CLS timber thickness (38mm) vs insulation board thickness (30 or 40mm)?

[KevinPeters]

Hi, I am planning to dry-line an exterior wall in my old house.

Something that is confusing me: there is a bit of a mismatch between the thickness of CLS timber and the rigid insulation board.

3x2" CLS timber is 38mm thick, Kingspan TP10 comes in 30mm and 40mm thicknesses.

I appreciate it's only a couple of mm, but which size should I use? If I use 40mm board for the 38mm timber, I would expect it would be difficult to pull the plasterboard so it makes good contact with the battens, but if I choose the 30mm stuff, I'm going to be left with an air gap.

What is the general recommendation here?

 

[pilsbury]

I bet that even if your battens are all perfectly in line, there will be variations in the wall. I recon you’d have a job getting the insulation to sit flush even if you could buy 38mm boards. I’d go for 30mm to be safe.

 

[KevinPeters]

Thanks Pilsbury, that was what my gut was telling me!

 

[Custom45]

Is it a big area you're covering? Go with 30mm and over the top of that between battens spray adhesive on a layer of bubble foil stuff? It won't make a huge difference, but every little helps.

https://www.wickes.co.uk/Thermawrap-General-Purpose-Foil-Wrap-Insulation-1000mm-x-7m--7m2/p/145707