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    • CommentAuthordereke
    • CommentTimeJul 14th 2017
     
    I am building an office down the end of my garden. As mentioned in this thread http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15127 I am building off a screw foundation.

    The building is 4x3m with a 3m wide opening for bifold doors.

    I was going to go with a 150mm floor filled with Celotex XR4150
    150mm timber stud walls again filled with XR4150 with 35mm Isolair sarking on the outside (http://www.pavatex.com/products/roof/isolair) and larch cladding.

    On the roof 150mm rafters with XR4150 between then and 52mm of Isolair on top. 4° pitch covered with sheet metal.

    Does this sound OK? Do I need some kind of vapour control layer?
    I was wondering about using 20mm Isolair internally as well instead of plasterboarding. Anyone know what the finish is like on it?

    The reason for the sarking board on the outside is for extra insulation but also I was under the impression I could fix the sheet metal and the cladding directly to it - is this the case or do I need to batten it as well?
  1.  
    Yes, you need a VCL. I prefer to use quilt or loose-fill insulation in TF.

    I have not used Isolair, but I have used its 'sister' products, and this makes me 90+% certain that you would have to batten the Isolair.

    With the lay-up you suggest you will only have cloaked the thermal bridges of the studs with 35mm of an insulant with a lambda only about half as good as Celotex. Isolair is breathable, which is great, but Celotex isn't, which isn't great! If sticking with PIR, why not 25mm PIR inside the studs, behind plasterboard? Forget the external insulation layer and fix battens for the tin directly to the studs, perhaps.
    • CommentAuthordereke
    • CommentTimeJul 14th 2017
     
    25mm of PIR doesn't sound like it would be very effective? Am I missing something?

    By my (probably wrong!) calculations 150mm of celetox and the 35mm of sarking should get me down to 0.12 U-Value which seems respectable to me?

    Regarding thermal bridging I am not to sure how to avoid this and not have the building fall over - are there any good resource that show the details of how to do this?
    •  
      CommentAuthordjh
    • CommentTimeJul 14th 2017 edited
     
    Posted By: dereke25mm of PIR doesn't sound like it would be very effective? Am I missing something?

    Do the U-value calculation for Nick's suggested buildup as well.

    By my (probably wrong!) calculations 150mm of celetox and the 35mm of sarking should get me down to 0.12 U-Value which seems respectable to me?

    Did you allow for the timber fraction?

    Regarding thermal bridging I am not to sure how to avoid this and not have the building fall over - are there any good resource that show the details of how to do this?

    The easiest way is to put most of the insulation as an unbroken layer over the outside of the studs. My sun room has 100 mm of grey EPS outside and rockwool between the 95 mm studs. Battens over the EPS to attach cladding, and a VCL and plasterboard inside.
    • CommentAuthordereke
    • CommentTimeJul 14th 2017
     
    Hmm I don't know about timber fractions! I'll have to look into that.

    What sort of fixings do you use to attach 100mm of insulation and then also the cladding? In my head I just see it falling off :confused:
    •  
      CommentAuthordjh
    • CommentTimeJul 14th 2017
     
    Posted By: derekeWhat sort of fixings do you use to attach 100mm of insulation and then also the cladding?

    The insulation is attached using 150 mm stainless screws through the battens. The cladding is nailed to the battens.
  2.  
    Dereke, I should clarify. I said: ''If sticking with PIR, why not 25mm PIR inside the studs, behind plasterboard? Forget the external insulation layer and fix battens for the tin directly to the studs, perhaps.''

    If I amend that to: ''If sticking with 150 PIR between studs, why not 25mm PIR inside the studs, behind plasterboard? Forget the external insulation layer and fix battens for the tin directly to the studs, perhaps.'', does that then make more sense (and smaller U value figures)?
  3.  
    ''Regarding thermal bridging I am not to sure how to avoid this and not have the building fall over - are there any good resource that show the details of how to do this?''

    My shed is built of 100 x 100 Segal-type frames with studwork between, but my studs are of 50 x 50, first vertically, then horizontally, so that I could lay 50mm sheep's wool first vertically, then horizontally, so that the 'through-thermal bridge' is limited to the main frame sections and 50mm squares, and at other places it is reduced to 50mm of timber.
  4.  
    ''Hmm I don't know about timber fractions! I'll have to look into that.''

    Imagine a 4m-long stud wall of 150x50 studs with uprights 2m tall (I have ignored head and sole plates for the moment) and you have 11 of them with insulation between. Imagine shoving them all together, so that your wall has (11 x 50mm) 550 x 2000 x 150 deep solid timber for the depth of the studs, and an area 2m x 3450mm of 150 Celotex. The Celotex area is further enhanced by 35mm Isolair, while the poor insulation of the stud fraction is only mitigated slightly by the Isolair. You need to do an area-weighted U value calc:

    (Area of each element x U-value of said element) + (Area of next element x U-value of said element) etc etc
    divided by Total Area of elements
    • CommentAuthordereke
    • CommentTimeJul 15th 2017
     
    Thanks Nick that makes perfect sense (the insulation and the timber fractions).
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