Moonshine Posted January 31, 2019 Share Posted January 31, 2019 I am looking at a potential roof construction, to put wooden boards (~9mm) above the rafters, and below the counter battens / battens and tiles. As the boards will be a Impermeable barrier i will be leaving a 50mm (BS5250) vented gap between the underside of the boards and the insulation. The question is, what type of boarding to go for, OSB or a treated ply? any thoughts Link to comment Share on other sites More sharing options...
Jeremy Harris Posted January 31, 2019 Share Posted January 31, 2019 We have 22mm OSB sarking boards nailed to the outside of our rafters, then 50 x 25 counterbattens nailed along the lines of the rafters, then a layer of roofing membrane, then 50 x 25 battens for the slates. Works very well, as it stiffens up the roof and makes it easier to work on. It also improves airtightness a fair bit and decreases "wind wash" through the insulation. We have no gap between the sarking and the cellulose insulation, as the OSB is sufficiently vapour permeable to not need it.. 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 31, 2019 Share Posted January 31, 2019 Two whys. Why do you think 9mm boards will be impermeable, they won’t why do you want an impermeable layer above your rafters. The only thing I can think of would be if you are having spray foam insulation so need a solid surface to spray against if not then on top of the rafters should be your vapour permeable layer, then your counter batten then battens. Link to comment Share on other sites More sharing options...
Moonshine Posted January 31, 2019 Author Share Posted January 31, 2019 8 minutes ago, JSHarris said: We have no gap between the sarking and the cellulose insulation, as the OSB is sufficiently vapour permeable to not need it.. Is it, i hadn't realised is was permeable, that is good. That would help reduce the total thickness of the roof, so i could have a 175/200mm rafter, cavity filled with 175/200mm mineral wool with OSB above and PIR below. Link to comment Share on other sites More sharing options...
Moonshine Posted January 31, 2019 Author Share Posted January 31, 2019 10 minutes ago, Russell griffiths said: Why do you think 9mm boards will be impermeable, they won’t why do you want an impermeable layer above your rafters. I was incorrect in my assumption that the OSB would be impermeable, as @JSHarris above the roof construction i am thinking is Tiled roof on 25mm battens Roofing membrane 25mm counter battens (running down roof) 18mm OSB above rafters 200mm rafters with 200mm mineral wool in cavity Vapour control layer 50mm PIR between battens below rafters Acoustic resilient bar below battens 2 layers of 15mm SoundBloc Link to comment Share on other sites More sharing options...
Jeremy Harris Posted January 31, 2019 Share Posted January 31, 2019 2 minutes ago, Moonshine said: Is it, i hadn't realised is was permeable, that is good. That would help reduce the total thickness of the roof, so i could have a 175/200mm rafter, cavity filled with 175/200mm mineral wool with OSB above and PIR below. Yes, OSB is sufficiently vapour permeable to be used on the outside. Our roof build up (warm roof, with rooms in the roof space) is, from the inside out: 12.5mm plasterboard 50mm service void (50 x 50 battens) VCL membrane 402mm deep I beam rafters, with pumped cellulose insulation between them. 22mm OSB sarking boards 50 x 25 counterbattens nailed along the line of the rafters to give a ventilated space Non-tenting vapour permeable roofing membrane 50 x 25 slate battens Slates or in-roof PV panel trays 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 31, 2019 Share Posted January 31, 2019 i would put the membrane directly on top of the sarking boards and then your counter batten but it works both ways, many ways to skin a cat i just like the look of the membrane on top of the sarking with lapped and taped joins i had the siga rep down this week and we were talking about this very thing. 2 Link to comment Share on other sites More sharing options...
Russell griffiths Posted January 31, 2019 Share Posted January 31, 2019 Like this. Link to comment Share on other sites More sharing options...
