Hi, I'm insulating between and under my rafters with 2" kingspan and am then going to fix plasterboard to the underside of the kingspan with good long drylining screws. The rafters are spaced by about 16"/40cm. What thickness of PB should I use? I'd fancied 9mm because the ceiling is a double height room and the thinner PB is a little lighter to lift into place.
Personally I would go for 12.5mm thick. I have never been a fan of 9mm, I think you will be better with the thicker stuff even though it is heavier and more difficult to position. ultimatehandyman
Hi skinny d Cant see any thing wrong with 9.5mm boards with a skim finish you get 1/2 hr fire resistance to the roof structure and you dont need any more sound qualities. Agreed the 12.5 is more stable for lifting into a horizontal position but 9.5 is ok. dont use 2400 x 1200 boards if you plan to fit on your own, get the smaller boards and you will be fine. The fact that your rafters are at 400 centre increase the stability of the 9.5 board anyway Cheers - jonah
Having said all that I agree with ultimate handyman - I would always slap up 12.5mm. Its not as whippy Cheers - jonah
Cheers for the replies. I'd initially thought 12.5 PB would be harder to position because of its weight, but your comments on its whippyness ring true with my experiences of my kitchen and bedroom ceilings. I'll be jointing rather than skimming the ceiling so I think I may need 12.5 for the fire resistance. Thanks for the advice, this forum is grand. Dave
Hi, I've just been thinking, if I have 50mm of kingspan followed by 12.5 mm of plasterboard and use the longest drylining screws that SF sell (75mm) then I'll only have 12.5 mm of screw tip in the rafter. Is this enough? I'm not sure but I think the next size up is 100mm and they are a fair bit more expensive and possibly overkill. Your comments will be valued as ever. Dave
Skinny d 12.5 isn't really enough to hold the board in my opinion. I've seen about that holding vertical pb to walls but it came off easy. A ceiling mmmmmmm from memory I think the 4" is the next one up but don't know ask at your locl merchant or google them. I think the 4" are probably nearly double the price of the 3" ones though Are you using 2 layers of 50mm kingspan for ecconomy of boards or because you have 100mm rafters and need to maintain the 50mm vented air space. If not then what about increasing the thickness in between and decreasing the thickness below. Alternatively nail!!! - not as good and more difficult I know cheers - jonah
Hi Jonah, I didn't think 12.5mm was enough, 4" screws it is then as I'm not a fan of nails for this job. I'm using 2 layers of 2" kingspan because I have 2.5-3.5" deep rafters with tyvek on top. I need a small (10mm?) airgap between the cold size of the insulation and the tyvek so figured 2" between rafters, 2" below, nice and simple. I could have gone with 60mm between and 40mm underneath but then this would have made my life a bit more tricky when ordering and sorting out the different layers of kingspan. I chatted the issue through with my BCO and could have put 4" kingspan either between the rafters (would need to batten rafters to increase their thickness to match and would have a "cold bridge" through the rafter) or on the underside (I have nice purlins under my rafters and want them showing so the less kingspan on the underside of the rafters the better). I hope this makes sense. Cheers, Dave
yeah - you are stuffed then unless someone else knows an in between length drywall screw What about screwing a 2" x 1" batten across lowest kingspan using 3.5" wood screws into rafters through kingspan, then fix pb'd to these battens using nice shorty length drywalls Its another option but have a look what difference an extra 1" makes to your exposed purlins. Chhers - jonah
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