How to Stop Heat Loss Through Your External Doors

An external door loses heat in three ways: conduction through the door material, air leakage through gaps around the frame and edges, and thermal bridging where metal components create a direct path for heat to escape. Air leakage is the biggest culprit in most homes.

A hollow-core door with perfect seals keeps a room warmer than an insulated door surrounded by gaps. Fix the leaks first, then assess whether the door itself needs replacing.

How to Stop Drafts Coming Through Your Door

Seal the gaps around the frame with weatherstripping, install a door sweep at the bottom, and caulk any cracks between the frame and the wall. Most heat escapes through air leaks, not through the door material itself. A wooden or steel door loses far less heat through conduction than through the invisible gaps that let cold air pour in and warm air rush out.

Air leakage accounts for 25 to 40 percent of heating and cooling energy use in a typical home, and exterior doors contribute a significant share of that loss. The sides and top of the frame compress weatherstripping each time you close the door, but that seal degrades over months of use. The bottom gap, often a quarter-inch or more, lets a steady stream of cold air sweep across the floor. Cracks between the doorframe and the wall structure open as the house settles or the caulk dries out.

Finding the leaks takes a minute. Run your hand slowly around the perimeter of a closed door on a cold or windy day; you will feel the draft where the seal fails. A lit incense stick makes smaller leaks visible: hold it an inch from the frame and watch the smoke. Any sideways movement marks a gap that needs sealing.

Three seal points stop the draft. Weatherstripping closes the gap between the door edge and the frame on the top and both sides. A door sweep bridges the bottom gap, where the door meets the threshold. Caulk fills the static cracks between the doorframe and the wall, which never move and do not need a compressible seal.

1. Measure the frame perimeter and bottom gap
   Measure the height of both side jambs and the width of the top jamb to determine how much weatherstripping you need. Measure the door width for the sweep. Check the gap width by sliding a ruler or your fingers along the closed door; compression weatherstripping works for gaps up to about an inch, and most exterior doors fall within that range.
2. Clean the surfaces
   Wipe down the doorframe with isopropyl alcohol or a degreasing cleaner to remove dirt, oil and old adhesive. The weatherstripping will not stick to a dusty or greasy surface. Let it dry completely before applying the new seal.
3. Apply adhesive weatherstripping to the frame
   Cut the weatherstripping to length for each side and the top. Peel a few inches of backing at a time and press the strip firmly into the doorstop or the frame edge, depending on your door type. Work slowly to keep the line straight. For better adhesion in cold weather, warm the backing with a hair dryer for about a minute before pressing it down.
4. Install a door sweep at the bottom
   Position the sweep so its rubber or brush edge just touches the threshold when the door is closed. Mark the screw holes, drill pilot holes if the door is solid wood or metal, and drive the screws. Some sweeps slide onto the door bottom and require no screws; others mount to the interior face. Test the swing to confirm the sweep does not drag or bind.
5. Caulk the static gaps
   Inspect the exterior side where the doorframe meets the siding or brick. Fill any visible cracks with exterior-grade caulk, smoothing it with a wet finger or a caulk tool. Do the same on the interior side where the frame meets the drywall, using paintable interior caulk if you plan to touch up the trim.
6. Test the seal
   Close the door. It should meet light resistance as the weatherstripping compresses, but it should not require hard force. If it binds, the weatherstripping may be too thick or poorly aligned; peel it off and reposition it, or choose a thinner profile. Repeat the hand or incense test to confirm the draft is gone.

For the weatherstripping step

Holikme Weather Stripping Door Seal, Shower Door Seal Strip,1.8″ W-33…

The Holikme weatherstripping seals gaps up to 1.2 inches and includes 33 feet across two rolls, enough for a standard exterior door with material left over for a second door or a window.

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What You Need

Weatherstripping that seals gaps and holds up through daily use

Door gaps

Holikme Weatherstripping Door Seal

Two 33-foot rolls give you enough length to seal multiple doors or windows. The adhesive backing grips firmly through temperature changes, and the flexible design conforms to uneven surfaces without peeling.

Check price on Amazon

How to Stop Drafts Through Door Locks and Keyholes

A keyhole or lock cylinder creates a direct air path through the door, funneling cold air into the room even when the door itself seals well. Yale locks and other cylinder locks are common culprits. The fix is an escutcheon plate with a built-in draft cover or a rotating keyhole cover that slides shut when you remove the key.

Older mortise locks and mail slots leak more air than modern hardware. For a mail slot, add a brush seal or a hinged flap on the interior side; the bristles or flap block airflow while still letting post through. Replace missing or worn exterior flaps at the same time.

Drafts along the hinge side of the door often come from an uneven gap between the door and frame, not the hinges themselves. Check whether the gap widens toward the top or bottom. If it does, adjust the hinge depth by adding a cardboard shim behind the hinge leaf, or apply weatherstripping along the hinge side of the frame where the door meets it when closed.

How Thermal Curtains and Door Curtains Reduce Heat Loss

A heavy, insulated curtain over an exterior door creates a dead air space between the fabric and the door surface. That trapped layer slows conductive heat transfer, the mechanism by which warmth moves through solid materials. The effect is most pronounced on doors with single-pane glass or large glass panels, where conduction losses are highest.

