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| Green Building Bible, Fourth Edition |
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These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment. PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book. |
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Posted By: davidfreeborough...So once installed the floor forms the principal tie member.
Posted By: CWattersI'm no SE but would the OSB floor be strong enough in tension? Normally the whole joist would be in tension but you can't do that with hangers. I think you would need some horizontal metal straps to tie the joists to the wall plate before putting down the OSB.I'm no SE either, I will be getting one before submitting plans. The idea is that the 18mm OSB floor will be fixed to the joists accordinging to a nailing schedule such that it transfers the tension load to the joists. It may be that straps can be used instead or in addition, that's a good point.
Posted By: CWattersHow do you fit gussets plates to the shear web of an I beam rafter? Presumably you pack out the sides rather than cut the flange? In which case why not use two collar ties bolted one either side of the rafter instead of one fat collar tie and gusset plates.Depending upon the load, you either nail the plates to one flange or you pack out the web & fix to both flanges. The former is better thermally, the latter can take higher loads.
Posted By: CWattersHow thick are your walls? At 45 degree pitch a 400mm deep rafter has a 564mm wide footprint. Are your walls really going to be that thick?Yes, as Nick says, they are that thick. One of the things I like about the I beam rafters is that it allows you to line-up the roof insulation with the cavity insulation at this junction.
Posted By: fostertomIt's all so straightforward with an air barrier of OSB over the rafter tops, insulation laid outboard of that, uniting with similar EWI, brick veneered if you must have a brick external finish.I agree that a true warm roof with all the insulation above the rafters has the potential to simplify this junction & makes sense if you are using external wall insulation. I don't agree that it's straightforward when targeting U values of 0.08 to 0.1. How would you fix counter battens to rafters through 400mm of insulation?
Posted By: fostertomLike inboard air barriers, wide-cavity cavity walls should be consigned to prehistory - they're not 'easy to build' - a completely different thing from the familiar trad cavity wall up to 150 {max) cavity width. An extremely expensive way of providing a brick decorative finish to effectively a single-skin+EWI wall.As you keep saying, but repeating the same message doesn't make it true. Where do you see the additional cost beyond 150mm? For each cost in a wide-cavity masonry wall there is an equivalent one in a solid wall with external wall insulation. With cavity wall construction you can shop around, with external wall insulation you are tied to the specialist vendors.
Posted By: davidfreeboroughHow would you fix counter battens to rafters through 400mm of insulation?http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5494 owlman answered that nicely. Anyway, half your insulation wd go between the rafters, a thinner layer over the top.
Posted By: davidfreeboroughWhere do you see the additional cost beyond 150mm?I see a saving in eliminating one whole skin (albeit partly thickening the remaining one) and width of found. I know you did a gd cost analysis previously - I must go back and look at it properly.
Posted By: davidfreeboroughwith external wall insulation you are tied to the specialist vendorsNot so any more - it's like u/f heating, become a commodity item that you can assemble bitza-style.
Posted By: fostertomWithout wishing to sound argumentative, that's different to what you originally said:Posted By: davidfreeboroughHow would you fix counter battens to rafters through 400mm of insulation?http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5494 owlman answered that nicely. Anyway, half your insulation wd go between the rafters, a thinner layer over the top.?
Posted By: fostertomIt's all so straightforward with an air barrier of OSB over the rafter tops, insulation laid outboard of thatIf the insulation is between the rafters then the air barrier needs to be below the rafters & trussed rafters will give you a sealing problem at the wall plate.
Posted By: davidfreeboroughIf the insulation is between the rafters then the air barrier needs to be below the rafterswhy?
Posted By: davidfreeboroughWithout wishing to sound argumentative, that's different to what you originally said:owlman came up with a very do-able solution to what was the only tricky bit of putting thick insul over the rafter tops (but not 400mm thick anyway - 200 max on top, the rest between the rafters). This way is definitely straightforward compared with, as you sayPosted By: fostertomIt's all so straightforward with an air barrier of OSB over the rafter tops, insulation laid outboard of that
Posted By: davidfreeboroughair barrier needs to be below the rafters & trussed rafters will give you a sealing problem at the wall plate.
