Thoughts on stuffing insulation behind boxes vs leaving air space?

I’ve found that just stuffing a full R-13 batt behind a standard 2.5-inch deep Carlon box is a waste of time. When you compress fiberglass that much, you lose the air pockets that actually provide the R-value. In my own basement, I started splitting the batts. I peel the fiberglass thickness in half, tuck about an inch of it behind the plastic box, and then cut the front half to fit snug around the perimeter. It keeps the insulation from being crushed but still provides a decent thermal break.

For the drafts, I skip the spray foam because it’s a nightmare if you ever need to add a circuit. Instead, I use those inexpensive foam gaskets that sit right behind the outlet cover. It’s a simple way to stop the air movement without making the electrical box a permanent part of the wall. Dealing with cold spots near the floor is frustrating, so taking that extra minute per box to split the insulation has really paid off for me.

I actually moved away from messing with the fiberglass thickness entirely because the real issue usually isn't the thermal loss behind the box—it's the air leakage through the wire holes and the box perimeter. I used to split my R-15 batts like the previous guy, but I still felt a draft coming through the toggle switches in the winter. Now, I focus on the seal rather than the fill.

If you really want to solve the cold spot, look into fire-rated putty pads, like the 3M or Hilti ones. You wrap the back of the plastic box before you even put the insulation in. It acts as a sound barrier and a thermal seal without you having to worry about how much fiberglass you've squished. In my experience, even a perfectly split batt won't stop a cold breeze if the box itself isn't airtight. I'd rather have a slightly compressed batt behind a sealed box than a perfect R-value behind a box that's leaking air like a sieve. I also add a foam gasket behind the faceplate once the drywall is finished. It’s a cheap way to stop the draft for good.

I'm still relly new to all this, so I'm trying to wrap my head around the heat dissipation point mentioned earlier. If I do split the insulation behind the box, is there an actual risk of the wires getting too hot, or is that mostly just a myth for standard home outlets? Also, I saw some 'putty pads' at the hardware store that supposedly wrap around the box. Does anyone know if those are easier to install for a beginner than trying to be surgical with the fiberglass? I just want to avoid that drafty feeling near the floor.

I stopped messing with fiberglass for this exact reason. If you switch to Rockwool Comfortbatt, the whole process is way easier. It’s dense enought that you can just use a serrated knife to carve a precise notch for the box. You aren't crushing it like those pink batts, so you keep the thermal protection without the wierd air gaps.

Regarding those putty pads, I always use 3M Fire Barrier Putty Pads. They aren't just for fire codes; they add significant mass and stop air dead. It’s way cleaner than spray foam and stays flexible if you ever need to pull wire later. It's the best way to handle those cold spots.