Spray foam can be a bear to remove at some later time, and it's hard to spray in sheet form, it's more for filling voids. Why not use spray adhesive and glue the fiberglas bats to the inside of the cap?
Also, unless air is circulating up there moisture shouldn't condense in that area. Check your clearance lights, antenna, and any other thing coming through the roof in that area.
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Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI
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