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Housing - Renovation: Resizing a window opening

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    • CommentAuthorwookey
    • CommentTimeMar 19th 2019
     
    I'm replacing the windows on the north wall (along with EWI). I need to change the size of 2 or 3 of them. One to avoid kitchen cupboards overlapping window, and to allow for the insulated floor to be higher, another to make the pointlessly large north-facing toilet window a sensible size.

    Question is, what's the best way of doing this? It's easy enough to add a row of bricks along the size to make it 220m smaller, but windows are attached to the wall and rely on it being solid. How does one tie the bricks in well enough to support the window fixings properly? Do I need to cut out the half-bricks and tooth them in (sounds like work). Or use one of those channels that bolt to the wall? Or drill in ties of some sort that go in the mortar layers? Suggestions welcome.

    This is also going to have to be a summer job, because now there is along pause between taking out the old window and putting the new one in (whilst one changes the size and waits for it to set). Not much to be done about that...
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      CommentAuthordjh
    • CommentTimeMar 19th 2019
     
    Posted By: wookeyQuestion is, what's the best way of doing this?

    I don't know whether it's the best way, but an alternative to brickwork might be to make a timber frame and bolt that to the existing wall. I'm concious that our windows are screwed to OSB boxes that are screwed to 2 no 2x4 (i.e. 4x4 equivalent) uprights, and they rest on OSB on straw bales underneath. So even though they're heavy triple-glazed and some are quite large, OSB and fence posts are strong enough to mount on :)
    •  
      CommentAuthorfostertom
    • CommentTimeMar 19th 2019 edited
     
    As Dave hints, an outboard-cantilevering OSB box to mount your new window into can be fixed to existing masonry and/or equally to any secure timber framework - studwork infill, or just a post fixed top and bottom.

    The latter can mean that re-shaping window/door openings, esp if it's a reduction in opening size, can be done just with the EWI blocks if thick enough (150, 200, ... 300 thick?) to have some strength. A post/transom, to define the new reduced opening jamb/cill or whatever then both restrains the free edge of the EWI and mounts the OSB box. Internally you're left with a curious recess (where the existing masonry hasn't been extended) which can be made into some shelves/bathroom cupboard, a curtain draw-back space, or anything ingenious.

    This is another benefit of EWI - window/door opening modifications become easy-peasy - even if only because the usual careful DPC/cavity tray etc work isn't necessary - the existing masonry becomes just a dumb lump that can be modified any way that's convenient.
    • CommentAuthorjms452
    • CommentTimeMar 21st 2019
     
    You could use these:

    https://www.screwfix.com/p/sabrefix-wall-starter-kit-stainless-steel/56037

    Green hat used something similar to this to build out a block wall to be covered by EWI.
  5.  
    I would go with dhj's suggestion of building in a timber frame to resize the opening and then EWI over on the outside and do what ever you want t the reveals on the inside.
    • CommentAuthorwookey
    • CommentTimeMay 3rd 2020 edited
     
    OK. So I did as suggested and built a timber frame (50x75timber) (faced on the outside with 18mm OSB so there is something to fix the EWI to. I put 60mm of XPS (Green bit) inside the frame to fill it. The cavity is closed with more XPS.

    So now I have a finished 'wall' in the filled-in bit ~90mm thick. The rest of the surround is masonry. The airtightness is the interior plaster. So the question now is how to 'finish' this bit of wall so as to get continuous airtightness from the window to the plaster.
    Here's a pic, hopefully, which should help:
    http://wookware.org/pics/online/greenbldg/resized-window-1.jpg

    So I could just plaster right across, connecting the wall plaster to the new reveal, but I'm not sure plaster will stick to XPS properly. Or I could tape an airtight sheet across and embed into plaster at old reveal. Or I could put a piece of plasterboard over it but then how do I connect that to the old and new reveals airtightly (plaster those and skim across the board too? use some blowerproof somehow (I've got a bit to try out)?)

    The plan for the windows is for them to be taped with plaster-in-tape to the reveals.

    This seems like it should be easy, but it's not totally obvious how to do it right. Suggestions welcome.
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2020
     
    So the bit on the left in the photo is the new wall? Are you going to use the revealed space for something - bookshelf, plants etc?

    I'd be tempted to put some Intello or something in, taped to the window at one edge and into the plaster elsewhere, and then cover with plasterboard or woodwool board and plaster over the top. I'd certainly have tape or membrane behind all the edges that are junctions, so you're not relying on the plaster never to crack there.

    I see Blowerproof as an alternative to tape and perhaps the membrane, but I've never used it myself.
    • CommentAuthorwookey
    • CommentTimeMay 3rd 2020
     
    yes LH side is the new wall. The OSB is covering the window-hole.

    Yes. Might as well put some shelving in the gap after it's finished airtight. Toilet reading, spare bogroll.

    I don't have any intello, and it seems like overkill to get a whole roll just for this. I do have breathable roofing membrane (Cromar vent3 https://www.cromarbuildingproducts.com/products/vent3-classic/, and something very similar from YBS). One wonders idly to what degree those membranes do essentially the same job as intello? I did wonder about the issues of burying bits of timber in the structure. It'll be butted up against the woodfibre EWI, which should dry slowly to the outside, and keep the inner timber above dewpoint except a few days/year. But obviously air leaks from inside could deposit enough moisture on very cold days to be an issue which is why I'm trying to make sure that there aren't any. A membrane is one way of doing that.

    I guess I'll do it with a piece of plasterboard, use blowerproof on all the joins (as I've got some), then plaster over that. Should work...
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2020
     
    The XPS is airtight, so I think you'd be OK just putting tape (or Blowerproof) over the timber and connecting the XPS with the plaster and window. I would try to put the airtight barrier behind the plasterboard. You can always make the plasterboard connections airtight too, but the chances of cracks appearing at the junctions eventually are quite high I think.

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