No, it's not at all. They're asking, you (legally) have to tell. If you don't, and it can be proven, you're in for a world of hurt. You could get the house back, trashed, in a decade *and* have to pay the current owner, big-time.
EVERY pro installer uses spray foam - to install it right.(the spray foam isn't the be-all and end-all, but is virtually "required" for a tight installation.) Use the "low expanding foam" only, or it will warp the window very badly.
In most cases the closed cell blocker, if used, is just to keep the foam in place. (so it doesn't run out all over either the inside wall or the outside trim) as it expands.
The R value would be pretty close (per inch), but the Backer rod does not make as sure a seal, and you will likely get more foam in the joint than you will get backer rod - so the total r value would be higher
Based on my experience of using backer rod - as backer rod for caulk - leads me to question its ability to hold foam in place.
It sure doesn't take much to push backer rod deeper into an open cavity than you might want it so I don't see how it would not just be pushed out of the way by the expanding foam.
I've used minimally expanding foam and seen it expand it's way out from behind nailed on trim. I can't imagine that backer rod wouldn't just get pushed right out of the way.
Am I missing something in how it would be used with foam?
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