This infrared photo of a quadruple switch box and a duplex outlet shows how cold air is leaking into the house through these boxes. The temperature readout on the switch box is just below 40°F! That's cold.
What to do?
The utility companies have been pushing the outside wall foam outlet and switch gaskets for years as a way to cut down on air infiltration. I though about it and realized that I don't have any at my house. Aha, opportunity. Let's find out if these things work. I walked to the local hardware store, bought a package of outlet gaskets and a package of switch gaskets, and walked home. I chose a triple switch box and two duplex outlets in my plaster walls, and double switch box and a duplex outlet in my drywall walls. In some cases the multiple switch boxes had overlapping gaskets. I set up my blower door and turned it on. I took infrared photos of the of the switches and outlets before and after gasket installation with the blower door running. They are shown side-by-side below.
Pictures of my outlets without Gaskets are shown on the left, then after gaskets on the right.
There is a definite thermal difference between pre-gasket installation and post-gasket installation. All infrared photos with gaskets have a higher temperature reading than those without the gaskets. Gaps outside the cover plate compromise gasket operation. They do not eliminate all air movement. Covers should be installed snug with the gaskets.
It appears that the gaskets do work, but not perfectly. I will install gaskets in all of my exterior wall outlets and switches. Consider repairs to openings that are larger than the cover plates.