How far back to sit my windows ?

[Big Jimbo]

If you can see the drawing my windows will sit into the cavity by 30mm. That's a bit rubbish with regards the thermal bi pass around the windows, and it's by using 180mm cills. I'd like my 70mm window frames to sit into the cavity by 60mm.

180mm is the deepest grey cill I can find ! 

When it comes to the upstairs which will have hardi plank cladding on a batten, the windows will sit completely outside the cavity which won't meet building regs. 

Frustrating.

What have others done ?

[nod]

50-60 external reveals 

[Kelvin]

I had this whole battle in April. As designed the windows were in line with the edge of the kit for both aesthetic reasons (deep external reveal) and thermal reasons (windows inside the thermal envelope of the building). I had 5 people on-site telling me that was wrong. The guy that owned the kit building company saying that the thermal performance reason was “bullshit Kelvin the architect disnae know whit he’s talkin aboot”. Two joiners telling me they’ve been installing windaes for 30 years and if it wis their hoose they widnae day it that way. They couldn’t even agree between them the best way to do it. One wanted to install them out to the edge of the cladding which would have been a disaster given the build up. Another wanted to bring them out to the edge of the firestop because that’s how they’d always done it. So there’s clueless me with a set of construction drawings none of them had bothered looking at it and five of them bickering between themselves about how to do it and an architect I couldn’t get on the phone. 
 

In the end I insisted they fit them flush with the kit fixed with brackets. This gives us a deep external reveal. The internal reveals were already very deep. The windows are inside the building. 
 

I also had a problem with the cills. The supplied cills weren’t deep enough at 80mm. Nordan didn’t do cills deeper than about 110mm so had to get some made. The next set were 150mm deep but far too wide and I refused to have them cut to fit (powder coated) I also pointed out to them that 150mm deep still wasn’t deep enough to allow enough of a drip overhang. I supplied the measurements myself and had them made to 170mm deep which were perfect. My cills are powder coated aluminium so a bit more flexible with dimensions. 
 

 

Edited August 15, 2023 by Kelvin

[Big Jimbo]

@Kelvin Well done for thinking about it. It seems that most people don't. Cold bridge, heat loss. damp and mould.

[Big Jimbo]

So with a cill of 150mm...Brickwork....A 30mm cill overhang....... your window frame will sit 30mm into the cavity. Building reg PASS, but shite.

So with a cill of 150mm...Render....Blockwork......A 30mm cill overhang......Your window frame will sit into the cavity by at most 15mm. Building reg FAIL.

With a cill of 180 cill.....10mm cladding.....25mm batten....Blockwork.... A 30mm cill overhang.... Your window frame will sit into the cavity by 15mm. Building reg FAIL

 

So, basically a large thermal bridge. Heat loss, damp, mould, etc...... It can't be just @Kelvin and me that have thought about this ? Come on guys, other than getting cills made, has nobody done better than above. Where are all the smart people when you need them ?

 

150mm cills are marketed as standard. They should be marketed as will just get you through building regs with brickwork, will fail in all other circumstances, and are basically shite.

[Iceverge]

@Big Jimbo

 

Have you looked into aluminium folded sills? 

[Big Jimbo]

@Iceverge Looks like i will have to. The currently available rubbish available should be taken off the market, and the window industry should wake up.

[Kelvin]

I used these guys: 

 

https://archital.co.uk/aluminium-cills/

 

Mine are the type 6 on that page. 
 

Worth adding that the match to my windows is just about perfect (RAL7016) 

 

 

Edited August 15, 2023 by Kelvin

[BotusBuild]

I'm being thick - where is the thermal bridge?

[Big Jimbo]

@BotusBuild Not being thick. I think it is something that most people dont consider.

If we wanted to limit our heat losses, we would line our windows up with the outside edge of our insulation Not put our windows further out, like in my drawing. They want us to all use fancy expensive tape to put our windows in, but allow us to have our window frames only sitting back in from the outside, 30mm into our insulated cavity. That means that the cold air outside, and the warm air inside only has to pass through 30mm of insulation. If we sat our windows further in from the outside, say that they were flush with the cavity insulation, the cold coming in, and the heat going out would have to pass through more insulation before it either got in, or in the case of heat got out.

[Big Jimbo]

@BotusBuild It's the reason why a lot of window reveals feel cold to the touch, and have damp and mould issues.

[Kelvin]

It’s a bit harder to achieve with rooflights especially if they need to be fitted to a kerb. We originally fitted our rooflights on a large kerb based on erroneous information. It made the windows look daft (stuck onto the roof rather than part of the roof) plus the whole frame was outside the building. Again much to the bemusement of the builders I insisted we reinstall them with no kerb as the Velux window already has a big enough upstand for the flashings for the standing seam roof. Nevertheless they aren’t nearly as thermally efficient as the NorDan windows fitted elsewhere. 

[joe90]

With mine I had brick cills laid by the bricklayer and mounted the windows on the back of the bricks (check reveals ) with a neoprene gasket between frame and brick. The other advantage of this is you don’t have to fiddle the windows into an opening, mine overlapped by about 15mm. 👍

[Iceverge]

6 hours ago, Big Jimbo said:

The currently available rubbish available should be taken off the market, and the window industry should wake up.

 

Given that the best window installed is about 6 times worse than the worst wall these days I completely agree. 

 

Ideally some kind of 360deg frame that a brickie would build into the opening . It would be made to the mm that would take care of the cavity tray, the reveals, the head and the sill. It would allow the finished window to click into place as well as any add ons like external flyscreens/ and blinds. It would site the window nicely in the centre of the cavity and provide a chunk of external insulation to the frames to really minimise thermal bridging.