Thermally broken doors

Thermally broken doors are steel security doorsets with an insulated break between the inner and outer skins, cutting heat loss while holding a forced entry rating. ArxBridge supplies and installs them across the UK for heated buildings that also need security.

• Low U-valuesReduced heat loss through the door
• LPS 1175 ratedForced entry resistance included
• Thermal breakInsulated barrier in leaf and frame
• External useBuilt for exposed elevations

The basics of thermally broken doors

A steel door is a thermal bridge: heat flows straight through the metal from the warm inside to the cold outside. A thermally broken door interrupts that path with an insulating barrier built into the leaf and the frame, so the inner skin stays warm and the outer skin stays cold. The door keeps the building’s heat in while keeping intruders out. The Fire range overview sets out the related fire rated products.

These are external doors for heated buildings: schools, hospitals, offices and commercial units where the entrance doors sit in the thermal envelope and the energy assessor counts every watt. The security rating, usually to LPS 1175, comes from the same steel construction that carries the break.

The thermal performance is designed in, not added on: the break runs continuously through leaf and frame, and the seals complete the envelope. Ask how the break is detailed at the threshold and the ironmongery, because those are the weak points.

Which rating do you need for thermally broken doors

The four lines are independent: thermal, security, fire and weather each have their own evidence. The fire ratings guide and LPS 1175 security ratings cover two of them.

Check the threshold. The thermal break is only as good as its weakest detail, and the threshold is usually it. Ask how it is broken before you specify.

Inside thermally broken doors

The leaf is two steel skins separated by an insulating core and a structural thermal break: a low-conductivity barrier that joins the skins mechanically while blocking heat flow. The frame uses the same principle, with the break running through the section. Together they give a doorset with far lower heat transfer than a standard steel door.

Seals do the rest: compression seals around the leaf, a threshold seal at the bottom and brush seals where needed. Air leakage undoes insulation, so the sealing is as important as the break. Ironmongery is thermally considered: through-bolted items get insulating sleeves or pads where the design allows.

The security construction sits inside this: reinforced lock and hinge zones, multi-point locking and a deep frame section. The thermal break does not weaken the door; the two systems are engineered together and tested together.

Thermally broken doors with combined ratings

Fire is the natural companion for compartment walls: the thermal break and the fire insulation serve different purposes, and a doorset can carry both where the evidence covers it. Fire rated steel doors covers the EI side.

Forced entry to LPS 1175 is standard on these doors: the security doors range sets out the classes. Security windows alongside keep the whole elevation consistent: a thermally broken door next to a poorly insulated window wastes the investment.

Acoustic performance can be added through seals and core density for schools and hospitals near busy roads. Each performance needs its evidence on the same doorset.

Typical uses for thermally broken doors

Education buildings use them on main entrances and plant access where the thermal envelope meets daily punishment. Healthcare estates use them on entrances and service doors where heat loss and security both matter.

Offices and commercial units use them wherever the entrance is in the heated envelope. High-value retail back-of-house doors pair thermal performance with attack resistance.

For internal fire compartment doors where thermal performance is not needed, the standard fire rated steel doors are the simpler choice.

Cost and lead time factors for thermally broken doors

The thermal break adds material and process steps over a standard steel door: the insulating core, the break profiles and the extra sealing. Higher security classes add steel in the reinforced zones.

Size and configuration matter: double leaf entrance doors with glazed side panels need the break detailed across every element. Special colours and finishes add time.

Energy calculations should be done before ordering: the assessor confirms the doorset meets the target, and late changes to the specification can mean rechecking the whole envelope model.

Specifying thermally broken doors

1. Set the thermal target. The energy assessor states what the doorset must achieve.
2. Set the security class. The security consultant names the LPS 1175 rating.
3. Note fire and weather. Add any compartment or exposure requirements from the strategy.
4. Demand the evidence. Ask for thermal calculations and LPS 1175 test reports on the same doorset.
5. Detail the interfaces. Agree threshold, seals and ironmongery with installation before manufacture.