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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.

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    • CommentAuthorwill t
    • CommentTimeMar 13th 2022
    Hi, I'm about to start a retrofit with an extension on my 1930s semi. The plan is to eventually insulation the whole house externally but before I do that I'm going to have the 50mm cavity insulated with bonded graphite EPS beads.

    The front and some of the side of the house cavity was filled with white fluff (mineral wool?) around 10 years ago by the previous owners but they only did half the job because of difficult access.

    To have this existing insulation taken out so the whole cavity can be filled with the EPS beads will nearly tripple the cost of the work. The company says they can leave it in and just fill what isn't already filled if I want but I'm wondering if it's a bad idea in the long run?

    Could the white fluff slump over time? It doesn't seem to have so far but if I'm externally insulating and covering I guess it will be very alkward to fix if it does slump.

    Is it worth doing just from a u-value point of view or is the money better spent elsewhere?

    Thank you!
    • CommentAuthorandyman99
    • CommentTimeMar 13th 2022
    I had exactly the same problem at my last house, although the lack of full fill was almost certainly not accidental. The companies that came out to quote for EPS beads refused to go ahead without removal of the existing mineral wool and as you have already found out this is expensive. I can sort of see their point, but in my case there was so little in the cavity that it really wasn't necessary to remove. Unless you live in a high risk area I would just go ahead with the EPS fill.
    • CommentAuthorRobL
    • CommentTimeMar 13th 2022
    I certainly wouldn't bother removing fluff, then refilling with beads. It is very important to fill the cavity before EWI though - any air leaks would reduce the EWI effectiveness. We have a mix of different CWI fills - mostly fluff done professionally over 10 years ago. I diy bead filled lots of places I found unfilled a couple of years ago when I EWI-ed. Our house originally had some brick, some render, and some trim panels - when I took the trim away (slatted white boards) there were huge gaps visible into the cavity. I foam filled these, drilled holes, then blew in beads. The rendered areas were unfilled too, a lot of them I had easy access too as I moved windows out.

    Most relevantly, I closed the top of our cavities - our house is a 1963 build, and none of the cavities had any closers at the top - which would have been a horrible thermal bypass especially with EWI. I had 2 layers of scaffolding fitted when doing the EWI- the upper layer was there so I could lift tiles and fit closers (I used 50mm XPS that I just happened to have, foamed into place, angled so water getting to the felt would drip away. As the felt had sagged so much over the years, I think if we had have ever actually had water getting to it, it would likely have ended up in the cavity :-( But it never happened :-)

    Perhaps a bit gung-ho, I foam filled the apex-es of our house, drilling lots of holes a foot away from the top, squirting in water, and foaming. I wanted to reduce the amount of air escaping at the top due to the complete lack of cavity closers and low fluffing there, and I didn't fancy removing those tiles - I'm not great with heights and didn't fancy clambouring around on the roof.
    • CommentAuthorowlman
    • CommentTimeMar 13th 2022
    I have a similar problem inasmuch as the original sheet polystyrene that was inserted at the build stage was very poorly done and also too thin for the cavity.
    On occasion when I have seen the cavity, because of window replacement etc., the white polystyrene was flapping about and I'd wondered if it's possible to fill/blow something alongside the existing stuff and what would be the best material to use.
    The other problem I have is the nature of the walls with lots of corners, windows and door inclusions in the walls and I wondered if, assuming something could be done, would partial treatment of only the good sections be any good.
    Unless you DIY I think the biggest problem you'll have may be getting a mainstream CWI co. to do something 'non-standard'. When I was trying to get a firm out to fill my (Victorian, and variable) cavities prior to EWI the first thing I did was tell them I did not want a CIGA guarantee, because I knew it would be so far outside the standard parameters to be a no-no for a guarantee.
    • CommentAuthorbxman
    • CommentTimeMar 13th 2022
    Am I right in thinking you are going to fit Exterior Wall Insulation (EWI)?

    Which will be rendered with a reinforced modern render there will be no chance of air movement through that render which will be 60,80.100 or even 120 mm away from your outer brick work, I know I have solid walls and that provides a thermal mass inside the insulation .
    I am confident that wind and water will not get through the render without it being physically damaged

    While you have a cavity wall will that still not provide a thermal mass for you inside your external insulation .
    I would be inclined to just crack on with the EWI

    I take it you are not going to leave the air bricks exposed when you have the EWI done ?

    Any way good luck and keep us posted as to how you get on .
    • CommentAuthortony
    • CommentTimeMar 13th 2022
    No real point in doing EWI if there is a possibility of draughts in the cavity, therefore it should be filled prior to EWI and thermography used to find any bits that have been missed. I worry about draughts getting into the cavity from below the g/f, service ducts, under sills, joist ends, porch or garage, eaves box.

    Re closing the cavity at the top, I join wall insulation to loft/roof insulation seamlessly. Closer seems to me to be leaving out wall insulation above it.
    What Tony said, and also heat will rise up through any air in the cavity to colder areas, such as the part of the gable wall that is adjacent to the loft, and so be lost.

    Cavity needs to be verified as fully filled, before doing EWI.
    • CommentAuthorRobL
    • CommentTimeMar 14th 2022 edited
    Here's a pic I took of our roof with a few tiles taken out. You can see the felt is a bit knackered, especially at the bottom, with lots of holes.
    • CommentAuthorRobL
    • CommentTimeMar 14th 2022 edited
    Pulling back the felt you can see inner and outer. There's no cavity closer. The EWI goes to just under the wooden boards at the bottom. Guess I could've removed them and joined up directly, but my back had had enough.
    • CommentAuthorRobL
    • CommentTimeMar 14th 2022
    Piece of XPS. It was foamed into place, joined up with white insulation above that had been stuffed in from loft side ages ago, with a 50mm gap above everything to felt above. I pushed any leftover bits of fluff I had into the top of the cavity just before I did this, last chance!
    • CommentAuthorwill t
    • CommentTimeMar 14th 2022
    Thanks for all your comments. I will leave what's there already then but get them to check with their camera or something while they're here. Just from feeling around at the top it doesn't seem to have slumped but at some point along the side I know it stops. They seem pretty happy to do whatever I want and know my end goal for the house.

    Haven't quite worked out the eaves detail yet because the loft will likely just have mineral wool insulation with cold roof for some years before possibly converting the loft and switching it to a warm roof later on.

    Just sorting out the cavity fill will do for now as now is a good time to make a mess of the internal walls (the height of the back wall + party wall means they might need to fill some of it from inside)
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