This will save you some utility costs and keep moisture from condensing in your walls and attic.
It only take about 3 minutes per outlet because there is no measuring or transferring of dimensions required.
This is a fun, satisfying, no cost, energy saving project. Cutting this stuff is a little like popping bubble-wrap. It's mildly addictive. I can quit any time I want.
Step 1: Simple Tools and Material
It's made of HDPE (High Density Poly Etheline) or LDPE (Low Density Poly Etheline)
Tools: Scissors, Exacto or utility knife, Screwdriver
For safety you can turn off the circuit breakers. (So I hear.) [ Mrs. Woodenbikes calls me "Sparky"] But I digress....
Thrill-seekers can leave the circuit breakers on and stand in a bucket of water while doing the project blindfolded.
I'm kind of middle of the road on the safety spectrum.
Step 3: Take off the outlet face plate
Hold it up to the back of the cover plate and poke the screw through it.
Step 4: Re-attach the cover plate
Then tighten the screw normally.
Step 5: Trim the excess gasket material
Step 7: All done. Now you have a happy and healthy outlet.
Repeat on all the other outlets in your home.
Even the ones on the interior walls that can act as chimneys for warm air entering through the outlets and rising into the attic via small unseen framing passages.