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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorbenawhile
    • CommentTimeNov 4th 2018
     
    We have moved into a 1950's house with minimal insulation.

    There is a conventional loft space between a hipped roof and plasterboard ceiling with 4” rafters. There is 4” mineral wool insulation between the rafters, which I want to increase by about another 6 inches, either using Celotex, Steico wood fibreboard or similar, or wool insulation roll, sheepwool or mineral, and then part board over for access and storage. If I use wool then I expect I will have to use plastic loft legs. I have also found insulated loft boards available from B & Q with 100mm insulation glued underneath, so we might try wool for the non accessible areas and insulated board for the storage areas.

    I have read that wool insulation over the ceiling joists in a cold loft should be covered by a membrane to prevent the circulating cold air from wind etc from “wicking” off warm air at the top layer of the insulation.
    What kind of membrane should be used for this, as I have also read conversely that a non permeable membrane will cause condensation on the underside as water vapour rises from the house through the ceiling?

    Also, for the boarded sections, will the board itself act to prevent heat wicking away?
    If I use Steico or Celotex, laid across the joists, should I put conventional loft board above that, to protect it, or below it to stop it being compressed where it stands on the joists?

    For what it's worth, currently the eaves, with fascia and soffit boards, are fully enclosed with no ventilation, so the only loft ventilation is by any accidental gaps between soffit and brickwork, fascia board and felt, and at joints in roofing felt under the concrete tiles.
    We have white coloured but probably aluminium framed double glazing, cavity walls but no c.w.i.
    • CommentAuthorjamesingram
    • CommentTimeNov 5th 2018 edited
     
    I believe some use a breathable roofing felt or a lighter version, I've heard it done vintage Scandinavia and the US not in the UK much though
    • CommentAuthorjamesingram
    • CommentTimeNov 5th 2018 edited
     
    Laying PUR board (celotex).over then boarding will work . It's currently gone up in price though and it e pensive. DIY shed do loft board feet which clip on existing joists and allow 300mm odd of quilt underneath this might be a more cost effective solution. Where there's no board go for 300mm + quilt. There's a few offers on at the moment £11 for combination rolls.
    It's also worth checking with your energy supplier to see if you can get it for free, if you're on any benefits you may. Once installed you can do your boarding work.
    • CommentAuthorCWatters
    • CommentTimeNov 5th 2018 edited
     
    <blockquote>There is 4” mineral wool insulation between the rafters..</blockquote>

    Did you mean the existing stuff is between the joists?

    4" deep joists aren't very strong so be careful how much stuff you go putting up there.
  1.  
    * Ah!*

    Started writing this just after the OP appeared, and the other comments were not there then. I think you mean it's insulated between the joists, not the (sloping) rafters, don't you?

    In that case ignore my best practice guidance re insulating rafters!

    Last night's post (not completed till this morning, had said:


    Is it the original roof? Is there (1) no sarking felt, (2)original '50's-ish bitumen felt, or (3) breathable membrane. If the 1st 2, 50mm gap between bk of tiles and insulation recommended, though if the tiles are good and gappy, you may get away with 25mm in scenarion (1). You want ventilation at each eaves roughly equivalent to a slot of 10-25mm (opinions vary - I often use 15) across the wholes elevation, as unobstructed cross-ventilation. Some like ridge vents as well but if it's an open loft space I find them rarely necessary. I see you have v little eaves vent. at present. There's another thread going on re round soffit vents. You may need *lots*! Slots are good, as far as I am concerned.
    • CommentAuthorCWatters
    • CommentTimeNov 5th 2018
     
    If the joists really are only 4" deep the max recommended span for use as a floor is less under 2m. You may want to think about putting in new deeper joists alongside the existing.

    Best practice is to raise the new ones up about 1/2" so they don't rest on the plasterboard and only bolt/nail them to the existing rafters at the ends or above a supporting wall. That way any load on the new joists doesn't bend the existing joists and avoids plaster cracks in the rooms below.
    • CommentAuthorjamesingram
    • CommentTimeNov 5th 2018 edited
     
    A lot of old houses have 4"/2" as ceiling joist with a centre binder to stiffen them up. If you sensible they can take light to medium weight storage.
    • CommentAuthortony
    • CommentTimeNov 6th 2018
     
    I always like to see a ventilation space above any quilt and below any storage deck. I do 50mm space and use 47x50 raising pieces

    Why store stuff though.
    •  
      CommentAuthordjh
    • CommentTimeNov 6th 2018
     
    Posted By: tonyWhy store stuff though.

    Indeed. We saw not having a loft to get full of stuff as a positive attribute of our eccentric roof design :bigsmile:

    And pretty much everything that was in our previous loft was either given away or discarded (recycled), not brought with us when we moved, so what was it there for?
    • CommentAuthorbenawhile
    • CommentTimeNov 9th 2018
     
    Thank you folks, this was very helpful, especially CWatters re stronger joists. Yes, I was talking about joists, not rafters, sorry. I have had trouble getting notifications hence the late reply. I have bookmarked this thread but can't see anywhere to choose notification method.
    James I have now had advice not to use celotex as it would be going over the cold side of the present fibreglass insulation between the joists and would cause condensation between celotex and fibregalss.
    Could you give me a link to the D I Y Shed loft board feet?
    •  
      CommentAuthordjh
    • CommentTimeNov 9th 2018
     
    Posted By: benawhileI have had trouble getting notifications hence the late reply. I have bookmarked this thread but can't see anywhere to choose notification method.

    Sadly, that's one of the shortcomings of this site. There are no notifications, and the RSS doesn't update; you need to visit the site regularly.

    James I have now had advice not to use celotex as it would be going over the cold side of the present fibreglass insulation between the joists and would cause condensation between celotex and fibregalss.

    That sounds like dubious advice to me. As long as the celotex is thick enough, the temperature on its warm side will be above the dewpoint so there won't be any condensation. You will get condensation on cold membranes, but not on warm insulation. Personally, I'd go with loft roll anyway.
  2.  
    Posted By: benawhileI have now had advice not to use celotex as it would be going over the cold side of the present fibreglass insulation between the joists and would cause condensation between celotex and fibregalss.



    Posted By: djhThat sounds like dubious advice to me. As long as the celotex is thick enough, the temperature on its warm side will be above the dewpoint so there won't be any condensation. You will get condensation on cold membranes, but not on warm insulation. Personally, I'd go with loft roll anyway.

    wot djh said +1

    Posted By: benawhileCould you give me a link to the D I Y Shed loft board feet?

    Google loft board feet and choose !
  3.  
    • CommentAuthortony
    • CommentTimeNov 11th 2018
     
    I just used 50x47 wooden props actually 300 long on top of mr trusses and 50x47 bearers, props at 800mm c/c
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