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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    If one is trying to maximise thickness of breathable 'wood fibre' insulation between rafters -ie full depth- is a small 'gap' left between each timber sarking (roof) board sufficient ventilation in lieu of a vented gap above the insulation? If so what sort of gap is required between the boards? Is this approach/detail acceptable to Building Control...I have heard it referred to as 'Scottish Practice'..!

    Related to the above…is a gap above 'wood fibre' insulation actually required if one uses a breather membrane such as 'Pro Clima' atop the roof boarding in a full roof retrofit? Context: Traditional building with Planning restriction so cannot add more insulation atop rafters and need to maximise headroom below.
    • CommentTimeApr 8th 2022
    Posted By: studiodub@mac.comRelated to the above…is a gap above 'wood fibre' insulation actually required if one uses a breather membrane such as 'Pro Clima' atop the roof boarding in a full roof retrofit?
    Can't speak for Scottish regs, or even current English ones, but my roof buildup goes from the inside:

    - plasterboard
    - Pro Clima Intello Plus membrane
    - 450 mm of Warmcel insulation and timber structure
    - 18 mm ply
    - (what is now called) Pro Clima Solitex UM Connect
    - aluminium standing seam

    So no gaps, and no breathability at the very top (no holes at all in the top). The UM Connect includes plastic wool that creates a small gap to allow condensation to run out. Building Control accepted it based on a condensation analysis done by Ecological Building Systems (the Pro Clima agent, using WUFI IIRC) and they also warranted the roof. No problems so far, as the man falling off a skyscraper said :)
    I would say that full depth insulation between rafters is more commonly seen in "Warm Roof" build ups, where there is an equal or greater thickness of insulation over the top of the rafters than in between them,
    (and by top I mean the outside bit, not the underside that would be the ceiling to the room)
    e.g. 100mm rafters with 100mm PIR between, then 100mm PIR over the top, fixed through to the rafters with counterbattens, then your breather membrane, then tile battens (if tiled roof).

    As opposed to the much more common situation (especially in retrofit) where the insulation is between and under the rafters, so 100mm rafters would only have 50mm PIR between with a 50mm air gap maintained at the top between the insulation and the roof membrane (or "roofing felt" if traditional) to ventilate condensation.
    then with additional insulation (e.g.150mm PIR) on the underside of the rafters i.e. the ceiling to the room.

    in a warm roof, the rafters are kept warm, in a cold roof the top of the rafters are cold but kept ventilated.

    I am not an expert and only have experience of warm roof on a recent barn conversion and cold roof on original farmhouse.

    Maximising insulation depth on an existing roof has considerations then via both routes, increase in external roof plane height, or loss of internal ceiling height, but in both cases its very important to consider condensation risk.

    If a new roof is being built, then warm roof has always made more sense to me.
    • CommentAuthorEd Davies
    • CommentTimeApr 9th 2022
    My roof build up in Scotland:

    Internal lining (plasterboard in places, timber in others).
    Service void.
    90mm PUR.
    235mm timber I beams with full-fill mineral wool between.
    22mm sarking boards.
    12mm vertical counter battens.
    Breather membrane draped over counter battens and sarking board.
    35x50 horizontal battens.
    Profile steel (“wiggly tin”) roof ventilated top and bottom and across from north to/from south facing surfaces.

    House designer said full fill would not be allowed. Building control was not convinced immediately, mostly because they didn't fully understand the house designer's description, but when I argued that the sarking boards were ventilated above and that all of the very vapour open mineral wool close to outdoor temperatures was within 75mm of a gap in the sarking board they were happy.

    If I'd been using woodfibre between the rafters, rather than mineral wool, I think they'd have been a lot less convinced and would probably have required the standard 50mm ventilation gap below the sarking board.
    For a bit more background, the building is a traditional form with 150mm rafters in a sensitive Planning context so need to maintain the roof profile as is ie only insulate between the rafters due to the latter with 150mm wood fibre and maximise headroom within. So my thought / question remains: would timber 'planks/boards' (called sarking in Scotland) with a small gap between -clad atop with a Pro Clima breather membrane- suffice with full fill insulation in lieu of a 50mm vented gap atop 100mm wood fibre and thereby worse U-value.
    • CommentTimeApr 9th 2022
    I think Ed gave the clearest answer yet to your question in his last paragraph. Namely, he doesn't think so, but you will have to check with your building control to know for sure. And FWIW I think you will need some well-informed organization with documentation behind you to succeed in that discussion.
    • CommentAuthorGreenPaddy
    • CommentTimeApr 11th 2022
    I'm sure this has come up previously, as I vaguely recall commenting on this, since I've been designing and building roofs with sarking and full fill insulation for at least 12 years. That is in Scotland, but the regs tend to be mostly identical, or slightly stricter in Scotland, due to our more challenging climate.

    Definition of warm roof versus cold roof has been picked up previously, but this link to a particular product, shows the roof build ups that they certify. AIUI, highly breathable membranes (that's not every roof VPM) can be substituted for this particular brand.


    Just check about the requirements for VCL - not sure what you're planning. I think there may be a need to control vapour levels passing through the roof void. I always use a VCL for airtightness, but check that aspect.
    Posted By: studiodub@mac.comin a sensitive Planning context

    Any more info in this respect, is the building Listed? Is it in a Conservation Area?
    what are the walls and roof covering made of?
    Update: Building Control say sarking detail with gaps between boards (in lieu 50mm gap above insulation) would require membrane BBA Cert allowing such-fair enough. That being the case have negotiated/had assessed with Steico to have-natural slate on breather membrane on 18mm WBP ply on 25x50 batten on a further breather membrane on the rafters with full fill wood fibre ie 25mm / not 50mm vent gap .
    • CommentTimeApr 26th 2022 edited
    What's the purpose of/requirement for a breather membrane underneath the ply? I'd have though all that will do is reduce the vapour permeability a bit more. (Plus raise the vendor's profits a bit / cost you more :cry: )

    edit: FWIW 18 mm ply with a breather on top and full-filled below with Warmcel is what we have. Our 18 mm ply is two 9 mm layers glued and screwed on top of each other with joints staggered so the whole thing is solid and smooth. We did that because the roof is curved so may be overkill for you.
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