Electrical – How to insulate a stud bay containing a subpanel

When we insulate the joist boxes in apartment buildings or commercial buildings we are required to close the boxes over the foam with drywall or flame retardant foam board. Since your job is for utility purposes here's my thoughts. After you spray a couple of inches of foam into each box, cut measured pieces of foam board that will snugly fit above your wall, between the joists and up against the 1" rise of the sill plate behind the wall. Put a few dabs of construction adhesive on the edges and fit them into place. This will do a couple of things for you. It will create a larger dead air space in your now insulated boxes, extra R-value and also seal the top of your wall against the foundation. Smaller pieces can be glued in under the joists to complete the wall seal. If you want to further seal and joints in the foam board, use some HVAC aluminum tape.

Roxul batts do not compress a lot, well not nearly as much as traditional fiberglass. They are also pretty rigid. They can definitely go 6 feet or so in a wall cavity. I have used them to do what you are doing, when there aren't things in the way.

I am guessing you believe you don't have cross bracing or electric/plumbing or obviously this won't work. I would unroll a batt, cut to height size (possibly cut to width), push up slowly starting at top of batt. If it snags and is resisting I would cut it. Then push up with broom or something similar and just keep on going until you fill it. Even if the Roxul slides a little you can stuff the bottom to hold it. The stuff really compacts well and holds its place. Why I tend to use it in basement joists.

Also if you place a small screw in the top side of your batt (just push it in so it isn't hanging out) you can use a magnetic stud finder to see where you are at.