Bad idea? I would do this in a heartbeat if I was building a new home and I had the option. Not sure if I would use closed cell again or open cell.
As for not making a difference, there is a huge difference when an attic is 150 degrees and 70 degrees. Also, I did my walls too and you have so much air leakage in your walls unless you have a log or timber frame home with 10”-12” logs. My buddy that converted his attic to closed cell from blown in insulation told me he cut his cooling expense each month by almost 1/2.
Closed cell is almost 2x more expensive to put in. The only thing about here in Florida is that I hear they replace the sheathing when they do a new roof. If that’s true I would spray closed or open cell insulation on a roof that will get replaced every 15 years. Maybe the roof would last longer with the open/closed insulation because you wouldn’t have the 150 degree heat under the sheathing
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