Kingspan/Celotex is what I used from seconds & co. got hold of 30mm to 50mm wedge shaped 1200m2 sheets, reversed them and et violin, 80mm sheet. £240 for 92m2 delivered. Looked at Isover rolls, spoke to a supplier regards the possible sponging/soaking up of water and they seemed to think it would not happen, and Isover is rodent replent as well. Was going down Isover route then fell over the deal from seconds and co. There's a bit from other folk on my garage build thread.