Ed Davies Posted January 31, 2019 Share Posted January 31, 2019 Another option is traditional sarking boards. 150x22 treated softwood. Not as much racking strength as OSB or ply and a bit more nailing but on a roof which is too steep to walk on it can form its own ladder for installation. https://edavies.me.uk/2018/01/sarking/ I'm not sure about the permeability of OSB. It seems to vary in odd ways between products and thicknesses so I think sometimes there's a surface layer which is impermeable with the inside of the produce being more permeable. Sarking boarding, because of the way it's installed and shrinks as it dries out has gaps between the boards of a mm or two so is intrinsically very vapour open. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted January 31, 2019 Share Posted January 31, 2019 OSB does vary, and I should have been more specific. Don't use OSB/3 or /4 for sarking, as it's not anywhere near as vapour permeable as OSB/1 or /2. 1 Link to comment Share on other sites More sharing options...
A_L Posted January 31, 2019 Share Posted January 31, 2019 Not sure I would be happy using OSB/OSB2 for sarking. https://www.norbord.co.uk/media/1297/sterlingosb-brochure-24pp-2015.pdf pages 5 &7 http://wpif.org.uk/uploads/PanelGuide/PanelGuide_2014_sect2-6.pdf page 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted February 1, 2019 Share Posted February 1, 2019 Scanda hause use an osb board that has holes drilled in it all over i have looked for it but it must be something they produce themselves. Link to comment Share on other sites More sharing options...
Crofter Posted February 1, 2019 Share Posted February 1, 2019 What effect does thickness have on vapour permeability? I used OSB3, but only 9mm. Link to comment Share on other sites More sharing options...
Ferdinand Posted February 1, 2019 Share Posted February 1, 2019 (edited) ... Edited February 1, 2019 by Ferdinand Not sure Link to comment Share on other sites More sharing options...
Ed Davies Posted February 1, 2019 Share Posted February 1, 2019 (edited) 3 hours ago, Crofter said: What effect does thickness have on vapour permeability? I used OSB3, but only 9mm. Good question. I'm confused: https://edavies.me.uk/2014/04/osb_vapour I'm not the only one, see the update link at the end to: https://www.greenbuildingadvisor.com/article/is-oriented-strand-board-as-impermeable-as-they-say I (pointed in that direction by @A_L., IIRC) and they independently came to the conclusion that it's the surface layers which provide the resistance to vapour. Having read all that I've put 9mm OSB3 on my gable ends with Protect TF200 over it. It'll then have cladding over that but with a good ventilated gap in between. On the other hand, @SteamyTea 's house has OSB on the outside with render directly on to it. It's been up since 1987 and hasn't rotted in the mean time. No idea of the thickness or flavour of his OSB (he didn't build it, not sure if he knows, he didn't mention it in e-mail). Edited February 1, 2019 by Ed Davies Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 1, 2019 Share Posted February 1, 2019 This has made me go back and check the spec for the OSB we have as sarking and as racking layer on the outside of our frame. It seems I was wrong about the specs. The sarking is 18mm OSB, T&G boards that seem to be specified for use as sarking boards, but there's nothing in the spec about what they actually are. I suspect they may well be roofdek, or similar, as looking at the photos the boards seem to be a lighter colour than the OSB/3 boards. Our racking layer wall boards are definitely 9mm OSB/3. Both the walls and roof have a ventilated space that should ensure that water vapour can escape from the boards OK. I may try and see if I can try and measure the RH at the surface of the boards, as it shouldn't be too hard to get a sensor up under the eaves and in contact with the board surface. Not sure how useful the data might be, but it'd be interesting to see the relationship between the RH of the air outside and that at the surface of the boards. Have to wait until summer, though. It's still snowing here, has been on and off since yesterday evening. Link to comment Share on other sites More sharing options...
