Dave Anderson Posted January 27 Share Posted January 27 We're about to appoint a builder to demolish an existing bungalow and build the shell for our replacement bungalow (in Lancashire). The design and drawings, building regs etc are all our own work, ie no Architect involved. I've seen a few threads on here discussing window position ie how much to overlap the cavity, and thinking this through leads to a few related queries. Diagram below: The walls are rendered blockwork (or stone to the front elevation), 150mm cavity with 100mm PIR, 100mm aerated blockwork with parge coat, and plasterboard on dabs. The windows are 100mm thick frames (Residence Collection R9).The diagram shows my proposed position. 1. This position gives 75mm overlap into the cavity, which does not seem to comply with the building regs requirement for 30-50mm overlap. Am I right to assume the 30-50mm can be regarded as a minimum? I think it is recognised best practice to push the window fully in line with the cavity, but I don't want to go that far. Does this position as drawn look OK to people? 2. I've read about window frame drainage, which can be face drained (ie with ugly holes showing) or through a stub sill, which I think gives a messy detail when overlapping a stone sill (or concrete). Is there a way of doing it as drawn? I'm struggling to get full engagement on this detail with the window supplier at this stage - the sales/managerial staff just seem to want to leave it to their surveyor to sort out once the walls are built. But potentially it could influence the shape of the stone sill, so needs to be decided now. 3. The cavity closers I have seen are quite flimsy. Are there some that are able to support the weight of the window? Otherwise it will have to be shimmed off just the 25mm inner edge of the stone sill. How are windows supported when they are placed fully within the cavity? Hope this all makes sense. I'd be really grateful for any advice or comments. Link to comment Share on other sites More sharing options...
nod Posted January 27 Share Posted January 27 Leaving 50 mil on the outside reveal for the render or face brick is the ideal and works well on the inside fixing straps Link to comment Share on other sites More sharing options...
Dave Anderson Posted January 27 Author Share Posted January 27 Thanks nod, so that would put my windows 50mm into the cavity which is ok with regs. And the windows would be better supported. And give us wider window boards! But I guess there would be slightly more of a cold bridge as the frame would not overlap the main body of insulation? Link to comment Share on other sites More sharing options...
nod Posted January 27 Share Posted January 27 Yes that’s fine 50 mil works great with a render bead Giving a 65 mil outer reveal Link to comment Share on other sites More sharing options...
HughF Posted January 27 Share Posted January 27 (edited) Is your builder any good? Pir in a cavity is a nightmare to get ‘good’… I’d go full-fill rockwool personally. Or blown bead. Edited January 27 by HughF Link to comment Share on other sites More sharing options...
nod Posted January 27 Share Posted January 27 32 minutes ago, HughF said: Is your builder any good? Pir in a cavity is a nightmare to get ‘good’… I’d go full-fill rockwool personally. Or blown bead. Very true We used full fill the first time round and pir this time Pir is better But has to be installed with care I’m not keen on blown beads as it tends to settle Link to comment Share on other sites More sharing options...
Dave Anderson Posted January 27 Author Share Posted January 27 I thought that issue might come up, having seen previous discussions about it on here. I think our builder is good and take pride in their work, and PIR is what they know. They were certainly not keen when I asked about 90mm PIR in a 100mm cavity with a view to slightly larger rooms! I might have a chat with them about rockwool. Link to comment Share on other sites More sharing options...
HughF Posted January 28 Share Posted January 28 8 hours ago, Dave Anderson said: I thought that issue might come up, having seen previous discussions about it on here. I think our builder is good and take pride in their work, and PIR is what they know. They were certainly not keen when I asked about 90mm PIR in a 100mm cavity with a view to slightly larger rooms! I might have a chat with them about rockwool. Ours were equally unhappy about the thought of 90mm in a 100mm cavity. We’re glad we went 150 and rockwool now, the thermal performance seems ‘good’ but it’s no passive house. Link to comment Share on other sites More sharing options...
Dave Anderson Posted January 28 Author Share Posted January 28 Anyone have any thoughts on my second query about window frame drainage? This picture shows an example of what we would like to achieve (ie no upvc stub sill), but there's no sign of any window frame drainage. Have they made a mistake or am I missing something? Link to comment Share on other sites More sharing options...
HughF Posted January 28 Share Posted January 28 Looks nice…. Not sure what they’re doing for drainage, as you say. Link to comment Share on other sites More sharing options...
torre Posted January 28 Share Posted January 28 Some stone cills are available with an upstand. For example a pic here. Depending on the profile of the R9 you may be able to sit them far enough forward on the upstand so they drain through the bottom of the frame. Thorvertone stone do something similar too Link to comment Share on other sites More sharing options...
Dave Anderson Posted January 29 Author Share Posted January 29 Thanks for the idea Torre Link to comment Share on other sites More sharing options...
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