From my pictures you can see that our engine bay already had insulation all around it. Since I had already purchased some (this guy:
Amazon.com: FatMat Hood-Liner 34" x 54" x 3/4" Thick Self-Adhesive Automotive Sound Deadening Hood Liner - Aluminum Face: Automotive )
I figured I'd double it up. From what I could tell the insulation Thor used was identical to what I had purchased except for the thickness (Thor's 1/2", my purchase 3/4"). I still have yet to take our camper on the road to see if it made a difference. Revving the engine to 2000 RPM didn't show any difference on the sound meter App I was using.
I really don't have an opinion of the spray sound insulation--it may work better as it is harder than the foam (but then "hardness" may transmit sound as well). There was a lot of area under there that I could have coated but I only chose to do the doghouse.
This insulation is self-adhesive; you just peel off the backing and stick it in place. Very Easy.
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