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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthorjfb
    • CommentTimeMar 19th 2019 edited
     
    I offered to have a look over a friend's plans for their extension.
    Overall I am fairly impressed .

    Looking at the foundation details I was wondering if it is possible to remove what looks like a cold bridge at the floor/wall junction. The 300mm wide trench blocks directly under the timber frame seem to break the insulation continuity from wall to floor.

    Is that something like Marmox thermablocks can resolve in this situation easily?
    Any comments welcomed
      Screen Shot 2019-03-19 at 21.09.16.png
    • CommentAuthorjfb
    • CommentTimeMar 19th 2019
     
    hoping this might make it easier to see the writing
      Screen Shot 2019-03-19 at 21.10.05.png
    • CommentAuthorgyrogear
    • CommentTimeMar 19th 2019
     
    er... I thought a strip foundation needed to be around 3 X the width of the supported wall ?

    gg
    • CommentAuthorDarylP
    • CommentTimeMar 20th 2019
     
    massive thermal bridge ... see the TRADA std detail...
      TF_BB_junction.JPG
    • CommentAuthorjfb
    • CommentTimeMar 20th 2019
     
    Daryl I have looked online for your detail but can't find it - there are many more standard details from Trada that seem more like the ones I posted. Do you have a more comprehensive image of the detail or a link?

    Is the essential difference that the sole plate is dropped down to the level of the insulation in the floor (doing away with the final trench block on my drawing) so that there is more overlap of the up stand insulation with wall insulation?

    Your detail is for a cavity wall not timber frame - does that have any particular impact on detail (and positioning of periscope vents)?
    •  
      CommentAuthorfostertom
    • CommentTimeMar 20th 2019
     
    Posted By: gyrogearI thought a strip foundation needed to be around 3 X the width of the supported wall ?
    Not a gd guideline. Bearing area (therefore strip width), regardless of wall width, depends on the bearing capacity of the soil at foundation-bottom depth. Building Regs gives guidance. Plus, as a separate over-riding rule, strip projection beyond the wall face, inward and outward, must not exceed the thickness of the strip.

    E.g. if weak soil under a 140mm wide wall necessitates a 600mm wide strip, then strip projection inward and outward needs to be (600-140)/2 = 230mm, therefore the strip must be 230 thick, and you may want to consider other foundation solutions.
    • CommentAuthorgyrogear
    • CommentTimeMar 20th 2019
     
    Thanks for that, Tom, I am much the wiser !

    gg
    • CommentAuthorgravelld
    • CommentTimeMar 20th 2019
     
    Looks like the sole plate is a bridge? Is this SIPs? It's a well known issue with a lot of SIPs systems. Could become a magnet for condensation (not literally :-D )
    • CommentAuthortony
    • CommentTimeMar 20th 2019
     
    I think that the sole plate will rot due to condensation forming under it on top of the block or dpc as it a very cold place and made worse by the vents ducts.
    • CommentAuthorjfb
    • CommentTimeMar 20th 2019
     
    tony - in the original drawing or daryl's or both?

    What do they do to prevent the problem if they want to use beam and block and timber frame walls ?
    (it is timber frame with blown cellulose infill I believe not SIP panels)
    • CommentAuthortony
    • CommentTimeMar 20th 2019
     
    External insulation down into the ground to mitigate the thermal bridge
    •  
      CommentAuthordjh
    • CommentTimeMar 20th 2019
     
    Have they considered replacing the whole foundation and floor with a passive slab? Much simpler and can easily be better insulated.

    The original drawing is better than the TRADA one from the point of view of the sole plate. Best to keep timber above floor level if possible. External insulation would certainly help with the thermal bridge. Thermoblocks would also help but you'd need to check with them about loading and about securing the timber frame to or through them.
    • CommentAuthorjfb
    • CommentTimeMar 21st 2019
     
    thanks djh - i think access is a real issue which is maybe why they have gone down beam and block route.

    I think I will suggest they incorporate something like this detail i have found elsewhere to mitigate the thermal bridging - though the periscope vents do make it all pretty awkward
      Screen Shot 2019-03-21 at 08.31.10.png
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