[^woody^]

.... assuming building control are not involved.

 

[Stu666]

.... assuming building control are not involved.

Not at this point.

The (flat) roof is already insulated, built in the 1980s I believe, and looks to be standard fibreglass, can't tell how thick it is without making a big mess.

Currently debating whether to go with Celotex or equivalent, minimum 50 mm, or whether to splash the cash on Kingspan K118. The specs looks better on paper, but how big a difference it would make (.019 W/mK vs .022 W/mK) I do not know. Also debating 57 mm vs 72 mm. I need approx. 14 sheets, which fits nicely with a pallet of 57 mm. Anything thicker, I'd need to order additional sheets, but with potentially better performance.