A mail slot or pet door is basically a hole in your front door. Cold air slips in around the flap all winter, and warm air sneaks in all summer. With a brush guard, a better flap and a little weatherstripping, you can seal it yourself in under an hour.
Quick answer: To draft-proof a mail slot, add a brush-style draft guard or a second inner flap on the inside of the door. For a cat or pet door, replace a worn flap with a magnetic or double-flap seal, then caulk or weatherstrip any gaps around the frame.
Why sealing small drafts matters
Small gaps add up. ENERGY STAR says that by sealing air leaks and adding insulation, households can save an average of 15% on heating and cooling costs (ENERGY STAR). A drafty mail slot or pet flap is one of the easiest leaks to fix because you can see it and reach it.
Sealing drafts also makes your entryway more comfortable and keeps out dust, pollen and bugs. The U.S. Department of Energy’s Building America Solution Center rates basic weatherstripping as one of the easiest home projects, with low material costs (DOE Building America Solution Center).
What you need
- Tape measure.
- For a mail slot: a brush-style mail slot draft guard or an inner flap kit sized to your slot.
- For a pet door: a replacement magnetic flap, a double-flap insert, or a weather-sealed pet door sized for your pet.
- Screwdriver or drill.
- Rubbing alcohol and a rag to clean surfaces.
- Exterior caulk or self-adhesive foam weatherstripping.
- Wood filler, if the door has damaged or soft spots.
How to draft-proof your mail slot or cat door
- Find the leak. On a cool or windy day, hold your hand or a strip of tissue near the slot or flap. If it moves, air is getting through. You can also look for daylight around the edges from inside.
- Measure carefully. Measure the opening width and height, plus the door’s thickness. Buy a guard or flap that matches. Guessing usually means a second trip to the store.
- Remove old or broken parts. Unscrew any worn inner flap, cracked pet flap or loose trim. If old screws are stripped or rusted, a drill set to a low speed can help.
- Clean and repair the surface. Wipe the area with rubbing alcohol so new adhesive and seals stick. Fill any soft or damaged wood with wood filler and let it dry fully.
- Install the new seal. For a mail slot, screw a brush guard or inner flap to the inside of the door so the bristles close behind the mail. For a pet door, fit the magnetic or double flap according to the kit’s instructions.
- Seal the frame. Run a thin bead of caulk or add foam weatherstripping where the pet door frame or slot trim meets the door.
- Test it. Check again for drafts and daylight. Make sure mail still drops through easily and the flap swings closed on its own.
- Let your pet practice. Prop the new flap open for a day or two, then let your cat or dog push through on their own. Treats help.
Common mistakes to avoid
- Buying the wrong size. Mail slot guards and pet flaps come in many sizes. Measure the opening and the door thickness before you shop.
- Skipping the cleaning step. Adhesive weatherstripping peels off fast on a dusty or greasy door.
- Blocking your mail. Very stiff bristles or a tight flap can stop mail from falling through. Choose a guard made for mail slots, not a door-bottom sweep.
- Making the pet door too hard to push. A strong magnetic flap seals well, but small or older pets may struggle. Pick one rated for your pet’s size.
Frequently asked questions
How do I stop a draft coming through my mail slot?
Add a draft guard to the inside of the door. Brush-style guards let mail slide through while the bristles block air, and inner flap kits add a second door that closes behind the mail.
Are magnetic cat flaps better for drafts?
Usually, yes. A magnetic strip pulls the flap snug against its frame so it doesn’t hang open or flap in the wind. Double-flap pet doors add a second layer that seals even better.
Can I just tape over an unused mail slot?
Tape is a quick short-term fix. If you never use the slot, a cover plate or an insulated insert on the inside looks better and seals more reliably. Check with your mail carrier first if mail still arrives there.
How much can draft-proofing save?
It depends on your home and climate. ENERGY STAR says sealing air leaks and adding insulation together can save an average of 15% on heating and cooling, and fixing a mail slot or pet door is one small piece of that.
Try it with our free calculator
Want to see how drafts and heating habits affect your bills? Use our free Home Energy Simulator to estimate your household’s energy use and compare simple fixes like air sealing.
Related guides
- How to Make a DIY Door Draft Snake in 10 Minutes
- How to Install a DIY Window Film Kit as a Renter
- How to Check Your Attic Insulation with Just a Flashlight
Sources
- ENERGY STAR, Why Seal and Insulate?
- U.S. DOE Building America Solution Center (PNNL), Add Weatherstrip to Windows and Doors
Written and reviewed by the Accessible Sustainability Alliance editorial team. Last updated October 5, 2026.
