pudding Posted October 19, 2019 Share Posted October 19, 2019 Hello, another little picture detail of my ongoing extension design. This time of the junction between the 140mm timber frame wall and flat roof. I've tried to wrap the whole lot in the PIR insulation and stopped the roof joists in the internal timber frame so as to minimise any cold bridging. Any comments or suggestions welcome Link to comment Share on other sites More sharing options...
Onoff Posted October 19, 2019 Share Posted October 19, 2019 Would you be better user thicker OSB vertically to take both your fascia and 25mm battens? Is the gap between the vertical 130mm pir and the roof 150mm pir, filled in the sketch by 12.5mm osb, not a cold bridge? Nice detailing imo aside from the questions. I might nick it for my garden room / dormer redo if it gets the OK here! ? Link to comment Share on other sites More sharing options...
pudding Posted October 19, 2019 Author Share Posted October 19, 2019 (edited) Hi, thicker OSB would be better how, for the fixings, or some other reason? Yes the gap between the 2 PIR sheets is filled by the vertical OSB sheet. It is a cold bridge, but my initial thinking was i needed a way to fix the 50x50 fascia battens some how. A better detail there i've just thought of is to stop that vertical OSB sheet at the top of roof joist level, so the horizontal PIR sheet can butt up against the vertical PIR sheet. Then for the fascia board fixing, the vertical 25x50 battens that the cedar cladding is fixed onto continues up to the top OSB sheet on the roof top, and then horizontal counter battens can be used, with the fascia fixed to those. Completely removed that little cold bridge, and those longer fixings for 50x50 arent needed, although just a few more long fixings for the 25x50 battens needed instead. Sweet, improvements already! Edited October 19, 2019 by pudding Link to comment Share on other sites More sharing options...
Russell griffiths Posted October 19, 2019 Share Posted October 19, 2019 I think you need to go back to the drawing board i will happily be corrected but I always thought that you are supposed to go most breathable on the outside and least breathable on the inside. If this is is the case your pir should be on the inside of your frame. Have you looked at frame companies to see what they think is best, most of the frames I have seen, have been full fill between studs with pir on the inside. Link to comment Share on other sites More sharing options...
A_L Posted October 19, 2019 Share Posted October 19, 2019 FWIW I think you need something under the PIR to support its edges and to spread any load (roofers) from above as it would concentrate at the 250mm joists. Why have a 150mm gap above the plasterboard? Fill with frametherm32 (or cheap loftroll) and use less/no PIR. Link to comment Share on other sites More sharing options...
pudding Posted October 19, 2019 Author Share Posted October 19, 2019 (edited) Hi Russ, The PIR on the outside of the frame could be substituted for EPS which I believe is breathable?I understand your point and have read about it several times. However, with a suitable VCL internally, is it necessary? I think if 100mm of EPS was used externally, then i could go with 25mm of PIR internally on the frame to compensate for the lower thermal performance of the EPS c.f. PIR, so the breathability decreases as you move out though the frame too? A_L, an extra 12mm OSB could be used under the 150mm PIR roof sheets yes. I didnt think it would be necessary though, as the roof joists will likely be 75mm wide at 400mm centres (awaiting structural eng calcs), so only 325mm open space, which for 150mm PIR, shouldnt flex or move, esp once 22mm OSB is fixed on top. The 150mm air gap above plasterboard and then 150mm PIR on top is to create a warm roof and ensure more insulation is above the wooden joists. If it was full fill of 250mm with frametherm 32 and then less PIR on top, then the dew point is brought down into the roof joist level. Happy to be corrected, as it would be cheaper and easier with only 50 or 100mm PIR on top. Edited October 19, 2019 by pudding Link to comment Share on other sites More sharing options...
Russell griffiths Posted October 19, 2019 Share Posted October 19, 2019 How are you going to fix your battens for the cladding through 150mm of pir, that will be a proper pain and costly. Link to comment Share on other sites More sharing options...
pudding Posted October 19, 2019 Author Share Posted October 19, 2019 (edited) Take 2. Extnernal 130mm wall PIR replaced with 150mm graphite EPS, and the 25mm batten and p/b internally replaced with 25mm PIR backed p/b, overall u-value not changed much with this :- Fixings through the what is now EPS for the battens. Any reason not to simply go with these - https://www.toolstation.com/forgefast-multi-purpose-self-drilling-wood-screw/p87754 or https://www.screwfix.com/p/timbascrew-wafer-timber-screws-black-6-7-x-200mm-50-pack/8628J?kpid=8628J&ds_kid=92700048793290424&ds_rl=1249413&gclid=Cj0KCQjw6KrtBRDLARIsAKzvQIEdinQ4_1xBcuui2HFGk8z1_YomGEKJtZX9zmmuMFkzraEwVVs--pwaAv7NEALw_wcB&gclsrc=aw.ds - just 200mm screws? Its only single storey so the amount of weight from the cedar cladding wont be huge, so sag and loading on the fixings shouldnt be a problem. But how do you know? Edited October 19, 2019 by pudding Link to comment Share on other sites More sharing options...
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