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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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.

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    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014
    The architect has suggested the following roof build up for our proposed new build......

    Vapour open roofing membrane draped by 8mm over rafters
    30mm air gap
    Foam insulation between rafters
    Foam insulation below rafters
    Plaster board.

    Target U-value of 0.11W/m2k

    The question is, does this leave the Timber on the cold side of the insulation and would it be better to have the Timber on the warm side by putting the insulation aboutve the Timber?

    Also would you go for OSB sarking over the whole roof?
    • CommentAuthorowlman
    • CommentTimeDec 5th 2014
    Personally I like OSB sarking as it gives a good firm surface for subsequent tiling, other benefits aside.
    If tiling, you'll need vertical counter battens before the actual tile batten, which together with the OSB thickness and insul. may affect the overall finished roof height, which may impinge on other aspects like flashings, design, and the interior too if you're working to an overall planning height restriction etc..
    • CommentAuthorringi
    • CommentTimeDec 5th 2014
    Are you limited on roof height due to planning or matching other buildings?
    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014 edited
    There is no mention of a height restriction in our planning approval.

    Would you place the insulation just below the sarking, rather than where it is now?
    • CommentAuthorJamster
    • CommentTimeDec 5th 2014
    Over top of roof = less cutting / fitting / filling of slabs between rafters but you will want a detail of how you join the roof insulation to your roof insulation; this could be an overhang of foam outside the final rafter. However, it might effect your timings, and it might up the cost of a roofer to do the work rather than yourself from inside.

    We went for in between in the end as we wanted to get the roof on before the weather completely got on top of us, but it did seem to take a while to fit and we used a lot of squirty foam!

    Another very good point that was made to me on here was that if you aren't using the space for a room in the roof, you are considerably increasing the volume of space in the house that you are committed t heating...

    My thoughts only as a self-build project manager mainly - some real hands-on experience will be along soon I a sure...!
    Is it single battens or double cross?
    • CommentAuthorCWatters
    • CommentTimeDec 5th 2014
    We built our house (room in the roof) with all the insulation between the rafters. I wouldn't do it that way again as there are too many gaps to be filled. I'd build a warm roof with all the insulation above the rafters or perhaps a SIP construction.
    • CommentAuthorEd Davies
    • CommentTimeDec 5th 2014
    What's the airtightness layer?
    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014 edited
    The air tight layer is under the plaster board.
    • CommentAuthorPaulJ
    • CommentTimeDec 5th 2014
    If you go for above and between rafters, you need at least the same thickness above as between. If you go for above only, the fixings for the battens can be very costly.
    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014
    Posted By: PaulJIf you go for above and between rafters, you need at least the same thickness above as between.
    looking at a previous email from the architect he suggested 1/3 above and 2/3 between the rafters. What is the logic for having the same thickness above and between?
    • CommentAuthorEd Davies
    • CommentTimeDec 5th 2014
    I surmise that the thinking on the half & half meme is something like that if the indoor relative humidity is around 50% then the dew point will be about 10 °C lower. Since the UK doesn't tend to have differences between indoor and outdoor temperature of more than 20 °C for extended periods limiting the surfaces exposed to condensation (the outside of the rafters in this case) to half the temperature drop should keep them dry most of the time. If they're cooler they could go many days, weeks or months with liquid water in place with consequent rot potential.
    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014
    So what happens if the insulation is between the rafters and below? Will the timbers be better protected?
    • CommentAuthorEd Davies
    • CommentTimeDec 5th 2014
    Yes, because the water vapour can escape (and the inboard insulation probably helps stop it getting into the wood in first place).

    “Foam” (I'm assuming PUR sheets rather than something sprayed as you don't distinguish from the stuff inboard of the rafters) between rafters with essentially no wind protection outboard of it would worry me as it's not obvious there won't be thermal bypass due to air blowing down the gaps between the foam and the rafters. It'd need tricky sealing which would be difficult to inspect.
    • CommentAuthorTriassic
    • CommentTimeDec 5th 2014 edited
    According to the drawing the whole roof insulation is Celotex or similar! I had wondered about fitting the insulation between the rafters, i agree fiddly, but doable. I had in mind OSB sarking to the exterior (silicone sealed to the rafters as per Viking House) to stop the wind.

    The whole roof is rather thin, so I'm not sure if the insulation between the rafters (150 deep) could be changed to rockwood or similar.
    • CommentAuthorringi
    • CommentTimeDec 5th 2014
    The option is to use breathable insulation above the rafters, e.g. wood fiber boards.
    • CommentAuthorwoodgnome
    • CommentTimeDec 5th 2014 edited
    How about
    Counter batten
    Non tenting membrane sealed at laps
    Wood fibre sarking
    Mineral wool full fill between rafters
    Polyurethane insulation
    Vapour Barrier
    Battens/counter battens/plasterboard/plaster.

    Edit..Thanks to David Freeborough.
    • CommentAuthorEd Davies
    • CommentTimeDec 5th 2014
    I'm using more-or-less woodgnome's suggestion except:

    - metal instead of slates,
    - timber boards instead of wood fibre sarking (with more membrane inside for additional wind protection),
    - rafters as timber I-beams,
    - vapour/air barrier outboard of the PUR (for protection from service fixings and compression/mechanical fixing of the laps)
    - not plasterboard in most parts of the house.

    235 mm I-beams with full-fill mineral wool between, 90 mm PUR inboard.
    Posted By: TriassicI had in mind OSB sarking to the exterior (silicone sealed to the rafters as per Viking House) to stop the wind.

    If you do OSB then I wouldn't recommend silicone sealing to rafters. Get T&G OSB and use a polyurethane D4 adhesive like Caberfix designed for the job.

    I found silicone sealing doesn't work well in practice, it invariably is VERY messy, a decent sized roof uses a huge amount of tubes of silicone and you have the problem of how you deal with the unsupported long edges.

    I'd pay the few quid extra for the T&G OSB 3 and save hours and do a neater job.
    • CommentAuthorwoodgnome
    • CommentTimeDec 6th 2014
    If using OSB getTG4. This is the smaller sized sheets that has T&G on all 4 edges. The bigger sheets only have T&G on 2 edges i think.
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