Performance
Why Mail Slots Let Cold Air In
A drafty mail slot is one of the most common complaints homeowners raise about their entry door, and the cause is physical, not incidental. A mail slot lets cold air in because it is one of the few intentional openings cut completely through an exterior door. Whenever pressure differences exist between the inside and outside of a home, air naturally follows the path of least resistance, and an unsealed mail slot provides one. Two forces are usually responsible: wind pressure and the stack effect.
The Stack Effect
Warm indoor air is less dense than cold air, so it rises. In a heated home during winter, rising air escapes through gaps and cracks near the top of the house, and the resulting lower pressure near the bottom draws outside air in through gaps near ground level to replace the escaped air.1 Entry doors are almost always at or near ground level, so any unsealed opening on one, including a mail slot, sits directly in the path of that inward pull.
Wind Pressure
Wind adds a second, independent force. Wind striking a house creates positive pressure on the windward side and lower pressure on the leeward side, and that pressure difference pushes outside air through any available opening on the windward face.2 An entry door and its mail slot are frequently on an exposed, unsheltered face of the house, exactly where this pressure is strongest.
Stack effect and wind pressure do not need to work together to cause noticeable air movement through a mail slot. Either one, acting alone, is often enough.
Why You Feel It Most in Winter
Stack effect and wind pressure are present year-round, but their strength is not constant. The stack effect scales with the temperature difference between indoors and outdoors: the colder it is outside relative to a heated interior, the larger that difference becomes, and the stronger the pressure driving air through low openings like a mail slot.1 A furnace or heat pump working harder to hold a steady indoor temperature widens that gap further rather than closing it, which raises the pressure difference instead of easing it. The result is a mail slot that produces barely noticeable air movement in mild weather but a distinctly noticeable draft once winter sets in, not because the mail slot changed, but because the forces acting on it did.
Why the Flap Alone Doesn't Stop It
Although the stack effect and wind pressure are different phenomena, they expose the same weakness: a mail slot designed to open for mail delivery rather than to maintain a continuous air seal. A standard mail slot flap is designed to swing open for mail delivery, not to hold a seal under pressure. Most flaps rest against their frame by gravity or a light spring, with no continuous gasket and no mechanism resisting the pressure differences described above.
Even when a mail slot flap appears closed, small gaps around its edges can remain. Pressure differences don't require a large opening; air naturally follows the path of least resistance, so even narrow gaps can allow measurable air movement over time. The U.S. Department of Energy notes that sealing gaps and penetrations, including those at door and window openings, is a standard step in reducing a home's air leakage.3 An unsealed mail slot is precisely this kind of penetration.
The Fix
Solutions generally fall into two categories: replacing the mail slot with sealed hardware or retrofitting the existing opening with an interior insulator. BOTA offers both approaches through the Brass-Seal Forged Brass Mail Slot and the BOTA Mail Slot Insulator. Because it replaces the existing mail slot hardware rather than retrofitting around it, the Brass-Seal can be used regardless of flap direction or cutout geometry. The Mail Slot Insulator is the tool-free retrofit option, installing inside the existing cutout and sealing it magnetically, and fits where the flap opens outward and the cutout falls within its adjustable range. See the Mail Slot Insulator explainer for how the seal works, or How to Stop Mail Slot Drafts for a step-by-step approach.
Key facts on why mail slots let cold air in:
- The stack effect draws outside air through low openings, such as an entry door, as warm indoor air rises and escapes higher in the house.
- Wind striking the exposed face of a house pushes outside air through any available opening on that side.
- Either force alone is often enough to move noticeable air through an unsealed mail slot.
- A standard mail slot flap is built to swing open for delivery, not to hold a seal under pressure.
- Sealing penetrations, including mail slots, is a standard recommended step in reducing home air leakage.
Frequently Asked Questions
Is it just my mail slot, or is this happening everywhere in my house?
Both. Stack effect and wind pressure act on every unsealed opening in a home, not just the mail slot. The mail slot is a direct opening in an entry door, making it a noticeable contributor.
Why don't newer mail slots solve this problem?
Many modern mail slots are built with stronger springs or magnets than older designs, but most still aren't independently tested for air leakage. A tighter-closing flap may reduce drafts, but actual air leakage depends on the entire sealing system rather than the flap alone.
Does a storm door help with this?
A storm door can reduce wind exposure at the entry door, but it does not seal the mail slot opening itself.
Will weatherstripping around my door frame fix a drafty mail slot?
No. Weatherstripping addresses the gap around the door's edges, not the mail slot cut through the door itself. The mail slot needs its own seal.
Sources
- Fine Homebuilding, "The Stack Effect."
- International Institute of Building Enclosure Consultants (IIBEC), "Water Penetration in Building Envelopes."
- U.S. Department of Energy, "Energy-Efficient Home Improvement Credit: Insulation and Air-Sealing Essentials," energy.gov, 2024.