Insulated plasterboard - thickness advice?

[gasket]

Hi folks - after a bit of quick rule-of-thumb advice.

We're converting a detached garage into a gym. Its a relatively recent single skin dense concrete block construction. Slate roof and we've bricked up the original garage door and have fitted a double glazed french door for access.

We've vaulted the ceiling so we're backing the ceiling plasterboard with 100mm cellotex/kingspan. No drama there, the thickness of the rafters dictates the insulation.

My question is about the walls. There is currently no insulation/cavity so we're going to dot and dab insulated plasterboard onto the (very dry) walls. I really don't want to have to batten the walls unless its unavoidable - though I do get a bit confused about moisture etc.

We don't want to lose too much space internally and I have a (possibly unnecessary) concern that thick insulated plasterboard is going to create a hassle when it comes to mounting anything on the walls - so I'm leaning towards a board with thin insulation (eg: 30mm), just as a nod to it being an insulated and usable (but not habitable) space.

All the advice online is based on it being a living space, so U-values seem a bit over cautions.

Just seeing if anyone had a suggestion on a sensible thickness of insulated plasterboard to use in this application? We have a small heater and a small air conditioner in there for the winter/summer months.

 

[Mr Chibs]

Think you may need 50mm PIR below the rafters for a vaulted ceiling. If you are doing building regs.

I wouldn't put anything less than 50mm PIR on the walls, even that is not a lot.
In winter it will be cold... if you are pumping iron you will warm up.
For a double garage you could easily do 100mm on the walls surely?

 

[jeds]

If you are not complying with regulations, one option is to dry fix PIR insulation and overboard with ply or OSB. You'd user a tack adhesive to hold the insulation in place and fix straight through the board into the wall with long screws. No need to plasterboard or plaster - the boards make a good enough finish for a gym and some people are going for it as a funky architectural, alternative finish in houses these days. Straight insulation is cheaper than thermal laminate and this method is very easy to fix. You can choose insulation thickness depending on what you can find on sale online - I would suggest a minimum of 60mm - and also board depending on cost and what you might want to fix to the wall. 15mm would do functionally but 18mm would be better.

 

[George_engineer]

50mm is a good compromise between insulation and cost, if not having to meet building regulations.

How did you go about vaulting the roof?

 

[^woody^]

The insulation type on the back of insulated PB is generally less efficient than PIR. So, if spending the same amount of money and losing the same amount of space, then I'd suggest you would want to be using the more efficient product.

And battens with full fill and face PIR insulation would be better (structurally, performance, less risk) than less efficient dot and dab insulated PB with an air gap - whilst occupying the same amount of space.