Doors and Thermal Bridges

A thermal bridge is a part of a building where heat escapes more easily than through the surrounding construction, because insulation is interrupted, a conductive material crosses the insulation layer or a junction is poorly detailed. Doors create several such bridges: the threshold where the floor insulation stops, the frame where it meets the wall, the door leaf itself if it is thin or uninsulated, and small but leaky points such as letterboxes, keyholes and cat flaps. Each bridge is small on its own, but together they make a door a cold spot in an otherwise insulated wall.

The visible sign is often condensation or a cold draught at the threshold or frame, especially on a frosty morning, and the hidden consequence is heat loss that does not appear in a simple door U-value, because that value describes the door leaf and frame, not the installed junction with the wall and floor.

Good door detailing starts with the door itself: an insulated core, a well-sealed frame and a threshold that is thermally broken, meaning a low-conductivity material interrupts the metal or concrete path from inside to outside. The frame-wall junction should be sealed against air leakage and insulated, not just filled with foam that shrinks or with mortar that conducts heat, and the floor insulation should run up to, and ideally under, the threshold without a gap.

Letterboxes and other penetrations deserve the same care: an insulated, draught-sealed letterbox, a keyhole cover and a sealed cat flap reduce both air leakage and the cold spot they create. Internal doors matter less for heat loss, but a door between a heated living space and an unheated garage, porch or cellar should be treated like an external door, with insulation and seals, because it faces the same temperature difference.

Check each external door for draughts at the edges and threshold, for condensation on the frame or threshold, for a letterbox that rattles or lets in cold air, and for whether the door closes and locks without force, which indicates frame distortion or hinge wear. Note the floor construction at the threshold, since a solid floor and a suspended floor need different threshold detailing.

Ask for the door U-value, the threshold specification, the frame-wall sealing method and the letterbox and hardware details in writing. A door quote that gives only a leaf U-value and no threshold or junction detail has not addressed the thermal bridge question.

In the EU, thermal bridging and door performance are addressed through national building regulations and product standards under the EPBD framework, with member states differing on how junctions are calculated in energy certificates and on minimum door U-values by climate. Some countries require detailed junction calculations for new homes, while others use default values that may overstate or understate the bridge.

In the UK, Approved Document L covers thermal bridging in new and renovated work, with accredited construction details and junction calculations used in energy assessments, and building regulations set door U-value limits. Devolved standards differ in detail, and fire door requirements can constrain threshold and seal choices, so those rules should be checked together.

A frequent mistake is fitting an expensive insulated door while leaving the old threshold and an uninsulated frame-wall junction in place, so the bridge remains and condensation simply moves to the frame. Another is sealing a letterbox with tape or foam that blocks post and is removed within weeks, instead of fitting a proper draught-sealed letterbox that works every day.

A third mistake is ignoring the porch or garage door: a heated home that opens into an unheated porch with a thin, leaky inner door loses heat through that inner door just as it would through a front door. Treat every boundary between heated and unheated space as an external door, whatever its name.

Keep the door U-value, the threshold and frame-wall details, the letterbox and hardware specifications, installation photographs before trim is fitted and the guarantee for the door and its seals. Those papers let a later installer or assessor see how the junction was detailed, not just what door was bought.

Ask a qualified installer: how is the threshold thermally broken, how is the frame-wall junction sealed and insulated, what letterbox and hardware are proposed, and how will the door be adjusted if it drops on its hinges. Junction answers show a door that was detailed, not just delivered.