Minimising thermal bridging at window sill, oh what to do now...

[cwr]

Hi Folks

Scratching my head on this one, I'm sure (hoping!) someone on here will have a bright idea...

So our original plan was to have aluclad frames which are about 120mm wide. the concrete window sills were fitted by the builder and sit about 70mm into the cavity, as per the detail below:

 

I was happy enough with this, however in a bit of u turn my other half now wants PVC frames, which are about 65mm wide. The result of which is this not so good detail, area of concern highlighted in red:

 

 

Sills are already in, so trying to figure out what to do.

 

-Try and cut back the sills by 30 or 40mm? Going to be awkward to say the least, and risk cracking or making a mess of them

-Chamfer the corner and get a little bit of insulation in?

-Live with it?

 

Cavities are 240mm and will be full EPS bead filled.

Thanks

 

 

[Dan F]

Can't you just sit windows further back inside the frame?  Either that, or get the aluclad windows and paint them in "uPVC white" ?

[cwr]

Quote

Can't you just sit windows further back inside the frame? 

 

Yes, could do, though did want a nice deep sill on the inside. And then would also need to close off the cavity on the outside somehow.

 

Quote

Either that, or get the aluclad windows and paint them in "uPVC white" ?

Lol, do you get that in the same shop as the tartan paint?

[Dan F]

15 minutes ago, cwr said:

Yes, could do, though did want a nice deep sill on the inside. And then would also need to close off the cavity on the outside somehow.

 

You could leave 20mm on cill, and 45mm on frame couldn't you? Your cill would still be as deep as it would have been with aluclad.

Edited July 8, 2021 by Dan F

[tonyshouse]

I don’t like the frame sat straight on the sill! Either a water bar needed or a stub sill . Stub sill will allow frame to move further inboard 

 

this will also allow you to lift the window board a bit too possibly 40 to 50mm and insert insulation under it near the stone sill 

[Russell griffiths]

Easy don’t buy the upvc. 

If your filling the cavity with eps beads you must be trying to build a quality house, I don’t see how 65mm upvc frames even come into the equation. 

Put them out of your mind buy the better windows and solve all the problems like you had before. 

[Oz07]

Stub cill or some kind of cill to achieve greater set back gets my vote. 

[cwr]

"Easy don’t buy the upvc. 

If your filling the cavity with eps beads you must be trying to build a quality house, I don’t see how 65mm upvc frames even come into the equation. 

Put them out of your mind buy the better windows and solve all the problems like you had before."

 

Yes if it was just my choice I would, but my other half is insistent on pvc....

 

 

So how about a strip of aerogel in conjunction with either frames raised up or further in?

[SimonD]

1 hour ago, cwr said:

Yes if it was just my choice I would, but my other half is insistent on pvc....

 

Why? Have you expained the ramifications and the difference in product? I've usually found that a quick conversation along the lines of cold bridge = condensatoin = black mold on those lovely new deep window cills, sorts the issue out very quickly!

 

Otherwise, just make an executive decision...