Mr Punter Posted February 1, 2019 Share Posted February 1, 2019 4 hours ago, Russell griffiths said: Scanda hause use an osb board that has holes drilled in it all over i have looked for it but it must be something they produce themselves. A while ago I read that to get the required permeability you had to drill a load of 2p piece sized holes all over the OSB. With the Scottish type softwood boards they have a gap between. Now it appears OK to use normal or T & G OSB. Not sure what changed. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 1, 2019 Share Posted February 1, 2019 Depends how vapour permeable you need the board to be, which depends on what's the other side of it and also how well ventilated the space is outside it. I doubt there is a "one size fits all" answer, and it probably needs a interstitial condensation risk analysis for each specific build up to determine whether any particular outer panel is OK or not. The challenge with that seems to be the wide variation in permeance for OSB, as that could well skew the results from any condensation risk analysis. Link to comment Share on other sites More sharing options...
scottishjohn Posted February 1, 2019 Share Posted February 1, 2019 I would follow the way kingspan do it on the sips roofs osb -membrane ,vertical battons then cross battons +slates I have seen some that like a second layer of membrane between the 2 sets of battons as another water shedding layer should n,t be a a problem if VPC is correct and edges tape sealed well. roof should be cold anyway with a draft up it If your OSB is warm --then insulation in roof is crap +air leaks and if you really want to be belt and braces use the peel and stick type of membrane on the osb , nothing getting through that from outside Link to comment Share on other sites More sharing options...
Crofter Posted February 2, 2019 Share Posted February 2, 2019 How much moisture is actually within a structure that has an impermeable polythene vapour barrier on the inside, and a breather membrane and rainscreen on the outside? 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 2, 2019 Share Posted February 2, 2019 On 02/02/2019 at 00:09, Crofter said: How much moisture is actually within a structure that has an impermeable polythene vapour barrier on the inside, and a breather membrane and rainscreen on the outside? It depends on the local conditions, as water vapour migrates in to the cooler outer layers of the structure from outside when the outside air is warm and moist, and then migrates out again when the conditions reverse. This can be quite a dynamic process (hence my concerns about the validity of steady-state condensation risk analysis) as conditions can change very quickly over the course of a few hours. For example a wet night that soaks the outer surface and ground, followed by a sunny morning with little wind that then tends to create warm, high humidity conditions in any external ventilated cavity, may cause water vapour to migrate inwards towards the cooler layers on the outside of the structure (cooled by lower temperatures the previous night, perhaps). As the humidity outside drops, this water vapour should be able to move back out again before the outer layers of the structure cool again. The trick is to ensure two things; that water vapour doesn't condense inside the structure (as the phase change energy needed to get liquid water back to vapour is high), and to ensure that water vapour can always move outwards through the structure quickly enough through the day to deal with the dynamic changes in outside temperature and humidity. 1 Link to comment Share on other sites More sharing options...
scottishjohn Posted February 2, 2019 Share Posted February 2, 2019 I see what JS is saying but you have to have some faith that a breathable membrane actually works --they are a one way device -they breath out wards,not inwards so providing there is an air gap above it and not trapping condensation or liquid water by having horizontal batons at sarking face instead of vertical ones,then cross batons for slates ,etc then air movement + heat build up will cause evaporation of water vapour from the surface of the membrane and by evaporation through the membrane from the sarking below . VPC on inside to stop water vapour from inside of house getting through the insulation to the outer cold surface of the roof. this is attention to detail on these layers cannot be over stressed -use correct tapes to seal all joins and overlaps in these membranes and don,t be skinny with overlaps vapour /condensation inside the VPC (in living area of house) ,is in the main caused by you living +breathing ,showering ,etc ,etc So it is down to having a good ventilation system ,which is why on air tight house MVHR or some sort of mechanical system is not a choice but a must Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 2, 2019 Share Posted February 2, 2019 Breathable membranes work both ways equally well, they aren't just a "one way valve", if they were then they wouldn't work well at all. Link to comment Share on other sites More sharing options...
scottishjohn Posted February 2, 2019 Share Posted February 2, 2019 how you figure that ? how can all makers of membranes say they will be water proof for up to 6 months,with no other layer above them , if they were not basically a one way valve ,as you call it. why bother fitting them if water passes both ways just as easy???? might as well go back to tar paper or roofing felt--which was just water proof - Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 2, 2019 Share Posted February 2, 2019 You are confusing water vapour permeability with liquid water permeability. The membranes won't let liquid water through (in either direction) but they do allow water vapour through (in both directions). 1 2 Link to comment Share on other sites More sharing options...
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