Hi all, me again!
More basic questions !

I’ve had a little square extension built , they have just done the shell for me, and I’m going to fit a window in to seal it up , but....... instead of having a plastic windowsill outside id like a stone cill to match the old pennant stone cills of the house

But how do I go about it so there’s no thermal bridge?

The wall is blockwork then a cavity with rigid insulation then another block, just standard stuff

So do I fit cavity closers on the left and right and bottom of opening, then fit the stone cill flush with the outer wall, fit the window so it’s on top of the cill and put insulation behind the back face of the cill?

I’ve looked at loads of cross section details online but it’s very confusing!

Or should remove some of the outer block wall to allow the stone cill to sit lower meaning the bottom cavity closer will sit over the cill a bit then fit the window?

Or do I make some sort of fake stone cill that fits over the outside and just gives the effect of a stone cill?

Can someone do me a sketch please! Can’t google anymore my brain hurts! Can’t see a way of doing it without a weakness in the insulation
Cheers all

Posted By: RobLThese guys make EPS fake stone sills:

https://passivesills.com/window-sill/" rel="nofollow" >https://passivesills.com/window-sill/

That’s great, ill see if they make one similar to mine, thanks

I used genuine pennant stone cills in exactly that situation. I bought second hand ones and disc-cuttered them down to 150mm wide so that their inside edge was flush with the inside edge of the outer leaf blockwork. I cut the blockwork away under the cavity closure, and mortared the cills in such that the lip of the cavity closure was resting on the top inside edge of the sill. then fitted the window as normal.
I also took the opportunity whilst I was trimming the cills to disc-cutter a V notch as a drip channel on the underside approx 15mm from the front edge.

Ideally don’t use stone sills but if you must, mitigate the thermal bridging at the sill

Posted By: Viking HouseCenter the cills on the outer block.

What do you mean? So the sill sits flush with the outer leaf and doesn’t poke into the cavity at all or centre the sill over the outer leaf so half the sill is outside and half the sill is poking into the cavity?

Could you do me a quick sketch by any chance please?

Yep that's pretty much it, My windows have small aluminium sub sills so the windows sit straight on the cavity closure centred in the cavity. will try and sketch up.

You can replace the insulated cill with a stone cill if you use the detail above.

Posted By: denianceLink doesn’t work

It does for me.

Posted By: tychwareltry this version

http:///newforum/extensions/InlineImages/image.php?AttachmentID=7863" alt="window detail-page-001.jpg" >

Thanks!!

The photo you posted appears to come from the product brochure, which also includes installation instructions (p7 et seq).

Posted By: deniance

Posted By: djhThe photo you posted appears to come from the product brochure, which also includes installation instructions (p7 et seq).

The instructions say just pop it in place and fix with screws but that seems a bit lazy, what if the insulation isn’t straight in the wall?, what if there are gaps?, how do I stop air leaking past it?

Just want to get it right the first time and I know my wall is not as perfect as the one in the picture😂

If the insulation board isn't flush with the wall find a way to fill the gap. Makes huge difference