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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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    • CommentAuthorMarkyP
    • CommentTimeJul 14th 2021 edited
    greetings GBF,

    i've been putting off my EWI plinth for about 4 years and hoping to get it done before the winter. I have PVC starter track and EWI to main facade which is long done and rendered, I am left with about 450mm of blockwork from DPC down to the top of the footing which I always planned to add EWI to. I will leave a small step detail to form a drip and have been mulling over what to cover the EWI with. The tops of the footings arent very deep and I am planning a gravel board about 300mm out to create a trench to fill with leca which will add a bit of insulation and create some drainage, I am on chalk so no other worries about drainage here. We will have a deck or just run the lawn up to the top of the gravel board. I know in the past we have talked here about concrete slabs, not sure these will work for me aesthetically or becuase I dont have deep enough backfill to hold them in place.

    I have been considering either a fibre cement roof slate or a class A cement panel (weatherkem) which is rated for wet conditions. I would fix the EPS EWI with adhesive and then considering using the adhesive to effectively bond the fibre cement slate or cement board to the EWI, using a notched trowel, basically tiling. I will paint the tiles or board with mineral paint in a lighter colour to match the render.

    The cement board is very waterproof, I think must have a lot of resin binder, it's not breathable. No idea on the vapour permeability of fibre cement slate. But not sure here being vapour open would really matter? The adhesive is frost proof and the EPS I think would tolerate being damp, the mortar joints in the tiles would allow a little outward drying.

    what do you reckon?
    The cement fibre board sounds like a good plan, fixed with the EWI adhesive and perhaps a screw through to the wall evey meter or so. If you don't like the colour of the cement board you could put the thin film acrylic render as used for the EWI on it.

    I don't know how much insulation value the Leca would add but gravel or sharp stone would work equally well.

    I don't think being breathable will matter here given that it is below DPC anyway
    • CommentAuthorbhommels
    • CommentTimeJul 14th 2021
    Posted By: Peter_in_HungaryThe cement fibre board sounds like a good plan, fixed with the EWI adhesive and perhaps a screw through to the wall evey meter or so.

    I would leave out the screw. My fake slate boards stay well in place without.
    I have a similar detail to the base of my cladding due to insulated raft foundation formwork.

    Whilst I've found a few 'robust' render solutions which have a bit more grit and are approved for below DPC, I suspect they could still be damaged under moderate impact.

    At the moment, we are looking at self-adhesive (fake lead) flashing strips since these are weatherproof, self-finished and can be moulded over the curved shoulder detail that we have to the EWI.

    Finding a decent primer for the adhesive has been challenging though. The proprietary primer from the flashing manufacturer is solvent based. Can I ask what product you are using as adhesive?
    • CommentAuthorCatherineD
    • CommentTimeJul 14th 2021
    I had foamglas (made from recycled glass) used below the DPC on my house after EWI was installed, The builders accidentally rendered over the top, which wasn't in the plan, but it seems to be OK (more than 10 years later)
    • CommentTimeJul 14th 2021
    Posted By: Doubting_ThomasAt the moment, we are looking at self-adhesive (fake lead) flashing strips since these are weatherproof, self-finished and can be moulded over the curved shoulder detail that we have to the EWI.

    We have Lacomet flashing at the base of our render, overlapping over the top of the vertical paving slabs below. It's curved over a piece of timber. We've found it difficult to join lengths of the Lacomet together, so the overlapped joints spring apart a bit, which looks ugly. I think real lead would hold the curved shape better. It's also fairly awkward forming the flashing into corners.
    • CommentAuthorMarkyP
    • CommentTimeJul 14th 2021
    One issue i've realised today is that sourcing sheets of the weatherkem cement panel isnt easy in small quantities, in fact it may be impossible without a huge delivery fee. I can cover 7.2 linear metres of plinth from cuts from a 2.4 x 1.2 sheet so only need a few. I may be going with plan B which would fibre cememt slates in the 600 x 600 format which I can cut to size, these are available at local builders merchants. They are only 4mm thick but wonder if with a decent adhesive layer being they will be resilient enough.

    for adhesive I will use the usual EWI basecoat/adheisve combination stuff. I used Baumit statcontact for my EWI but may use ceresit this time as I can source a few bags locally. I'll have to test it on the slates to check they bond OK, I guess an exterior tile adhesive might be an alternative. The slates will be smooth so not much key.
    If you are not expecting mechanical damage why not use the usual EWI adhesive with the glass mesh embedded ?
    • CommentAuthorMarkyP
    • CommentTimeJul 14th 2021
    I did consider it and I can plaster a bit so it's not out of the question for me DIY but I just dont fancy the rendering job and then the thin coat to finish, "tiling" with cement panel or slate it will be quicker and will I think be easier to get a decent finish.
    • CommentTimeJul 14th 2021
    Apart from burrowing insect/rodent attack I don't see need for any finish to buried EWI. Sharp aggregate fill should ward off rodents, and gardening plastic fine mesh on the face of the EWI against insects.
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