Hang the curtain rod as close to the ceiling as practical and let the curtain reach the floor. The longer the air column, the more insulation it provides. A curtain that stops short or leaves gaps at the sides allows convection currents to circulate, defeating the purpose.

Thermal curtains work best on rarely used exterior doors: a basement exit, a side door to the garden, or a back entrance you open once a week. Frequent opening breaks the air seal and lets the trapped cold air spill into the room. They supplement weatherstripping and sweeps but do not replace them; air leaks around the frame still need sealing first.

When to Replace the Threshold or Upgrade the Door

Most heat loss and heat gain occurs around an entry door, not through it. Air leaks at the edges matter more than the door material. But once you’ve sealed the perimeter, heat transfer through the door itself becomes the issue—especially with hollow-core doors or old single-pane models that conduct heat straight through.

If the threshold is cracked, warped, or worn flat where the door sweeps across it, replace it. An adjustable threshold with a vinyl compression bulb seals the bottom gap better than any sweep can. You thread screws through the threshold from the top and turn them to raise or lower sections until the bulb compresses evenly against the door bottom. Replacing a worn threshold stops more air infiltration than layering sweeps on a surface that no longer makes contact.

Hollow-core exterior doors and old single-pane doors with no thermal break lose heat through conduction even when gaps are sealed. Heat moves through the material from the warm side to the cold side, and a hollow door offers almost no resistance. Modern steel or fiberglass doors with polyurethane foam cores have R-values of 5 to 6, compared to 2 to 3 for solid wood and under 2 for hollow-core. A thermal break in the door frame—a non-conductive strip that interrupts the metal path—prevents the frame itself from conducting heat outdoors in metal door systems.

Storm doors add an insulating air buffer and work well over older wood doors you want to keep. The air space between the two doors slows conductive loss and blocks wind. Choose a storm door with a low-E glass panel for winter and a screen insert for summer.

How to Reduce Conduction Heat Loss Through Single-Pane Glass

Heat moves through single-pane glass by conduction: warm room air heats the inside surface, heat conducts through the glass, and the outside surface radiates it away. The process is continuous. Single-pane glass has a U-factor around 1.0 to 1.1, meaning it loses about one BTU per hour for every square foot of glass per degree Fahrenheit of temperature difference. Double-pane insulated glass has a U-factor of 0.3 to 0.5, cutting loss by more than half.

You have four practical options. Replace the glass panel with an insulated glass unit—a job for a glass shop if the panel is large or if the door uses a sealed frame system. Add a storm door to create an air buffer without altering the original door. Apply a low-E window film to the inside of the glass; the metallic coating reflects some radiant heat back into the room, though it does not stop conduction the way an air gap does. Or hang a thermal curtain over the glass when the door is not in use.

Secondary glazing—a removable acrylic or polycarbonate panel mounted an inch or two inside the existing glass—creates a dead air space without replacing the door. You attach the panel to the door frame with magnetic strips or a snap-in track. The trapped air between the original glass and the secondary panel slows conduction. It works well for French patio doors with large glass areas where storm doors do not fit or where you want to preserve the door’s appearance from outside.

How to Manage Heat Loss in Multi-Unit Buildings and Flats

In flats and apartments, the hallway side of the door opens to an unheated or poorly sealed communal space, increasing the temperature difference across the door and driving heat loss through every gap in the frame. You lose heat through the building fabric of the flat wherever warm interior air meets cold surfaces or escapes through air leaks, and the front door is often the largest single source.

Seal the interior side of the frame with adhesive weatherstripping even if you cannot access the exterior. A door sweep installed on the inside face of the door stops the draft at floor level. Both work from one side and require no permission.

A heavy thermal curtain mounted on the interior side creates a buffer zone when the hallway temperature drops well below the flat temperature, common in older buildings with uninsulated corridors. It traps a layer of still air between the curtain and the door, slowing conduction and blocking convection currents.

Shared buildings often restrict door replacement or alteration. Focus on weatherstripping, sweeps, and interior measures that do not change the door structure. Schools managing heat loss through sealing air gaps follow the same approach: weatherstrip accessible frames, sweep thresholds, and caulk static cracks where the frame meets the wall.

Related Heat Loss Issues: Extract Fans, Chimneys, and Air Bricks

Extract fans and unused chimneys leak warm air continuously, even when not in use, creating a thermal escape route as powerful as an open window. The same stack effect that pulls smoke up a chimney pulls heated room air out through an unsealed flue or an unshuttered fan opening.

To stop heat escaping through your extract fan, install a model with a motorized damper that closes automatically when the fan switches off, sealing the duct. Retrofit a manual vent cover over the interior grille if replacing the fan is not practical. Heat loss through a chimney not in use stops when you fit a chimney balloon inside the flue or cap the chimney pot from above; both block the draft while remaining removable if you reopen the fireplace.

External air bricks provide ventilation for underfloor spaces and must not be fully blocked or you risk damp and timber rot. Use adjustable vent covers that reduce drafts while maintaining airflow, or replace fixed grilles with models that close partially in winter. For broader strategies, see heat losses in the home through vents and how to stop heat loss through unused vents.

Frequently Asked Questions