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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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    • CommentAuthorrsk1
    • CommentTimeJan 8th 2019
     
    In my offshot kitchen i plan to pull down the ceiling to open up the roof space, and insulate the single pitch roof. Am considering using wood fibre fixed to the rafters since space is not in short supply, but i don't know what membrane or permeability i need on the inside. The roof is felted, which i guess is not very vapour permeable, so the risk is moisture building up behind the insulation, on the rafters. I have heard that the inner membrane needs to be 3 times more permeable than the outer, but don't understand why: if the aim is to shed moisture outwards, surely it needs to be the other way round? Please could someone explain the moisture / permeability considerations in this context??

    Alternatively, do I need to ventilate the roof, and how would i do it? replace the ridge tiles with something vented?
    • CommentAuthortony
    • CommentTimeJan 8th 2019
     
    If there is a void it needs to be ventilated

    I don’t like the concept of shedding moisture through the roof, if something goes wrong it could lead to condensation, smells, rot or drips.

    In the early days of breathability I heard it said five times more permeable on the outside than on the inside so with felt and no ventilation you would have needed a vapour barrier, now they would insist on breathable sarking.

    I would say don’t do it without ventilation but ensure airtight to the room.
    •  
      CommentAuthordjh
    • CommentTimeJan 8th 2019
     
    Posted By: rsk1I have heard that the inner membrane needs to be 3 times more permeable than the outer

    No, no, no. The usual wisdom is the other way around - 5 times greater resistance internally. But so-called breathable constructions can safely be designed with different arrangements.

    The roof is felted, which i guess is not very vapour permeable, so the risk is moisture building up behind the insulation, on the rafters.
    do I need to ventilate the roof, and how would i do it? replace the ridge tiles with something vented?

    I'm not quite clear what covers your roof. Is the external face of the roof covered in felt or tiles? By felt do you mean roofing felt or a membrane? What are the layers in detail? What's underneath the ridge tiles, and any other tiles? What's underneath the felt?

    You probably do need to ventilate the roof but exactly how will depend on the construction.

    What's an offshot kitchen, BTW?
  1.  
    ''What's an offshot kitchen, BTW?''

    Come to Yorkshire and we'll show you! What might be called a 'back addition', but usually contemporaneous with the house, not an afterthought. Typically on a terrace, but sometimes also on semis, a single or 2-storey part, narrower than the full rear elevation, normally leaving a window to look out of the main rear room next to the offshot, with the back door out of the side of the offshot. In a typical 4m-ish - wide terrace, the offshot can be as little as 2m wide.
  2.  
    Posted By: Nick Parsons''What's an offshot kitchen, BTW?''

    Come to Yorkshire and we'll show you! What might be called a 'back addition', but usually contemporaneous with the house, not an afterthought. Typically on a terrace, but sometimes also on semis, a single or 2-storey part, narrower than the full rear elevation, normally leaving a window to look out of the main rear room next to the offshot, with the back door out of the side of the offshot. In a typical 4m-ish - wide terrace, the offshot can be as little as 2m wide.

    AKA a lean to ??
  3.  
    Watch the weight with wood-fibre. If you are looking at Bld Regs compliance (a U value of 0.18) you'd want 220mm approx of wood-fibre with a lambda of 0.042W/mK. That's heavy! (I can look it up if you want). You could maybe cross-batten (again, adding weight), use flexi (light) wood-fibre between the battens and rigid under. Suppliers will offer advice re use of WF in moist areas.
    •  
      CommentAuthordjh
    • CommentTimeJan 10th 2019
     
    Posted By: Peter_in_HungaryAKA a lean to ??

    No, AKA what my grandma had in Bacup. Thanks for the explanation, Nick, BTW. Before running water, all the 'services' were out back in the yard behind the main house. Kitchen, loo, mangle, coal store etc. As Nick says just a long thin rectangular part of the building behind the main part, with a yard outside that led onto a path across the backs and then a garden behind that. (Yes, she lived in a more expensive house than the back-to-backs that dispensed with the garden and just had another row of houses instead.)
    • CommentAuthorrsk1
    • CommentTimeJan 10th 2019
     
    Roof is artificial slates, roofing felt, 2 x 3" rafters 3m long with approx 2 x 5" purlin halfway. The weight is a good point: how to know whether the roof can take it, without having to pay an engineer?

