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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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    • CommentAuthorlineweight
    • CommentTimeJan 15th 2019
     
    With domestic rear extensions in particular, the scenario often arises where you want to build right up to the boundary line, where a neighbour has already built right up to the line on their side.

    It's usually required by BC that whatever wall you build continues to comply if the neighbour were to demolish their extension, and your wall then became an external wall.

    You can do a standard cavity wall, outer brick leaf built overhand very close to the neighbour's wall.

    That eats up quite a lot of potential floorspace though.

    I guess you can also do a blockwork wall with internal insulation.

    But is there any way to create an 'externally' insulated wall in this situation?

    I assume the answer is 'no'.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 15th 2019
     
    Prefabricate the wall panel on the ground (stud) and erect it. Masterboard or something facing instead of render finish.
    • CommentAuthorlineweight
    • CommentTimeJan 15th 2019
     
    Posted By: fostertomPrefabricate the wall panel on the ground (stud) and erect it. Masterboard or something facing instead of render finish.


    Would require some careful thinking about what happens along its bottom edge...I guess you would build up to DPC in masonry and work out some kind of drip detail that would overhang once the panel was vertical, or something like that.
    • CommentAuthortony
    • CommentTimeJan 15th 2019
     
    Ignore BI and put insulation against their wall, screw in ties and only build new internal skin.

    I wrote an article about this called extend a party wall
    • CommentAuthorJeff B
    • CommentTimeJan 15th 2019
     
    <blockquote><cite>Posted By: tony</cite>Ignore BI and put insulation against their wall, screw in ties and only build new internal skin.

    I wrote an article about this called extend a party wall</blockquote>

    Wow, I wouldn't like to try that! I've only lived in a detached house so the issue has never arisen but my son's neighbour objected to him breaking up the concrete pads to replace fence panel posts, even though the panels were falling down onto his side!
    • CommentAuthortony
    • CommentTimeJan 15th 2019
     
    The party wall of a terrace or semi is a shared wall.

    I think when extensions are built in terms of best use of space, sustainability and common sense the party wall should be extended by the first person to build with the neighbour having a right to use the wall

    Sometimes a party wall is 225mm and the width plaster to plaster in two rear extensions 725mm wasting 0.5m of space and rendering it unusable
    •  
      CommentAuthorfostertom
    • CommentTimeJan 16th 2019
     
    There's a strong Party Wall procedure in London, but elsewhere ikt's much vaguer. Discuss with Building Inspector and hope common sense will prevail.
  1.  
    Why not suggest to your neighbour that you put insulation up fixed to his wall and then build your single skin creating an insulation sandwich (taking care to stop air wash on your side of the insulation) and sell this to your neighbour as an advantage to him as he gets a wall insulated free.
    • CommentAuthorCWatters
    • CommentTimeJan 18th 2019 edited
     
    With domestic rear extensions in particular, the scenario often arises where you want to build right up to the boundary line, where a neighbour has already built right up to the line on their side.


    I suspect in many cases the problem isn't so much the wall but how the two roofs join/seal. Especially if the neighbours don't want anything to overlap their side.

    If their wall is high enough the best way might be to cut into their wall and insert flashing, but I don't think you can do that unless it's a party wall (which it won't be if it's entirely on their side?)
    • CommentAuthortony
    • CommentTimeJan 18th 2019
     
    I have done it lots of ways, even with a gutter running the length of it, best avoided if at all possible
    • CommentAuthorlineweight
    • CommentTimeJan 18th 2019
     
    Yes, how the roofs join is often tricky. It tends to be different in every instance, according to what's physically there and how co-operative the neighbour and/or their party wall surveyor (if there is one) is.

    Easiest is to try and end up with a parapet higher than the roof each side. Then each side can flash into it independently. When you have two walls back-to-back you can easily end up with a very clunky parapet though.
    • CommentAuthorlineweight
    • CommentTimeJan 18th 2019
     
    Posted By: Peter_in_HungaryWhy not suggest to your neighbour that you put insulation up fixed to his wall and then build your single skin creating an insulation sandwich (taking care to stop air wash on your side of the insulation) and sell this to your neighbour as an advantage to him as he gets a wall insulated free.


    In this instance we presume the neighbour's wall is already insulated (relatively recent build). But the issue is the BI wants something that still functions/complies if the neighbor's extension is taken down. That includes wind load structurally (not such a big deal to get around). If you create a sandwich then if the neighbour's extension comes down then the insulation is going to go with it. I'm trying the argument that EWI could (at least in theory) be installed, from the neighbour's side, in this scenario.
    • CommentAuthorlineweight
    • CommentTimeJan 18th 2019 edited
     
    Posted By: CWatters
    If their wall is high enough the best way might be to cut into their wall and insert flashing, but I don't think you can do that unless it's a party wall (which it won't be if it's entirely on their side?)


    I think the technical term in this scenario - when it's not a party wall - is that you 'enclose upon' their wall.
    •  
      CommentAuthordjh
    • CommentTimeJan 18th 2019
     
    Posted By: lineweightEasiest is to try and end up with a parapet higher than the roof each side.

    Not so easy to produce a thermal-bridge-free solution, though.
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