For best results, you’ll want at least a 20 degree temperature difference.
This can be achieved by heating your house up on a cold day or turning on the air conditioning when it’s hot outside.
Mark any spots where you see a temperature differences as probable locations of missing insulation.
Common areas where you’re likely to find missing insulation include the space around outlets and switches; the edges of attics where the outside walls meet the roof; and unfinished garages, which if connected to the rest of the house can siphon huge amounts of heat during the winter.
Insulation in wall cavities can also settle over time, causing gaps to appear.