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  1.  
    Hi all,

    I asked this question many months back but I'm still after some clarification and reassurance.

    I'm keeping an area of my timber framed bungalow as original as possible for multiple reasons. The original buildup was direct to stud ship lap weatherboarding, 3" frame and internal lining. The frame will be thickened up with a new 4" stud work internally to strengthen and give more insulation and services capacity. One side of the house is having the old weatherboards covered up by foam fixed EPS and render (this worked well on a test room).

    This question relates to keeping the original weatherboards exposed. The building has stood for 80 years and the original boards and timbers are mostly healthy except where water has ingressed around rotted windows. My intention is to leave these external boards as they are (they'll be painted), and the stud bays insulated.

    I've read on one US website about leaving an air gap behind the insulation in direct fix weatherboard applications in order to allow drying and provide air tightness to the internal. Sounds sensible. I could fix celotex (foamed around for air tightness) or similar with an air gap behind, and fill the rest with glass wool before membrane and plasterboard, or staple tyvek with an air gap behind it and fill the bays with glasswool.

    Any helpful suggestions? Naturally I don't want to rot away a structure that's done quite well without my intervention so far.
  2.  
    Anyone?
    • CommentAuthorEd Davies
    • CommentTimeMar 7th 2017
     
    Since nobody else seems to be responding I'll have a go: Celotex sounds like a bad idea to me as it's pretty much vapour impermeable. How about wood-fibre board of some sort to give a rigid but vapour open layer to hold the mineral wool out of the ventilation space? You could even use pegboard, perhaps.
    • CommentAuthortony
    • CommentTimeMar 7th 2017
     
    I would form a 15 to 20mm void behind the weather boards with vertical battens three per bay, staple building paper to them from inside then fill everything with quilt, inside this vapour/air barrier combined and internal linings over that.
  3.  
    Thanks both.

    Could I use Tyvek house wrap in the bays over the battens? I've got plenty lying around.

    After filling the remainder of the bays with something vapour permeable, could I finish with a layer of foil backed celotex / kingspan over the studs internally perhaps combined with plasterboard or battens then boards, in order to bring the U value up safely?
    • CommentAuthortony
    • CommentTimeMar 7th 2017
     
    Yes, sure, that sounds like a great plan
  4.  
    What's the technical reason not to use rigid impermeable insulation between the studs, out of interest? Is the concern that the studs at some point may contain moisture due to the weather conditions and particular build-up and be unable to dry out?
    • CommentAuthortony
    • CommentTimeMar 8th 2017
     
    Probably but most of North America has quilt filled studs so it must be a misconception.
  5.  
    '' Is the concern that the studs at some point may contain moisture due to the weather conditions and particular build-up and be unable to dry out?''

    I recently had to dismantle a section of timber-framed wall which was insulated between and over posts with Pu. A combination of dross-filled ('clear') cavity and incorrectly-detailed perimeter drainage had allowed moisture in, and (although I have no objective proof for this) the PU seemed to have held the moisture tight to the posts, damaging them badly. I had never been particularly happy with rigid plastic insulation in TF, but am even less so now.
  6.  
    I guess the traditional approach of quilt does seem more sensible then!

    With drying in mind, is it worth leaving it vapour open to the inside to allow complete breathability in case of moisture build up, or would this simply encourage it? Intelligent membrane might make sense.
    • CommentAuthorringi
    • CommentTimeMar 8th 2017
     
    Posted By: alexoftheorchardWhat's the technical reason not to use rigid impermeable insulation between the studs,


    After you have done it once cutting the insulation to fit, with each stud being at a different angle and spacing, you will know.......
    • CommentAuthortony
    • CommentTimeMar 8th 2017
     
    I would strongly recommend vapour barrier on the warm side of the insulation, not vapour open.
  7.  
    Thanks to all.

    Will implement the recommended accordingly. Now to figure out how best to flash around new uPVC windows...
    • CommentAuthortony
    • CommentTimeMar 10th 2017
     
    I usually used a head drip, uPVC overfascia, at 20 to 25 tilt being careful bot to obstruct the sashes opening and two more bits that mastered the ends of the cladding boards and sat on the sill board now wider by about 65mm overall.

