Window positioning on cill

[cwr]

Hey folks,

What are the thoughts on how much a window frame (with triple glazing) should sit on a concrete cill, with the aim of minimising thermal bridging? Is 25mm of the frame enough to take the weight? 

 

We're going for trad block cavity walls, but with a 240mm cavity. Wondering about using concrete cills intended for timber building that have a narrow, circa 25mm, wide strip for the window to sit on. The rest of the frame would overhang the cavity and have the immediate space filled with PIR to minimise thermal bridging. Hope that makes sense!

Cheers

[craig]

Cavity closer, don't just have the window sitting on 25mm of stone cill and overhang the cavity.

[Mr Punter]

I agree with @craig although they will probably be quite pricey / non-stock item at that width.  BC like to see them and they are more robust than DPC.

[cwr]

Yes plan to have PIR closer beneath frame where it's not on the cill. Aim to minimise the overlap of frame to cill to minimise thermal bridging.

 

Main concern is are PVC window frames going to be robust enough or do they need better support?

 

Something like this (Not show is support for PIR or inner cill). Cavity insulation will be EPS bead. 

[ruggers]

On 09/07/2019 at 20:32, cwr said:

Yes plan to have PIR closer beneath frame where it's not on the cill. Aim to minimise the overlap of frame to cill to minimise thermal bridging.

 

Main concern is are PVC window frames going to be robust enough or do they need better support?

 

Something like this (Not show is support for PIR or inner cill). Cavity insulation will be EPS bead. 

 

What option did you go for? 

Did you use a cavity closer under the ply internal cill and if you did, did the outer part of the closer, sandwich between the underside of the window frame and on top of your concrete outer cill?

[cwr]

Ended up pretty much as per the drawing above, except with 100mm pir, and osb instead of ply.

 

We also went for Rationel aluclad frames instead of pvc.

 

Standard concrete cills, though might have been better with ones with a narrower part for the window to sit on.

[ruggers]

My cavity wall construction is 100 Face brick - 150 partial fill cavity with 100mm PIR - 100 inner block.
I was looking at cast stone slip cills for the front of my build that provide a 50mm front over hang, but I want to sit the back of the 65/70mm deep window frame so that it's 25mm into the cavity to reduce thermal bridging and to give a deeper reveal on the outside of around 60mm.

 

I'm not sure if I need a 150mm cill or a 175mm. The passive cill image attached has a 200 cavity and full fill blown bead, but it shows the cill set back into the cavity and the frame set back on top of that leaving a 100mm external reveal.

 

Did you run DPC under the concrete cill only, or differently?

 

 

Edited Tuesday at 13:24 by ruggers

[Iceverge]

1 hour ago, ruggers said:

150 partial fill cavity with 100mm PIR -

 

 

DON'T DO IT!!!!!!!!!!!!!!!!!!!!!!!!!!!

[ruggers]

10 minutes ago, Iceverge said:

 

 

DON'T DO IT!!!!!!!!!!!!!!!!!!!!!!!!!!!

I need a disclaimer because I've had to post this lots of times, but I can't have blown bead with facebrick in a higher wind driven area on the West coast, no one uses it and the local BC won't like change. The plans have been approved a long time ago now. Going to use T&G boards.