Go to Forum Home Building Refurbishment and Retrofit External Insulation & Window Sill extensions

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    • #31356
      Anonymous

        I have a 1925 solid wall house, currently rated a tragic G1. I'm on the cusp of finalising my order for new good quality DG UPVC (I know, I know) windows and the only thing that's coming between me and no howling draughts is deciding on the window sill detail.

        As part of my overall plan, I'm going to have 100mm of phenolic external insulation and will therefore need extended window sills and I know the standard approach is to simply put powder coated aluminium over the existing sills. However, I spent months agonising over my choice of windows because the house is in a lovely canal side location and whilst it's not of any great architectural merit itself, I can't help feeling that I can either improve matters aesthetically or make them a lot worse, depending on my choices. I finally chose some Evolution Storm white wood grain windows because they have a pretty good u value, won't rot (sorry, but I have advanced experience of rotten wood windows) and look as near as I can get in maintenance free to the ones already there. They come with matching wood effect sill, so it seems rather counter intuitive to ruin the whole effect by bodging aluminium over it. However, predictably, the window company doesn't cater for EWI, so I can't just order a deeper matching sill.

        I got quite excited thinking about having a UPVC stub sills and concrete sub sills, as many of the canal cottages around me have stub sills and brick/stone sub sills. However, it didn't take me too long to realise that concrete sills would be lovely great heat dispersing thermal bridging fins, not to mention the logistics of cantilevering 13cm over the render in a solid wall.

        Andy (Simmonds) kindly put me onto the idea of plastered EPS and suggested that I post here for further ideas and advice. I'm going to ring the EWI company tommorrow, but I just wondered if anyone had any experience with this sort of retrofit product? My limited understanding of the product is that it's not going to have the thermal bridging difficulties of concrete or stone and it looks pretty good, but I wasn't clear about the practicalities – most notably what kind of sill I should be asking for with my upvc windows? I'm no architect, but my impression is that the houses around here have stub sills and then masonery sub sills, but I don't know whether it would be possibly to install a window with a stub sill, wait for better weather, then install the EWI and retrofit the sill – sounds unlikely to me.

        The only other alternative that I've come up with is to stick with the powder coated aluminium extension
        sills and have windowboxes to hide them – but that doesn't work around the side of the house and I *will* notice 😉

        Any thoughts/advice or experience with window sill extensions would be greatly appreciated – even if just to say that I'm obsessing unecessarily and aluminium looks & functions fine!

        Thanks,

        Tania
        I

      • #37394
        Paul Buckingham
        Participant

          Hi Tanya,
          I'm currently working on a house with a similar problem. My problem involves sash windows though, which are fitted inside the building so the positioning of the windows are pretty much fixed, yours sounds like you could actually move your windows forward and bridge between the existing wall and the EWI and eliminating or reducing thermal bridging around the window. One way I have been looking at to do this is make an 18mm plywood box to line the window opening from the internal wall to the limit of the EWI which would give you a solid base to mount the window into and allow you to fit the window forward of its original position and eliminate the need to extend the window sill.
          I'm not sure if this is any help to you, if you have any further questions about this please PM me.

          Paul.

        • #37395
          Nick Grant
          Participant

            Hi Tanya

            Attached is what a couple of friends did with timber sashes and EPS+render that I think looks fine in aluminium. However is not the usual extrusion but custom made. Better if this doesn't fix back to the wall for the thermal bridge reasons you mention.

            Not in a position to advise re actual details of fixing and ensuring no leaks.

            Nick

          • #37396
            Anonymous

              Hi ,
              anybody got a link to a UK supplier for alli. sil extensions , been using German one , but would prefer UK .
              thanks

            • #37397
              Mike Whitfield
              Participant

                I have had them made by a local sheet metal business and they were cheaper than imported, with only 2 week lead time. If you're drawing up ones with upstands on the ends, remember to make sure they weld them to the upstand at the back….Mike

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