    I dont like admitting to having used uPVC but this was down to the specifier.
  8.  
    Do you have an example drawing Tony? I've looked at plenty of stuff online but nothing specific to UK windows (mostly yank stuff) on a weatherboard house.
    • CommentAuthortony
    • CommentTimeMar 11th 2017
     
    I can do a freehand chronic sketch and may be able to find a photo
  9.  
    Yes please Tony.
    • CommentAuthortony
    • CommentTimeMar 11th 2017
     
  10.  
    That's just a shortcut link I'm afraid Tony.
    • CommentAuthortony
    • CommentTimeMar 11th 2017
     
    This detail also works with slate or tile hanging

    https://dl.dropboxusercontent.com/u/446863/20170310%20Window%20detail.pdf
  11.  
    Perfect, thanks Tony. That's much as I'd expected but the head detail seems particularly sensible.

    I'll stick in some bits of house wrap prior to window and bits of fascia to give additional protection, and use a 150mm sill to bring any water well away from the boards. Given the hefty eaves overhang, the house is protected quite well regardless. Amazingly, the completely rotted out windows haven't let in all that much water over the years. Only the bottom couple of weatherboards were knackered, and that's due to heavy 1:1 mortar at the base of the pebbledash prohibiting natural drainage (since removed).
  12.  
    Posted By: tonyI would form a 15 to 20mm void behind the weather boards with vertical battens three per bay, staple building paper to them from inside then fill everything with quilt, inside this vapour/air barrier combined and internal linings over that.


    Going back to this thread, I've had a re-think. How about covering the original uninsulated 3" studwork with Tyvek entirely internally, building the new 4" studwork up against it filled with glass wool, and covering over with 50mm celotex or similar, battening off for services and pboard? This will save farting around with battens in the old stud bays and give me a more airtight build-up, utilising the original wall as a kind of elaborate weatherboard cavity. Any thoughts?
    • CommentAuthortony
    • CommentTimeMar 19th 2017
     
    This is up to you, you will loose more internal space and don't forget the vapour barrier
  13.  
    The new 4" stud work is happening regardless to stiffen the structure and better support the roof. The compromise is a few cm of glass wool, and it'll probably be much more air tight and a lot less hassle this way.
    •  
      CommentAuthordjh
    • CommentTimeMar 19th 2017
     
    I haven't really thought this through but how about stapling a breather membrane around the inside of the original studs and dipping into the space between the studs. You can staple the membrane to the sides of the studs to get a more accurate size of cavity. Then add the new studs inside and fill the whole lot with rockwool or whatever. Then a VCL and a service cavity if you want, again filled with rockwool.
  14.  
    Posted By: djhI haven't really thought this through but how about stapling a breather membrane around the inside of the original studs and dipping into the space between the studs. You can staple the membrane to the sides of the studs to get a more accurate size of cavity. Then add the new studs inside and fill the whole lot with rockwool or whatever. Then a VCL and a service cavity if you want, again filled with rockwool.


    That was pretty much the original idea, but I'm trying to simplify by regarding the original studwork as the outer breathable layer as there's very little insulation depth there anyway (assuming I allow for an air space). The new studwork will butt up to the old, this time filled with glass wool, with a thinner layer of rigid insulation over the top and battens for services. Considering the u value of the east side of the house is going to be about 0.14, it's going to cost me a bit of inside space on the west side (with the weatherboards) if I want to preserve the original external aesthetic whilst keeping the whole building well insulated.
    • CommentAuthorIan1961
    • CommentTimeMar 21st 2017 edited
     
    <blockquote><cite>Going back to this thread, I've had a re-think. How about covering the original uninsulated 3" studwork with Tyvek entirely internally, building the new 4" studwork up against it filled with glass wool, and covering over with 50mm celotex or similar, battening off for services and pboard? This will save farting around with battens in the old stud bays and give me a more airtight build-up, utilising the original wall as a kind of elaborate weatherboard cavity. Any thoughts?</blockquote>

    Sounds like a good plan to me assuming that you have enough space internally to accommodate the build-up. That spec is pretty much what I used on a new build that I've just finished except that instead of the 3" studs that you will have supporting the weatherboarding we had just 1". (Also our studs were 140mm instead of your 100mm)
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