Long time lurker, first time poster. I’m in zone 5A, outside of Boston. We’re renovating the (unvented) top level of my 1920s bungalow. (The town assessed it as a 1.5 story home, I would not argue with that characterization.)

That level had previously been finished, but poorly, and (among other changes) we’re now adding significant insulation for the first time, 5.5″ closed cell spray foam.

Last year (early 2023) I spotted some mold growth on the roof decking and rafters in the knee wall area. It must have started after we had some insulation installed (summer 2022): 18″ of cellulose and fiberglass batts on the knee wall floor, plus 2″ XPS boards along the wall. As I came to learn from reading this forum, the installers made the mistake of not air sealing the top plate. 

Fast forward to last week, the contractors finished demolition, I pointed out the mold (that I knew was there) to the GC. It’s limited to an area that’s maybe 150 sq ft of the roof deck in the knee wall, maybe 200 sq ft at most. I’m trying to decide between two possibilities. 

1) The GC said we really ought to encapsulate the mold before the closed cell foam gets applied. He told me to buy 10 gallons of BIN shellac primer, which he says he uses regularly in his mold remediation business. It costs about $650 if I get the big buckets. He would apply it to the affected area within the scope of work of the project, so no change order / additional cost above what we’re already paying. The offer seems more than fair to me. (10 gallons is almost certainly a huge overestimate for the affected area, but I figure I can always return any unopened containers.)

2) That said, I have read that cc foam itself kills and encapsulates mold very effectively. I brought this up with the GC, and he told me it’s really better if we do the encapsulation, just to be safe.

The encapsulation won’t cost all that much. And I don’t hate the idea of having one more protective layer. But is it going to do anything at all? Is there any upside to encapsulating the mold first? Are there any downsides to encapsulating with shellac primer that I’m not thinking of?