Insulating sash window boxes

[NCXo82ike]

Hi all, I came across this post on another forum about reducing the cold bridging across sash boxes. I wanted to see if there was any experience on here. https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16668

I am refurbing my old sash windows in a 1905 mid-terrace. The main plan is retrofitting vacuum insulated glazing, draft stripping, and replacing the mastic joints. We'll be adding internal wall insulation, either Thermactive plaster or PIR, with a plan to boost ventilation (MVHR) for moisture control within a major refurb.

We have a single storey bay window with 3 sashes in. I was putting expanding foam behind the mastic where the joint was large, and it seemed to be an endless void. Turns out the boxes aren't complete and are open to the stone on the external face. This is a big cold bridge, with only 20mm window frame and ~100mm stone between inside and out. Obviously foam in here could bust out the boxes and would definitely stop the weights running.

 

I was wondering however about sheathing the weights in plastic pipe e.g. 40mm waste pipe, and filling the remaining cavity with loose fill insulation. Any experience with this/recommendations/cautions?