Mail slots help to keep you out of the rain and the cold. The mail carrier drops the mail right into your house, and you can pick up your mail without putting on your shoes. The disadvantage of having a mail slot is that it also allows warm or cold-conditioned air out and a small amount of noise in. There are basically three ways to insulate your mail slot. Temporary insulation lets you use the existing slot, but provides weaker insulation than the other methods. Permanent insulation will fill the slot and make it unusable, but will do a great job of insulating your door. Or you can install a new highly insulated mail slot, which uses magnets to hold the doors closed and has weather stripping along the openings.
Measure your mail slot's interior length, depth and height.
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Cut a piece of styrofoam or batt insulation to the dimensions you measured in Step 1.
Place the insulation into the mail slot.
Cut weather stripping to line the top, bottom and side edges of the styrofoam or batt insulation.
Line the side of the insulation on the inside of the door. Remove the insulation as needed to use the slot.
Measure and cut the styrofoam or batt insulation and place it into the mail slot, as you did in Steps 1 through 3 of Section 1.
Caulk along the edges of the Styrofoam or batt insulation instead of using weather stripping.
Caulk both the interior and exterior slot doors closed.
Installing a New Insulated Mail Slot
Unscrew and remove the exterior and interior faces of the mail slot.
Remove the mail slot cartridge.
Place the new cartridge in the open spot in the door.
Use the electric screwdriver to screw the faces of the new slot securely onto the inside and outside of the door.