Many homeowners install plain foam gaskets between the cover plate and the switches/receptacles. However, those gaskets don’t always seal well. Infrared thermograph images of those outlets will show cold air still entering (which means warm air escaping) the house.
We’ll show you more effective methods that permanently seal those leaks. Sealing is easy to do and you can complete the entire house in about four hours. The materials cost less than $25, one of the best investments you’ll ever make! All you need is a caulk gun, caulk and aerosol foam. Here’s how to do it.
Gaps around ceiling boxes must be sealed with an “intumescent” fireblocking caulk or foam. Ordinary spray foam burns too quickly, opening the gap and creating a chimney effect that feeds the fire. Intumescent caulk or foam, on the other hand, swells when heated so it prevents that airflow. Regular caulk or foam can be used on wall-mounted boxes.
If the electrical boxes aren’t mounted flush with the drywall, adding a box extender will make them easier to seal. They’re available at most home centers for single, round and multiple-gang boxes.
Intumescent foam works best to seal large gaps (larger than 1/8 in.) around ceiling boxes. But the foam drips out as it expands and is difficult to remove after it cures. So turn off the power and double-check with a voltage sniffer. Then remove the light fixture, spread a drop cloth to protect the floor below, and inject the foam (Photo 1). Intumescent fire caulk doesn’t drip like foam, so you don’t have to remove the light fixture or cover the floor.
Sealing wall boxes is a two-step process: First seal the cable intrusions and then the gap around the box. To locate open box knockouts, shine a light inside to see where the cables enter the box. That way you’ll know where to inject the foam. If you can’t spot the entry points, turn off the power and pull the switch or receptacle out far enough to see.
Next, enlarge the gap above and below the box (if needed) to provide room to insert the foam straw deep into the wall cavity. Then seal the wire intrusion openings with foam (Photo 2). We used intumescent fire-blocking foam to seal the wire intrusions as an added measure of safety, but it’s not required by code. You can use regular spray foam. However, never seal the intrusions from inside the box—that doesn’t meet code and you’ll have to remove the sealant if it’s ever inspected. Finish the job by shooting ordinary foam around the box (Photo 3).
Sometimes wall-mounted electrical boxes are recessed because of an installer error or new tile installed over the old layer. You can seal around the gap with caulk, but that doesn’t solve the problem of the box not being flush with the wall. That’s where a box extender comes in handy. It provides solid mounting for the switch or receptacle and makes the box easier to seal.
Start by sealing the wire intrusion following the procedure above. Then turn off the power (check it with a voltage sniffer) and slide on a box extender (Photo 4). Then seal the gap around the box extender (Photo 5).
Fix Leaky Can Lights
If you have older recessed lights in a ceiling under an attic space, you’re likely losing a lot of heat through the holes in the housing and around the base of the fixture. Caulking those openings or covering the fixture with an airtight box can create a fire hazard. The easiest and best solution is to buy an airtight LED retrofit light/baffle kit. Just remove the bulb, screw in the threaded adapter and snap the LED unit in place—no more air leakage.