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General: Residual air gap or in cavity 50 or 25mm?

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      CommentAuthorJustin
    • CommentTimeNov 29th 2008
     
    I’m trying to make the best of a fairly standard wall specified for a small extension. This a little job on a small site which needs to interface in several places neatly with 1970’s wall, so it can’t go ever so thick. Hence fairly standard 103 brick, 95 cavity, 100 thermalite block is suggested. Common specification for this is PU partial fill with 45mm Kingspan, maintaining nominal 50mm air gap. I see this is a common combo, with unspectacular performance (though meeting part L1b minimum U of 0.3). It crops up most places for small brick extensions.

    But Kingspan tell us that cavity residual air gap can go down to 25mm on <12M high work, yet searching building approval applications on .gov websites, this freedom seems to be rarely made use of. Is this thought to be for improved ventilation, or do specifiers really fear that workmanship will be so poor to the extent of requiring such a large margin? (It seems that for new build, a NHBC guarantee requires these big margins). I have no particular interest in NHBC, but shall be watching stuff gets done properly myself.

    So it sensible for a small extension where I’ll either be doing work myself, or watching my brickie closely, to push this limit more closely and specify (for example) 60mm PU, leaving a nominal 35mm gap, (not to fall below 25).

    I see that that Kingspan put it this way:

    "The specifier may either:
    • design a cavity width by consideration of the dimensional tolerances of the components which make up the wall by reference to the British
    Standards relating to the bricks, blocks and insulation boards, or use the data from their respective manufacturers.
    In addition, allowance may need to be made for the quality of available building operatives and the degree of site supervision or control; or
    • design a nominal residual cavity width of 50 mm (a residual cavity nominally 50 mm wide will be required by the NHBC where normal
    standards of tolerance and workmanship are adopted)."
    • CommentAuthor10100
    • CommentTimeNov 29th 2008 edited
     
    i have been looking at this myself recently, and Building Regulations Part C 5.13 B stated you need 'a cavity at least 50mm wide'

    im gonna question my BC inspector, as Celotex and Kingspan specify their products with a Low E 25mm cavity, where it seems in practice you need a 50mm which makes there insulation not fit in a 100mm cavity. (to a good u-value)

    note, it also lists a BS standard, i will see if i can get that now and see what it says.

    edit
    says in the BS

    a cavity is usually at least 50mm wide, and further on in the BS - partial fill cavitys should have a minimum target of a 50mm air space. but there are other BS' about different insulation types.
    •  
      CommentAuthorJustin
    • CommentTimeNov 29th 2008
     
    Hi 10100
    I agree it says 50mm in Part C. Also it says "Urea-formaldehyde foam inserted into the cavity should be in accordance with BS
    5617:1985109 and be installed in accordance with BS 5618:1985110." (I have no access to those BS unless I buy them).

    Kingspan says "a minimum 25 mm cavity is recommended," So I'd like to know what you find out.
    • CommentAuthorFred56
    • CommentTimeNov 29th 2008
     
    Building regs demand that you meet the requirements of the regs. The technical solutions it proposes are satisfactory options but there may well be others. I'd reckon that if Kingspan have a BBA cert or similar that approves their 25mm cavity then the BCO will accept it. NHBC is another matter, if you want their warranty, you'll have to meet their spec.
    • CommentAuthor10100
    • CommentTimeNov 29th 2008 edited
     
    i will get the BS' in a minute and check what they say

    i cant seem to get 5617, but 5618 is a little over whelming! it has some nice calculations to work out the cavity width required.
    •  
      CommentAuthorJustin
    • CommentTimeNov 29th 2008
     
    Hi Fred,
    Thanks.

    For the exposure zone I'm in (2..3), If I take a 95mm cavity, then Part C recognises that I could chose either full fill material (which I take to be glass/mineral wool), or partial fill material (which I take could be polyisocyanurate if I chose that). In the latter case, the insistence of a 50mm air gap (so limiting the foam to only 45mm thickness) actually makes the glass batts more thermally efficient (and probably cheaper) than the PIR foam.

    Taking the Kingspan recommendations on does give the performance edge back to the PIR foam, but only at the risk of upsetting BCO and having a 25mm residual air gap? For sure I'd better just check with BCO next week.
    • CommentAuthor10100
    • CommentTimeNov 29th 2008 edited
     
    i'd agree with asking the BCO Justin, as i know when i have done Celotex calculations it has said in the u-value calculation to use a 25mm low e cavity (but, 50mm for Zurich and NHBC)

    so i guess the 25mm with celotex is ok with Building Control, but i bet that will also depend on the Building Control you use!
    • CommentAuthorpmusgrove
    • CommentTimeNov 30th 2008
     
    I hate to show my ignorance but how does this need for an air gap match with the AECB fully-filled cavity design for their CarbonLite Silver standard? Also the NBT show designs with no cavity other than a 25mm air gap externally between the cladding and external woodfibre board.
    • CommentAuthorMike George
    • CommentTimeNov 30th 2008 edited
     
    I looked into this some years ago and the issue [then] with the 50mm airgap was down to stipulations in the BBA Certification.

    Edit. Checked this and there is a little more explanation here http://www.ribaproductselector.com/Docs/8/06738/external/AG943047.pdf?ac=

    see pt 7.7 regarding exposure

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