The 20% off is on only some J-M batts. The price of insulation is so all over the place I'm not sure the sale is any sale at all. I am in the market for insulation, R19 or so for my soon-to-be-built garage, and I've searched extensively. Right now the J-M sale can get me R21 at about $0.54 a sq ft, but I can get Certainteed R19 a Menards for $0.29 sq ft. But, that is for only one size of the kraft faced -- the other packages are much more. I guess it is moot, as there are no Menards in my state!
I can get Roxul R15 at $0.79 sq ft, and R23 at $1.14 sq ft. If either of those approached the price of fiberglass I'd be all over it.
Jeff