Retrofit spray foam

[Stones]

I've seen (American) videos of retrofit spray foam insulation being injected into a Timber Frame to fill up potential voids, the idea being the foam expands between plasterboard and whatever insulation is in there (glass/mineral wool) to form a continuous air tight layer immediately behind the plasterboard.

 

Working on the principle that when mixing insulation types, specifically glass/mineral/earth wool type and PIR type board insulation, you would normally install the less vapour permeable first then work outward.  I can see how the above retrofit would fit that principle, but do wonder if it's a good idea to compress wool insulation too much.  Assume you would also have to take account of whether whatever is behind the glass wool is vapour open or not i.e. normal TF make up with breather membrane okay, but if you would end up creating a sandwich of PIR board, wool then retrofit spray foam, that wouldn't be good (although retrofit EWI seems to work effectively on that basis)?

 

Purely academic, just interested in collective thoughts on the subject.

[Temp]

I tried injecting spray foam into a stud wall in a garage once. Went round the other side to find the foam had expanded so much it had pushed the plasterboard off the wall so the wall now looked pregnant.

[kxi]

I once saw 'mini' spray foam blowing out a brick wall into the street below. Given this is on a major bus route I suspect the mortar on these Victorian buildings was pretty much dust.

Edited December 6, 2019 by kxi
Zoomed image