-Is it a good idea to fill the air space behind the 703 with Ultratouch R-13?
Sure.
Is it a good idea to fill the cavity completely with the ultratouch or just use 1 layer to still leave a gap between the wall and the trap.
Filling the space is better.
Would I be better off using 4" 705 FRK and forget about the ultratouch.
Either way works well. If cost is a concern, it's better to have more 4-inch traps straddling corners than fewer filled corners.
I have alot of Ultratouch leftover so I'm looking for ways to use it!
In that case use it to fill the corners.
--Ethan
