Well, not quite that solid. Actually, mostly spongy, but still rock and still déjá vu.

I am talking about 35 bundles of rock wool (also known as mineral wool) that I picked up at the Chicago Green Depot. The bundles don’t look very happy sitting in the corner and are sort of asking for our attention.

The three and a half inch thick rock wool batts are the final component in our insulation assembly. They add an additional R-15 to the R-6.5 of the one inch closed cell foam and the R-7 of the two inches of open cell foam. That brings the total R-value of the insulation components up to 28.5.

Why did we finish the insulation assembly with rock wool and not just continue with spray foam?

Rock wool is significantly more economical compared to spray foam. The batch I bought for the 1st floor cost $0.17 per board foot. Open cell foam runs around $0.40 and closed cell foam around $1.00 per board foot.

The rock wool batts are an ideal medium to install between framing. The contiguous gap between the framing and the brick wall on the other hand is easier to fill with a medium like polyurethane spray foam (SPF).

There are a number of other good reasons which led to the decision to use rock wool. If you like, you can read up on them in a previous post.

We had a good test run installing the rock wool in the garden unit about a year ago. Cathy actually has become sort of a master in fitting the batts between the joists, studs and around obstacles like PEX tubing. I set her on the job and before I knew it, she had finished the ceiling.

The walls were a little more involved as I first had to cut back the excess spray foam, so that the batts would actually fit. Our friend Anne was kind enough to help with the installation and we blew through half the building in one day.

With the 1st floor insulation finished, it should now become a little cozier, even though we don’t have the radiators up and running yet.