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

10 Jun 2012
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United Kingdom
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?
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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.
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