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Green Building Bible, Fourth Edition
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    • CommentAuthorrytech
    • CommentTimeJul 29th 2017
     
    Hello,

    I am new here so please accept may apologies if I have posted this in the wrong place.

    I wondered if anyone may have advice on how best to calculate the timber bridging factor where the
    Engineer specifies the use of double studs? I make it as 2 x 38mm studs =76mm
    600 cts - 76/600 = 0.127 or 12.7%
    400 cts - 76/400 = 0.19 or 19%

    The default factors within the U-Value conventions are 12.5% and 15% so the above seems a little low? Any advice would be gratefully received.

    Many thanks,
    Ryan
    • CommentAuthorgoodevans
    • CommentTimeJul 29th 2017
     
    Just a guess but do you not need to take into account horizontal timber as well.
  1.  
    Is this a TF house? Look for horrible detailing on sole plates.
    •  
      CommentAuthordjh
    • CommentTimeJul 29th 2017
     
    What do you mean by 'double studs'? Two studs with one inboard of the other? Two studs fastened together side-by-side (why would that be necessary except at specific places?). Why has the engineer specified their use? Which U-value conventions are you referring to? What is your overall target/goal?
    • CommentAuthorrytech
    • CommentTimeJul 29th 2017
     
    Yes, its a timber framed timber clad construction. The panels were detailed with studs doubled up (side by side) at 600mm cts. I believe this is for racking resistance for wind loading etc.

    The U-Value conventions are the 2006 edition by Brian Anderson (BR 443)

    I guess the default values will have the head and sole plates accounted for too.
    • CommentAuthorgoodevans
    • CommentTimeJul 29th 2017
     
    I'm no expert for timber frame but would diagonal members or a structural skin (e.g. OSB) not resist racking (in my day we called it a shear wall). If memory serves me right only one 'bay' in each plane needs this shear stiffening.
    •  
      CommentAuthordjh
    • CommentTimeJul 30th 2017
     
    As Paul says, racking resistance is usually provided by a suitable panel, such as OSB, or sometimes by diagonal members (as in roofs). The aim is to triangulate the structure. Additional vertical members do nothing to triangulate it so racking resistance seems like an unlikely explanation.

    BR443 is not a particularly good basis for thermal calculations, IMHO.
  2.  
    Posted By: rytechEngineer specifies the use of double studs? I make it as 2 x 38mm studs =76mm


    Posted By: rytechThe panels were detailed with studs doubled up (side by side) at 600mm cts.

    A bit off topic but could someone explain the advantage of double studs rather than 1 x 76mm stud. Could it be the availability (or cost) of 38mm timbers over 76mm timbers and are the doubled studs nailed together.
    •  
      CommentAuthordjh
    • CommentTimeJul 30th 2017
     
    Posted By: Peter_in_HungaryA bit off topic but could someone explain the advantage of double studs rather than 1 x 76mm stud. Could it be the availability (or cost) of 38mm timbers over 76mm timbers and are the doubled studs nailed together.

    Partly availability, yes, since double studs are normally fairly rare. A double stud is nailed or screwed together, but even so it is not as stiff as a single piece of wood so orientation matters. It is slightly less likely to have a flaw since it is kind of poor-man's laminated. In some circumstances it can cause an airtightness trap for the unwary since air can pass between the two studs, but with good design this isn't an issue.
    • CommentAuthorCWatters
    • CommentTimeJul 30th 2017
     
    Is it double studs everywhere or just around openings and in the corners?
    • CommentAuthorrytech
    • CommentTimeJul 30th 2017
     
    Double studs to gable panels.
    • CommentAuthorneilu
    • CommentTimeAug 4th 2017
     
    Just ring up the technical department of whichever insulation you are intending to use and they will do you a U-value calculation over the phone and email it to you. I do this all the time with my work.
    One item you'll need to know is the type of breather membrane that will be installed. Low emissivity membranes give better U-values.
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