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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorSprocket
    • CommentTimeAug 13th 2018 edited
    I know, I know. I've read the "CWI removal" thread from 2011 but bear with me a moment...

    Our new project is a 1970s two storey detached property with cavity walls. First thought would normally be EWI.
    But although it is unlisted this house is built from bath stone and is in a conservation area that tends to take this sort of stuff pretty seriously (Bath).

    We are dealing with the roof first as it is leaking already - I don't see any major problems there.
    It's not particularly draughty but glazing upgrades are being considered. Swapping the 464 DG sashes for something better looks pricey but probably worthwhile.

    But 250m2 (ish) of walls with a U value of something like 0.5 is a big concern. I can't EWI (because bath stone in conservation area) and it looks like IWI would be a right pain and compromised by ceiling voids, several window bays, and lots of windows.

    So I am still drawn to the compromise of adding CWI as a quick fix to get that 0.5 U value down. It is looking like that or just live in it as-is and probably burn a lot of gas and put up with cold spots.

    Any thoughts?
    • CommentTimeAug 13th 2018
    We had CWI on our last cavity-walled 1970s house, and it definitely improved the situation. No problems that I remember. So I would go for it; just make sure to find a good installer. What width is your cavity? Can you look inside it with an endoscope camera to see what it's like?
    • CommentAuthorgyrogear
    • CommentTimeAug 13th 2018
    Is there no way to partially EWI ? For example, one facade (or two) where you could get away with it ? Worth making the application ?

    Also, "it is not particularly draughty" - that might imply that there are *some* draughts - they would need nailing for starters !

    You don't mention floors - any insulation improvement possible in this respect ?

    Finally, what chance of improving passive solar gain (sunspace etc.) ?

    Guess it all depends on the available budget...

    Posted By: gyrogearIs there no way to partially EWI ? For example, one facade (or two) where you could get away with it ? Worth making the application ?

    Certianly try getting PP for EWI on rear / side elevations but IMO on point in EWI unless the caiity is properly filled. Al thel cavities that I have known have had gales blowing through them !
    How do you get the U value as low as 0.5 for an un-filled cavity wall?

    112 brick/50 cavity/112 brick is generally considered t have a 'base case' U value of around 1.5W/m2K. 0.55 after CWI.
    • CommentAuthorSprocket
    • CommentTimeAug 13th 2018
    Oops. I pasted wrong number. Should have been 1.0

    I had been playing with this online cavity wall calc:-


    0.5 was, like you say, for cavity filled. Still, it's not great is it? :-/
    The last time we used CWI it was inside between studwork so we managed 100mm.
    Next time I'm there I will go to the roof and measure the cavity and see if I can get a look inside it.
    • CommentAuthorgravelld
    • CommentTimeAug 13th 2018 edited
    I feel that "typical" numbers for a cavity wall are going to be very very rough estimates - a lot will depend on the level of air ingress.

    I think you need to step back and work out a long term goal and then a plan of how to get there, that will tell you whether and how the cavity should be treated.

    Shouldn't the insulant be breathable in a property like this?
    • CommentAuthorSprocket
    • CommentTimeAug 13th 2018 edited
    Goal currently is to do enough now (before moving in) that I won't later-on wish I had done more.

    Air leakage is a big guestimate currently. It was never designed to be proper airtight but I don't notice any draughts anywhere. Of course I've not seen it in winter yet but exterior mortar is thin and pretty tight, roof has sarking board in the pitched areas, and it is all fairly sheltered. Existing sash windows fit well and are not in bad condition (almost seems a shame to replace them). So I think it's probably quite good for what it is. No much point doing a pressure test though.

    I like to put numbers into a spreadsheet... hoping to get a feel for how far to go with everything else.
    Roof is probably the worse currently (it is big and it leaks anyway) but I could get that down to maybe 0.12 (theoretically) with foamglas above the flat roof rafters and celotex between if that looks worth the trouble. And I *could* swap out all the 464 2G (U value maybe 3.0ish) for wider 2G or 3G... I'm looking at prices on that.

    So it is looking like the walls are by far weakest link. All that other stuff would have looked far more worthwhile if the walls could do much better than 0.5. I was thinking about filling with EPS; but it may be moot already... this afternoon I came across an old specification sheet for heating in the place that suggests the walls have already been cavity filled that assumes a U of 0.5 but I wasn't aware they had been done and I can't see any telltale drillmarks on the exterior walls and there are a couple of cavity wall ventilation vents inside that have small draughts which is puzzling so I'm going to have to peer in with a torch and camera to see if it was ever done and what with.
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