Posted By: davidfreeboroughOne of the things I like about the I beam rafters is that it allows you to line-up the roof insulation with the cavity insulation at this junction.
David
I was going to ask how you get the rigid insulation in but then I remembered you were going for mineral wool. Should work well.
Is your BCO likely to insist you to ventilate under the membrane/sarking boards?
Posted By: CWattersIs your BCO likely to insist you to ventilate under the membrane/sarking boards?The Tyvek Supro BBA certificate doesn't require any ventilation as long as if sarking boards are used the insulation is at rafter level. The BBA certificate for Pavatex Isolair woodfibre sarking board requires ventilation of the batten cavity above the boards, but not the rafter space under the boards. If this isn't good enough for the BCO then there's no point using the woodfibre sarking board & I'll probably just use Tyvek Supro pulled tight with taped laps. However, I'd prefer to use the sarking boards to keep the wind out of the insulation & to temporarily store moisture while conditions don't allow it to be dissipated by the Tyvek.
Posted By: fostertomThe classical way to use OSB as an air barrier is to put it on the warm side of the insulation with vapour open board on the outside of the insulation. You seem to be proposing putting the air barrier half way through the insulation. What are you using as insulation? How will you stop warm moist air reaching the less warm OSB air barrier? Will the 200mm of insulation outside the OSB air barrier be vapour open?Posted By: davidfreeboroughIf the insulation is between the rafters then the air barrier needs to be below the rafterswhy?
Posted By: davidfreeboroughHow will you stop warm moist air reaching the less warm OSB air barrier? Will the 200mm of insulation outside the OSB air barrier be vapour open?
Posted By: djhAn inboard air barrier is important for thermal performance though.Why inboard? As long as there's a continuous air barrier preventing movement of air inside to outside it doesn't matter where it goes in the sandwich.
Posted By: fostertomtiles/slates on battens over metallised breather felt, shiny side down, draped over 35x50 downslope battens on 100 EPS or wood fibre on 9 OSB3 glued and screwed (as air barrier cum slight vapour check) to 145 high rafters with blown Warmcel or sprayed Icynene between (as supplementary, long-term robust, un-puncturable air barrier), plasterboard and skim beneath.Interesting build-up. What's the vapour resistance of 100mm of EPS? Is it really negligible compared to the outboard breather? Even 100mm of woodfibre board starts to have a very significant vapour resistance.
Note no VCL - this is a breathing construction, with the 'inboard' vapour check (the OSB) about 5 x the vap resistance of the outboard breather.
Posted By: davidfreeboroughPosted By: CWattersIs your BCO likely to insist you to ventilate under the membrane/sarking boards?The Tyvek Supro BBA certificate doesn't require any ventilation as long as if sarking boards are used the insulation is at rafter level. The BBA certificate for Pavatex Isolair woodfibre sarking board requires ventilation of the batten cavity above the boards, but not the rafter space under the boards. If this isn't good enough for the BCO then there's no point using the woodfibre sarking board & I'll probably just use Tyvek Supro pulled tight with taped laps. However, I'd prefer to use the sarking boards to keep the wind out of the insulation & to temporarily store moisture while conditions don't allow it to be dissipated by the Tyvek.
David
Posted By: davidfreeboroughInteresting build-upThank you - I think! Do you agree it solves problems plus enables hygroscopicity benefits?
Posted By: davidfreeboroughWhat's the vapour resistance of 100mm of EPS? Is it really negligible compared to the outboard breather? Even 100mm of woodfibre board starts to have a very significant vapour resistance.145 Warmcel 6.5 MNs/g, 9 OSB 1.95, 100EPS 15 or, better 100 woodfibre 2.5. EPS still checked out OK last time I did the condensation calc but I'm hoping budgets will stand wood fibre in future.
Posted By: davidfreeboroughWill you core drill the plasterboardyes - plus the installers are used to tacking scrim as necessary where there's leak holes.
