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How to install insulation around electrical wires

When you have wiring and outlet boxes in exterior walls, you need to take a few extra steps when insulating and installing vapor barrier.

Electrical boxes have holes which can allow moist air from inside to penetrate through the switchplate cover and back into the cold surfaces beyond the insulation. Obviously, you can’t cover the front of them with vapor barrier, so you need to put the barrier behind the box. You can purchase special boxes for this purpose, and caulk where the wires enter the box, or you can cut a 12″ to 18″ square of vapor barrier and wrap it around behind. Ideally, you should do this when you’re installing the box; then you can simply put a hole through the plastic for the wires and caulk it after. But it can still be done after the fact; just slit the plastic to allow the wires through, and then tape up the area with housewrap tape.

When you install the vapor barrier on the walls, cut a hole for the outlet and tape the two sections together to seal gaps. (A bead of acoustical sealant as a second barrier is good, too.)

Insulating’s also affected by the wires and boxes. The insulation needs to fill the space to achieve it’s R-value. If you simply shove the fiberglass insulation against the wires, it will leave an uninsulated gap behind the wires. You have a couple of options. You can slit the back of the insulation to the depth of the wires, at the height of the wires. Then, make sure the wires sit in the slit when you install the insulation. You can also separate the layers of insulation and slide it up under or down over the wires; or separate it into two layers, and put one behind the wires, one over the wires. It can help to cut the batt into smaller sections when doing this, since they’re easier to work with.

Cut out a small section for outlet boxes to avoid compressing the insulation, or loosely stuff behind it.