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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Products: window render beads

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      CommentAuthordjh
    • CommentTimeApr 20th 2018
     
    Back in 2012, we had a discussion of aluminium window sills, which included some discussion of self-adhesive render beads to join the render to the windows.

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9731

    This is an update about the render beads. I used the Wemico ones that were discussed (I see their website now actively blocks me from looking at it because I don't have Javascript enabled, so I'm not going to post a link). They were very good at build time, working well to finish the render neatly and simultaneously protect the window. For the most part they still look good and are working well.

    However, there's one window (actually a sliding patio door) that faces south and gets a lot of sun where the render bead has pulled away from the window and opened a gap of a few mm. So I'm going to need to fill the gap with some mastic or silicone etc. I'm just letting the area dry out in the sun.

    I think what's happened is that the render has moved a bit, presumably because the straw behind it has moved a bit. So I don't think any amount of stickiness would have kept it in place, or another way to put it something had to give and I'm glad it was the adhesive.

    And I'm very glad my sills did have upstands on the end! (not that there's one under the door)
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      CommentAuthorfostertom
    • CommentTimeApr 20th 2018
     
    I don't for a minute trust the double sided foam tape that's supposed to provide the ultra-critical watertightness (for the next 50?/100? years) at that vertical junction where driving rain will sluice it with concentrated water.

    I see the tape as something to hold the render bead in place for a few weeks max until it can be embedded in the render. Then the real watertight seal is applied, into the ideal 3mm gap that the foam tape creates, between hopefully scrupulously cleaned surfaces - a gunned bead of quality silicone mastic of recommended depth:thickness ratio.

    This is not what the likes of Wemico recommend - they really do think their DS tape is good enough alone.

    The only problem with the above, is that the DS tape is sometimes not recessed back into the joint depth but comes flush with the render face. Worth checking on your chosen make/model.
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      CommentAuthordjh
    • CommentTimeApr 20th 2018 edited
     
    Posted By: fostertomThen the real watertight seal is applied, into the ideal 3mm gap that the foam tape creates, between hopefully scrupulously cleaned surfaces - a gunned bead of quality silicone mastic of recommended depth:thickness ratio.

    Actually, I doubt that would have stayed sealed either in my case. In general the foam tape that is part of the bead does seem to have held up for a few years now at any rate.

    Given that silicone is notorious for pulling away from the substrate after a while, are there any other suggested ways of sealing the gap?
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      CommentAuthorfostertom
    • CommentTimeApr 21st 2018
     
    Posted By: djhsilicone is notorious for pulling away from the substrate after a while
    Is it, if ideally dimensioned, prepared and applied? We see some heavy-duty uses of such sealants that look very good longterm. Like edge-to-edge glass half way up a skyscraper.
    • CommentAuthorgravelld
    • CommentTimeApr 23rd 2018
     
    Thanks for the report @djh.

    Surely silicone doesn't have the flexibility of a foam tape. I'm not sure what does cause silicone to pull away after a while, I assumed it was the failure of adhesion given movement.

    Of course, it's probably also useless talking about "silicone", there's such a vast difference in performance between different products.

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