SBMS Posted June 23 Author Share Posted June 23 36 minutes ago, JohnMo said: Full story Posi rafter, above was all sarking boarded, breather membrane and slates on 45 Deg roof and Sarnafil membrane on 12 Deg roof. Inside was under drawn with counter battens to increase the depth from 256mm to 356mm. Full filled with open cell spray foam. VCL double sided taped to battens, stapled and all staples and joints aluminium taped. VCL draped down walls and bonded to wall. Counter battens (50mm service cavity for lights) then plasterboard. When I battened the posi rafter, I did it to get a good thermal break. So used a small section of batten on a long section to form a 100mm Wall drape can be seen on wall. Counter battens @JohnMo great pictures and explanation thanks v much. What u value did your roof makeup achieve? Did you consider keeping your 256mm rafter depth and using an insulated plasterboard - and if you did how come you decided against it? Is 256mm the maximum posi rafter depth or could you have gone deeper? @Iceverge if we went blown cellulose what depth gets us to 0.1? I’m thinking of doing what John did but either increasing the depth with counter batten, or lining with an insulated PB to get to the u value. Link to comment Share on other sites More sharing options...
Oz07 Posted June 23 Share Posted June 23 1 hour ago, JohnMo said: Full story Posi rafter, above was all sarking boarded, breather membrane and slates on 45 Deg roof and Sarnafil membrane on 12 Deg roof. Inside was under drawn with counter battens to increase the depth from 256mm to 356mm. Full filled with open cell spray foam. VCL double sided taped to battens, stapled and all staples and joints aluminium taped. VCL draped down walls and bonded to wall. Counter battens (50mm service cavity for lights) then plasterboard. When I battened the posi rafter, I did it to get a good thermal break. So used a small section of batten on a long section to form a 100mm Wall drape can be seen on wall. Counter battens Interesting thanks. So your pozis go side to side not wall to ridge or am i seeing it wrong? Link to comment Share on other sites More sharing options...
Oz07 Posted June 23 Share Posted June 23 56 minutes ago, SBMS said: @JohnMo Is 256mm the maximum posi rafter depth or could you have gone deeper? @Iceverge Ive had 300mm before for floor Link to comment Share on other sites More sharing options...
JohnMo Posted June 23 Share Posted June 23 1 hour ago, SBMS said: Did you consider keeping your 256mm rafter depth and using an insulated plasterboard - and if you did how come you decided against it? Mainly decided against it as we have some large down lights and they need 50mm depth. Didn't want to digging holes in the insulation to install. 1 hour ago, SBMS said: Is 256mm the maximum posi rafter depth or could you have gone deeper? That was the design that came from Pasqual (maybe misspelt) so kept with it. We have quite big roof overhangs and didn't want to mess up the look outside. You can get deeper. I did look into blown cellulose, but was getting lots of demands from them, which made it hard to do. So dropped in the end. The company we used for assembly of the roof also do spray foam, they did all the interstitial condensation calcs and all looked good. So the same guys that did the joinery also did the spray foam. 11 minutes ago, Oz07 said: So your pozis go side to side not wall to ridge or am i seeing it wrong? 45 Deg roof wall to ridge. 12 Deg, wall to wall. 1 Link to comment Share on other sites More sharing options...
SBMS Posted June 23 Author Share Posted June 23 22 minutes ago, JohnMo said: did look into blown cellulose, but was getting lots of demands from them, which made it hard to do. What were they asking for? Link to comment Share on other sites More sharing options...
JohnMo Posted June 23 Share Posted June 23 Just now, SBMS said: What were they asking for? They wanted a 50mm ventilation air gap minimum. The way slate roofs are done in Scotland are Sarking board, breather membrane then slates attached directly to the sarking board. So for the 45 Deg roof we would need to add battens and counter battens then slates. Which need all the roof drawings redone and re approved by BC. For 12 Deg roof same thing but then I would have to add another deck to support the Sarnafil membrane. Link to comment Share on other sites More sharing options...
SBMS Posted June 23 Author Share Posted June 23 (edited) 1 hour ago, JohnMo said: They wanted a 50mm ventilation air gap minimum. The way slate roofs are done in Scotland are Sarking board, breather membrane then slates attached directly to the sarking board. So for the 45 Deg roof we would need to add battens and counter battens then slates. Which need all the roof drawings redone and re approved by BC. For 12 Deg roof same thing but then I would have to add another deck to support the Sarnafil membrane. Got you. Useful to know whilst we are pre drawings stage. Does anyone else know if this is a common ask with cellulose blown roofs?MBC profile doesn’t seem to have a 50mm ventilation gap. Edited June 23 by SBMS Link to comment Share on other sites More sharing options...
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