    Re: asking a suppliuer about WF, that is a good idea. I was just hoping someone could explain/confirm the principle of inner membrane needing to be n times less permeable than the outer membrane
    • CommentAuthortony
    • CommentTimeJan 11th 2019
     
    This is an old idea and is basically saying that if you allow moisture into the structure or insulation then it has to be five times easier for it to get out than to get in.

    In your case if there is a void it should be ventilated, with the impermeable sounding sarking in my opinion you should use a vapour barrier internally.

    I would shy away from organic insulation, going for something failsafe instead.

    For a small roof the weight should not be an issue, what size are the purlins and what span?
  4.  
    Posted By: rsk1The weight is a good point: how to know whether the roof can take it, without having to pay an engineer?

    There are tables of timber structures used in buildings covering timber dimensions and spans for various loadings. If you look at the tables and apply your timber sizes and spans to the tables then this will give you the supportable loads.
    In anticipation of the next question - I don't know where to get a copy. I have a copy given to me 30 years ago but I don't suppose the strength of timber has changed much in that time - but the assumptions may have so the tables might be out of date.
    A quick look at my tables with the limited info. you provided above it looks like your roof is constructed to support a dead load of not more than 24,41kg/m2 but there are too many assumptions for that figure to be used with confidence. However that is the lowest loading figure that could be properly constructed but you don't know if the roof was constructed with due regard to the tables or just put up and it will probably be OK.

    Posted By: tonyI would shy away from organic insulation, going for something failsafe instead.

    +1
    •  
      CommentAuthordjh
    • CommentTimeJan 11th 2019
     
    Posted By: Peter_in_HungaryI don't know where to get a copy

    First two hits on google for span tables pdf seem likely
  5.  
    ''I was just hoping someone could explain/confirm the principle of inner membrane needing to be n times less permeable than the outer membrane''

    Although this is theoretically applicable on any plane the idea of the 5:1 ratio was, I believe, developed for walls. In my view, for a ventilated roof, it's a different kettle of fish. I agree (particularly in a kitchen) on the desirability of a VCL on the warm side, but as you get to the cold side, what have you got? In performance terms, not a membrane which is too resistant to water vapour, but a ventilated void which is hardly resistant to WV at all - *as long as you can adequately cross-ventilate it - and I can already see a potential problem there as you may have an 'in' but not an 'out' for ventilation air, depending on what goes on at the top of the (monopitch?) roof.
  6.  
    http://www.greenspec.co.uk/building-design/ventilated-roof-insulation/

    Something like this. You'll want a 50mm ventilation gap between the existing felt and the insulation, and then 150-250mm of insulation depending on type and desired efficiency. So 200-300mm total, ie much more than the existing depth of the rafters.

    You could add a few deeper rafters and/or purlins to support the plasterboard, and insulate between them. The weight of the extra timber and plasterboard will likely be much more significant than the weight of insulation.Add some ventilation slates to get air in at the top of the roof, and at the eaves if there isnt a gap there .

    Or retain a flat ceiling and insulate upward from it?

    Get a local builder to advise you on typical build up, or the building control officer, no need for an engineer.
    • CommentAuthorrsk1
    • CommentTimeJan 12th 2019
     
    The rafters are 50 x 80mm at 450 spacing, 2.7 m long . The purlin is 60 x 160mm, 2.3m long. The roof is monopitch with angle ridge tiles at the top. I'm thinking that the purlin will count for a lot in terms of weight bearing.

    I would not bother insulating between rafters, fix insulation directly to the them.

    In terms of ventilating the roof, there is currently no gap at top or bottom. I could add ventilation slates, but how many would I need? with no lateral airflow across the rafters, wouldn't I need a ventilation slate betweenn every rafter pair? What other options are there for ventilation?
    • CommentAuthortony
    • CommentTimeJan 12th 2019
     
    What is the roof pitch?
    • CommentAuthorrsk1
    • CommentTimeJan 12th 2019
     
    35 degrees
    • CommentAuthortony
    • CommentTimeJan 12th 2019
     
    I would put a small piece of flat ceiling in at the top say 600mm wide and cross ventilate above that then three tile vents will do the job for you up there or high level vents just under monopitched ridge tiles through the wall
    • CommentAuthorrsk1
    • CommentTimeJan 13th 2019
     
    These both sound like good solutions..
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