Russell griffiths Posted March 10, 2018 Share Posted March 10, 2018 So I’m starting to sort out details and every answer creates another question so talking to a truss manufactures he’s leading me towards a truss rather than a metal web joist so question is how can I create a warm roof setup with trusses, I want to avoid insulation on top of the trusses and just have it below the level of the 18mm osb that will form the roof covering. Hope that makes sense. Link to comment Share on other sites More sharing options...
A_L Posted March 10, 2018 Share Posted March 10, 2018 58 minutes ago, Russell griffiths said: so question is how can I create a warm roof setup with trusses, I want to avoid insulation on top of the trusses and just have it below the level of the 18mm osb that will form the roof covering. This depends on your idea of a 'warm' roof. Mine is that there is a continuous layer of a substantial proportion of the total insulation above the trusses to reduce the temperature differential across the trusses and have them and everything below the insulation at near internal temperatures. Obviously this not the construction you want. If by a 'warm' roof you mean everything below the trusses at substantially the internal temperature then leaving a 50mm air gap on the topside of the insulation, running up to a ridge vent, or sometimes by fully filling the truss depth with insulation and using a suitable breather membrane below the roof covering. A VCL below the trusses would be necessary and I would put at least some insulation below the trusses to reduce thermal bridging through them. Link to comment Share on other sites More sharing options...
Temp Posted March 10, 2018 Share Posted March 10, 2018 (edited) "Warm roof construction" has all/most of the insulation above the rafters so that the rafters are on the warm side where no condensation should occur. Hence no ventilation of the rafters is needed. If the rafters or trusses penetrate the insulation it's a "cold roof construction". With a cold roof construction there must be some method of allowing water vapour to escape from the cold top side of the rafters. Either a ventilated void or a vapour permeable membrane. Both methods produce a "warm loft". The problem is that I think OSB is not considered vapour permeable... So there probably should be a ventilated void between the OSB and insulation. https://www.thenbs.com/knowledge/vapour-permeable-underlays-for-slate-and-tile-roofing Quote When laid over a material which has a high resistance to water vapour, e.g. plywood, orientated strand board (OSB) or particleboard, a vapour permeable underlay will not reduce that high resistance. A vapour permeable underlay laid over open jointed softwood boarding may be considered as providing a low resistance. When the underlay is laid over rigid boarding, counterbattens may be deemed necessary to prevent any penetrating moisture building up behind slating/ tiling battens by allowing free drainage down the roof slope. Counterbattens are typically not required with an unsupported underlay; where there is sufficient drape between rafters. So could you use open jointed softwood boarding instead of OSB? Edited March 10, 2018 by Temp Link to comment Share on other sites More sharing options...
ProDave Posted March 10, 2018 Share Posted March 10, 2018 You should, with the correct insulation over the rafters, be able to make this a warm roof. BUT the complication you will have is all the bits of the truss sticking out into the roof space will make it very hard to seal and make the roof structure air tight. Expect to use a lot of air tightness tape sealing around every bit of the trusses structure. Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 10, 2018 Author Share Posted March 10, 2018 I may not be explaining very well I will re think and try to do a sketch to explain. Thanks. Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 10, 2018 Author Share Posted March 10, 2018 Just now, ProDave said: You should, with the correct insulation over the rafters, be able to make this a warm roof. BUT the complication you will have is all the bits of the truss sticking out into the roof space will make it very hard to seal and make the roof structure air tight. Expect to use a lot of air tightness tape sealing around every bit of the trusses structure. This I think is what I am worried about. Will find some pics and come back. Link to comment Share on other sites More sharing options...
Pocster Posted March 10, 2018 Share Posted March 10, 2018 36 minutes ago, ProDave said: You should, with the correct insulation over the rafters, be able to make this a warm roof. BUT the complication you will have is all the bits of the truss sticking out into the roof space will make it very hard to seal and make the roof structure air tight. Expect to use a lot of air tightness tape sealing around every bit of the trusses structure. You mean like this ? then I’m going to put 50mm pir over the lot . Followed by aluminium foil tape on all the joints in the pir and around any protruding timber . It’s a lot of work ? Link to comment Share on other sites More sharing options...
ProDave Posted March 10, 2018 Share Posted March 10, 2018 ^^^ Yes that is the point. If you are trying to make the house truly air tight, hen there is a LOT of work sealing all that insulation and sealing around every timber that makes up the trusses. Link to comment Share on other sites More sharing options...
A_L Posted March 10, 2018 Share Posted March 10, 2018 Tongue and grooved 18mm OSB should be reasonably airtight on its own, could always put silicone in the groove. Maybe this is a case for an airtight breather membrane outside the OSB so that you only have a problem if the air-tightness layer comes 'in-board' (at around the gutter level) Maybe a case for fully adhered membrane https://www.proctorgroup.com/air-barriers https://www.dorken.com/en/our-products/products/residential/delta-vent-sa.php 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 10, 2018 Author Share Posted March 10, 2018 All this fussy detailing is what I am trying to design out i will post up my alternative thoughts. Link to comment Share on other sites More sharing options...
ProDave Posted March 10, 2018 Share Posted March 10, 2018 8 minutes ago, Russell griffiths said: All this fussy detailing is what I am trying to design out i will post up my alternative thoughts. Ridge beam hung roof will solve all your woes. 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 10, 2018 Author Share Posted March 10, 2018 16 minutes ago, ProDave said: Ridge beam hung roof will solve all your woes. Yes you are bob on. Its a mono pitch so thinking I joists. Details to follow if I can find a pic. Link to comment Share on other sites More sharing options...
Pocster Posted March 10, 2018 Share Posted March 10, 2018 I assume trusses are cheaper ?? Link to comment Share on other sites More sharing options...
Alexphd1 Posted March 10, 2018 Share Posted March 10, 2018 ridge beam was cheaper for us, but if we was paying for labour probably not. Link to comment Share on other sites More sharing options...
PeterW Posted March 10, 2018 Share Posted March 10, 2018 Mono pitch will be much cheaper with a metal web or I-Joist system from a ridge board. The only issue you will have will be the internal ceiling will follow the slope but that is easily fixed. Link to comment Share on other sites More sharing options...
Alexphd1 Posted March 10, 2018 Share Posted March 10, 2018 we are planning to spray underside with open cell insulation. 500m2 one day work, cut out lot of faffing with insulation....... thats the plan anyway! Link to comment Share on other sites More sharing options...
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