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As part of a large interior refurb job my clients want to remove a fairly typical deep bay window with a walk in rectangular bay. The finished bay internal will be about 2.6M x 1.0M. The window will most likely be a Swedish 3G unit of one description or another. For the supporting wall below the window what construction should I specify to the builder, in order not to negate any of the advantages of the 3G window? The clients would prefer a brick finish to the exterior. Any help would be gratefully received. Tony???
300 wide fully filled cavity, same U value as the rest of the walls with particular attention to mitigation of an thermal bridging resulting from design
I prefer to see window behind the brick cladding and no thermal bridges from cavity closers, lintels or sills.
If you're pushed for space, then Xtratherm CavityTherm full-fill PIR insulation is another option which would give you a slimmer wall but with traditional-ish cavity construction. It's a bit more pricey than normal PIR insulation, but I've just used it to rebuild a bit of cavity wall, using some k-value = 0.11 W/mK aircrete blocks as the inner skin (there are a couple of near-identical blocks on the market), and it's worked quite well.
Full-fill with ecobead "platinum" is another option, but the cavity will need to be a little wider.
Take care to get the builder to stop any convection air circulation around the blocks (e.g. I got my builder to stick the edges of the insulation blocks together using some MS polymer sealant and/or low expansion foam), particularly at the top, and edges of the extent of the PIR. Really emphasise this, it's easy to cock up!
It's preferable to sit the window back in a bit, as tony says, but alternatively - if the window must be inline with the outer brick skin, then sitting it on a ~50mm thick bit of XPS or high density EPS, or foam glass (depending on weight of window) is an alternative (hiding the thermal bridge course with some quadrant underneath the sill).
Thanks Tony, thanks Tim. Your last suggestion of sitting the window on an insulating course is interesting Tim esp. as this would have knock on implications for the sizing of the windows. As for sitting the window back off the face of the brickwork I don't know how this would pan out yet. I'm considering alu clad timber windows, but the Swedish windows I've looked at come without any cill detail and the alu cill is a locally, (UK) fabricated extrusion, which is surface fixed to a groove in the window.
"Is there not a problem with building control to have a cavity fully filled? Don't they require 50mm of air?"
Good point. That depends on the finishing of the masonry, the fill material, the location (exposure to driving rain etc. including whether the wall in question faces into the prevailing wind or not), and the local building regs in force (I'm only familiar with the England + Wales regs). Also, (not a regs-issue per-se but...) for new build, the NHBC may also stick their oar in I think (especially in Scotland when it comes to cavity fill - no full-fill allowed by them I believe BICBW). There's a map of zones (for the whole UK) and a corresponding cavity-fill table in England+Wales building regs Approved Document C (page 34, 35).
The building regs info is general advice, and the individual manufacturer can get an independent testing establishment to carry out tests which may allow more permissive usage (i.e. fuller fill under harsher conditions) than those in approved document C (if they have features in their construction+installation which further-prevent water from travelling over to the inner skin when compared to "standard" products). For an example, see the CavityTherm BBA cert and FAQ pages, and also the Springvale Ecobead certificate, and the Knauf Dritherm BBA cert.
Note that this isn't meant to be a particular endorsement, and that other full-fill insulation products exist, these are just a few which I happen to know about!
And use low-conductivity wall ties. And think about how the bay wall will join the existing wall - make sure there is no thermal bridge there. And the lintel. (how are these done for bays?)
Posted By: wookey............... And the lintel. (how are these done for bays?)
Most windows are not loadbearing so stout cornerposts supporting a load bearing ring beam would be the obvious solution if the roof was likely to be very heavy, or the bay very deep. Maybe even a centre post too if the bay was very wide. I'm looking at another solution though.
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