Posted By: CWattersSeveral makes of membrane allow it to be used over insulation or sarking boards without ventilation underneath but I hear some BCO still won't let you get away without it. Perhaps worth making your case when you submit the drawings so that any entrenched views can be handled before you start building.Thanks for the tip. I guess they're just going by what they learned in the 1970s & 1980s, but I get the feeling that some BCOs are not open to the changes required to make our buildings energy efficient. They should be leading the way in educating people, not standing in the way. Perhaps we need to be re-training them to understand these issues?
Posted By: fostertomI wasn't being sarcastic. I like the simplicity of the structure & the way that trussed rafters can be used, while still sealing the air barrier to the masonry. With Warmcel & woodfibre board it offers good breathability & hygroscopicity, & the hygroscopic insulation is exposed to the room in a way that allows it to regulate the humidity & temperature of the internal air.Posted By: davidfreeboroughInteresting build-upThank you - I think! Do you agree it solves problems plus enables hygroscopicity benefits?
Posted By: fostertom145 Warmcel 6.5 MNs/g, 9 OSB 1.95, 100EPS 15 or, better 100 woodfibre 2.5. EPS still checked out OK last time I did the condensation calc but I'm hoping budgets will stand wood fibre in future.100mm of woodfibre board is extremely expensive compared to an equivalent thickness of mineral wool & 100mm of EPS doesn't allow a fully-breathing roof. You still getting breathing to the inside, but not in any meaningful way to the outside. It certainly doesn't follow the 5:1 principle of ensuring that the materials on the inside of the insulation have 5 times the vapour resistance of those on the outside.
Posted By: fostertomIt's all so straightforward with an air barrier of OSB over the rafter tops, insulation laid outboard of that, uniting with similar EWI, brick veneered if you must have a brick external finish.Do the rafters terminate at the wall plate, so that the OSB can be sealed to the masonry? If so, how are the fascia & gutters fixed? Do you use tapered counter battens?
Posted By: davidfreeboroughDo the rafters terminate at the wall plate, so that the OSB can be sealed to the masonry?yes
Posted By: davidfreeboroughIf so, how are the fascia & gutters fixed?short extension rafters fixed on top of the OSB, in the outer insulation zone - good opportunity for decorative 'exposed rafters' - I typically use 3x4 doug fir with scroll-shape end, no fascia, gutter on top-of-rafter brackets. Eave (and verge) soffit board of 18 doug fir faced shuttering ply laid across the tops of the 'fake' rafters. These 'fake' rafter ends are laid out to miss the structural rafters beneath, to avoid an all-wood thermal bridge right thro the insulation. Noggings between, on the arris line where the roof and wall OSB meet, to carry the 'fake' rafters' load sideways to the adjacent real rafters, and an inboard binder to hold the inboard ends of the 'fake' rafters down.
Posted By: fostertomI typically use 3x4 doug fir with scroll-shape end, no fascia, gutter on top-of-rafter brackets. Eave (and verge) soffit board of 18 doug fir faced shuttering ply laid across the tops of the 'fake' rafters.Nice detail. How do you support the verge soffit board? Is there a gable ladder in the insulation zone fixed to the OSB/rafters?
Posted By: davidfreeboroughHow do you support the verge soffit board?As you can see, an outboard verge rafter on purlin end. The purlin is also a short 'fake' - over to yr design ingenuity to support it! In the pic, they went and fixed the ply face-grain the wrong way - it shd 'span' across the rafters therefore 1200 lengths cut across the 8x4. Doug fir faced cheap shuttering ply is harder to get nowadays - and isn't fair faced so you need to be allowed to take your pick of the stack at the timber yard! Put a nice 6mm bevel on the abutting edges where visible, going up the verge.
Posted By: davidfreeboroughDo you ventilate above or below the sarking membrane? Have you had any issues with the BCO requiring ventilation as mentioned by CWatters?No, and not so far - I think it's just BCO ignorance, which shd be overcome.
Posted By: davidfreeboroughI assume it works because its the insulation itself which is providing the vapour barrier ... (unlike a polyethylene barrier, for example)That's right, and is perfectly OK. We don't need to think of thin barriers doing the resisting, and expecting the layers between to be inert. It's not the absolute resistance of any layer, thick or thin, but the slope of the gradient (i.e. resistivity) across it, which should start steeper inboard and progressively flatten as you go outward.
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