Finished up the insulation over the weekend and I'm very happy to have this job behind me. The inspector came and signed off on the bracing and fire stops required on the west wall (garage door wall) so I could get it finished and ready for drywall. I had plenty of extra Rockwool, so I decided to double insulate the double wall- insulation against the sheathing, then an air gap, then foil radiant barrier across the entire wall, then another air gap, and finally the interior Rockwool layer. Since this will get the worst of the summer sun, I figured anything I could do to help keep out the heat would be worth the effort.
I asked in another thread about the viability of adding a foil backing to mineral wool insulation to give it both thermal and radiant blocking attributes, but the question I asked wasn't accurate. I wanted to know if the performance of 3.5" thick wool plus radiant barrier (and a ~1" air gap in the 2x6 walls) would provide a better result than buying 5-1/4" thick wool. The reason that question is not really relevant is that the thicker insulation was more than 3 times the price of the 3.5" thick stuff. I called around several places and got the same answer, so that was out of the equation since it would take a hundred years to make up the cost by electricity saved. So I decided the radiant barrier on the 2x4 wall Rockwool would work well. Lowes had it on sale 15% off which was a nice find, since nothing has been on sale for this project.
Step one was laying out the batts on a leftover piece of OSB with the foil barrier on a PVC pipe and a pair of rollers to make it easier to slide out. I sprayed three stripes of 3M 77 spray adhesive on the wool and with my sub 10% humidity it was ready to adhere to the foil immediately.
The stripe down the middle helped when I needed to cut the top or bottom of the batt off, and the rest of the foil wouldn't fall off. Rolling out the foil without it drooping down into the adhesive was a bit of a challenge, but wielding a ripped 2x4 made it work. Creasing the gaps between the batts with my board helped cutting them apart.
My snap blade Olfa knife was a good choice for cutting the foil- cutting aluminum is not easy on a cutting edge, so by having the snap blade extended out 2" or so would give it lots of edge area to dull before becoming useless.
Rinse and repeat for about 230 more batts.
After caulking the bottom plate, around the spray foam on the doors and windows to seal against air ingress and bug migration, I am ready for drywall. The blown cellulose wouldn't effectively get under my OSB platform in the attic, so I put some Rockwool under them and secured them with electrician's pull tape so they wouldn't fall before the drywall is mounted. I installed the extra pieces of R38 fiberglass that were left over from the 'hallway' insulation. I filled up 10 bags of insulation scraps and took care of tools and other stuff littering the garage, and now it's ready. It is eerily quiet in here now- hopefully the drywall will make noise bounce a little more because this stuff absorbs everything and feels really odd just walking around.
