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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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    • CommentAuthorAMG
    • CommentTimeJun 12th 2016
    Hi everyone,
    Apologies for the long post. You helped me previously with window details.

    I've just moved into my new build house that I have been constructing over the last few years.
    It's an open plan living dining room.
    The house is externally insulated apart from one side because a neighbouring wall (3 storey) is only 300mm away from my external wall. There is no space to get anything between the houses. As a consequence, this wall is internally insulated by constructing a timber stud wall with insulation in between. The make up of this side elevation is as follows render, block work, 100mm cavity, breather membrane, insulated stud wall, vcl, insulated plaster board, plaster.

    Up until we moved in, no issues had been identified regarding any damp or smell. However, once we moved in a musty smell was apparent coming form the sockets. Taking sockets off, it was clear that builders had cut vcl to sink back sockets. I then bust open the back of sockets and you can clearly smell the damp stale musty air. I could not inspect the cavity between the breather membrane and externall wall. However everything appeared dry on the internal face of the breather membrane.

    I have subsequently re insulated and repaired the vcl later. The smell has reduced significantly but it seems like there is still an odour which suggests that there is background leakage of air into the house. The building inspectors suggested drilling 50mm holes from the external wall so that the internal 100mm cavity can ventilate?
    If this did not help the it would then require busting open the whole wall and doing an instrusive survey. .

    Not sure how good the drainage is between the two houses (the 300mm) gap. Both houses have their foundations and the ground is made up. Possibly the rainfall is not draining away and that the smell is coming from the ground up.

    The floor is a block beam with insulation and a screed for UFH.

    What would you suggest re next steps? Make a hole to ventilate the cavity between false stud wall and external block work? Rip open internal wall and start again with possible tanking of floor/wall etc but not sure what system to use if this route is recommended?

    I feel hugely disappointed and blame myself for not picking up on this earlier. I just did not see it smell anything.
    If you cannhelpnwith any thoughts or advice that would be much appreciated.
    Is it possible to remove a socket box and look into the cavity with an endoscope camera. I got one for a fiver a few years ago and it's been very useful.
    • CommentAuthortony
    • CommentTimeJun 12th 2016
    Is the beam and block underfloor void ventilated?
    • CommentAuthorAMG
    • CommentTimeJun 12th 2016
    Will see if i can insert an endoscope. Just need to figure out how to repair breather membrane somehow afterwards. Yes subfloor ventilated with air bricks all round perimeter.
    Seems to make sense to ventilate cavity, top and bottom of the external wall and hope that it gets rid of any stale air.
    • CommentAuthortony
    • CommentTimeJun 12th 2016
    Is it a drains smell. Could you smoke test the drains?

    It would very unusual for a cavity wall to smell, there are millions of them in the UK that don't smell, what is different about yours?
    • CommentAuthorAMG
    • CommentTimeJun 12th 2016
    No drains in that area and smell limited to that specific area. Could it be dew point is in cavity so there is damp air with possible mould grow giving rise to musty smell??

    Thanks for your comments so far.
    • CommentAuthortony
    • CommentTimeJun 12th 2016
    Shower or bath anywhere above?
    • CommentAuthorAMG
    • CommentTimeJun 12th 2016
    Bedroom above, that has the same construction as living room below re cavity wall and false insulated stud.
    The sockets in that bedroom had the same smell coming through and having now repaired vcl layer, all seems relatively ok (just paranoia left). Only options re smell I can think is outside water getting into subfloor and for whatever reason not being ventilated away. Alternatively somethings within the cavity wall and moisture being formed/trapped.

    I'll see if I can get a humidity sensor and try correlating readings in cavity wall when it rains. Might help to shed light and to What's happening?
    • CommentAuthordickster
    • CommentTimeJun 13th 2016
    If it's rainwater down the 300mm gap that's adding to the problem, an extreme solution might be to attach a 300mm roof over the gap? It might just help.
    The VCL and insulated plasterboard will be preventing the timber studs from drying towards the inside. If the unventilated cavity is at or close to dewpoint then the studs won't be able to dry towards the outside either. So in winter moisture will be trapped in the cavity and in summer it will be driven back into the studs. Over time this could lead to the frame moisture content getting to mould inducing levels.

    A timber frame would typically have a ventilated cavity on the outside. So the standard solution would be ventilation at the top & bottom of the masonry wall. If the 1st floor goes across the cavity then ventilation would also be required above and below it.

    If this is not practical then you could look at changing the build-up. If the VCL was replaced by a variable resistance membrane such as Intello and the insulation/insulated plasterboard replaced with breathable insulation then this would allow drying towards the inside during the summer. If the timber studs were replaced by metal studs then moisture content is a non-issue, but they would obviously form a thermal bridge unless twin frames were used.

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