Hi all, This is probably a really daft question: I need to use 100mm Kingspan within studwork to achieve the required U-value (0.28) as part of a garage conversion, however, CLS studwork timber is quoted as being 50 x 100mm but comes as 38 x 89mm, what do people normally do? Use 80mm insulation and additionally use insulated plasterboard? Or use larger studwork (if it exists!) so the 100mm can be used? Thanks, Jason
I've used the 47 x 125 C24 timbers with 100mm PIR insulation boards in the past. In fact if you go for 125 timbers, you could increase the PIR insulation thickness to 120mm. The additional cost of the slightly thicker boards would be offset in less than 5 years of thermal efficiency gains, so it's usually worth doing. (FWIW, I've just had my first winter inside of a granny flat with 200mm externally insulated walls, and it barely required any heat at all.)
I am creating a 50mm cavity as there is only a single brick skin currently, so I guess I could have 11mm of the insulation hanging out the back of the studwork in the cavity? Not sure if the 89mm studwork affects the U value. The Kingspan U value calculator claims 0.28 is achievable with 140mm studs and 70mm insulation, however, with 89mm studs and 70mm insulation only 0.30 achieved (80mm insulation is not an option).
Use 89mm CLS as it’s cheaper than buying 4x2 and double board the face, ie screw rips of plasterboard to the face of the studs then go over it later with a full sheet of plasterboard, this will accommodate your 100mm Kingspan. Use longer screws for your plasterboard.