How to Install Batt Insulation

Batts are sold for 16-inch and 24-inch on-center framing. Buy the width that matches your studs; trimming 24-inch batts to fit 16-inch bays wastes material and usually leaves ragged edges that gap.

Depth matters as much as R-number: R-13 fits a 2x4 wall, R-19 or R-21 fits a 2x6 wall, R-30 needs at least 8.25 inches. Cramming an R-19 batt into a 2x4 bay gives you roughly R-13 performance and a lumpy wall. The batt label is not the whole wall either: studs conduct heat, so the wall assembly R-value calculator gives the effective R of the finished wall, and the ROCKWOOL insulation calculator covers mineral wool batts if you want the fire and sound benefits.

Measure the bay, add about half an inch, and cut on a scrap sheet of plywood using a sharp insulation knife and a straightedge board as both a guide and a compressor. Dull blades tear fiberglass and make dust.

A slightly oversized batt friction-fits and stays put. A short batt leaves a gap at the top plate, and a 5% gap can cost you 25–30% of the assembly's effective R-value.

Peel the batt into two layers along its thickness. Slide the back half behind the wire or pipe and lay the front half over it. Stuffing a full-thickness batt behind a wire compresses it and leaves a void in front.

Around electrical boxes, cut a pocket so the batt wraps the box on all sides rather than bunching in front of it. Seal the gaps around the box and the top plate first; the caulk calculator works out how many tubes the joints take.

Kraft or foil facing always goes toward the heated side of the assembly. In a wall that means facing the room. Use inset stapling to the sides of studs or face stapling to the stud edge — but do not staple so tight that the batt bulges.

Under floors over a crawl space or garage, hold unfaced batts up with insulation support rods (lightning rods) every 12–18 inches. Sagging batts that drop away from the subfloor do almost nothing.

Long sleeves, gloves, safety glasses and an N95 or better respirator. Fiberglass itch is avoidable. Rinse in cold water afterwards — hot water opens pores and drives the fibers in.

Doing a garage next? See how to insulate a garage.