Thermal Breaks in Metal Windows and Doors

A thermal break (or thermal barrier) is a material of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials.

When it comes to metal framed windows and doors a thermal break is essentially a thermal barrier in the conductive metal with a material of low thermal conductivity. This then stops the conduction of heat through the metal framing and to the external side of the installation.

Why is a Thermal Break Important?

Thermal Break technology is important when it comes to metal framing systems as it separates the frame into two separate interior and exterior pieces joined with a less conductive material. This ‘break’ in the metal reduces temperature transfer across the framing system and ensures the system achieves modern thermal performance values.

The concept of a thermal break is similar to that of a double or triple glazed unit; creating a thermal barrier against heat loss by introducing a low conductive material to the system makeup. In insulated glass panels such as double glazing, this is a gas filling and spacer bars. In framing this is the ‘thermal break’.

These thermal breaks are generally made of a substantially rigid, low thermal conductive polyamide or polyurethane material, which are naturally good thermal barriers. The thermal break material is then mechanically locked in the metal framing to create a thermally broken system.

steel windows and doors to modern house
steel look patio doors to modern extension

What is the Relationship Between a Thermal Break and Thermal Performance?

If a metal frame does not have a thermal break in it you will experience high levels of heat loss through the framing. This will reduce the Uf value of the system (the thermal performance of the frame) and subsequently reduce the overall thermal performance of the window/door (the Uw value).

If a thermally broken window/door system is not used in most cases it will be impossible to achieve the thermal performance requirements for the project.

In order to adhere to modern thermal performance requirements (and to achieve Building Regulations minimum requirements for thermal insulation) a thermally broken frame should be used in conjunction with an insulating glass unit with Ug value 1.1 W/m2K as a minimum. You can achieve this Ug value by using a double-glazed unit with a low e coating and argon gas filling.

For more information about Uf and Uw values see our technical article ‘What is a U Value?’

Where Should Thermally Broken Systems be Used?

For useable, warm environments a thermally broken system to all external framing is a must. Aside from the relevant Uw values required by modern building regulations, using a non-thermally broken framework to an internal space can lead to freezing on the internal framing in colder months and condensation build up internally on the cold metal surfaces. This can lead to mould and damp to build up on internal building finishes such as real wood flooring and curtains.

In short, thermally broken systems should be used anywhere there is a difference in climate on either side. This could be between inside and outside, but it could also be between an indoor swimming pool environment and a living space.

thermally broken steel patio doors
slim aluminium sliding doors with thermal break

Thermal Breaks in Aluminium Framing

As aluminium framing systems are generally modern systems, almost all of them will have a full thermal break within them. It is always worth double checking this just to ensure the aluminium system is fully thermally broken and highly insulating.

You can tell if a system is fully thermally broken either by a framing cross section or taking a look at the thermal performance values of the system. In general, if a system has a Uw value of 1.5 W/m2K or better it will have a thermal break.

Some examples of thermally broken aluminium systems:

minimal windows® sliding doors

Sieger® Architektursystem Windows

Sieger® 120 Bifold Doors

HIRT Swiss Descending Windows

Apertio Glass Entrance Door

Thermal Breaks in Steel Framing

As steel systems are a more traditional framing material many of the systems available on the market were developed many years ago (before modern thermal performance requirements were put into place) and therefore there are many steel systems available that are non-thermally broken.

Careful consideration must be taken when specifying steel and architectural metal framing systems to ensure they are fully thermally broken and give you the thermal performance you need for your project.

Some thermally broken steel systems:

Mondrian® CWS Slim (50, 65 or 75)

Mondrian® CWS XL (50, 65, 75 or 85)

Mondrian® FR (fire rated steel systems with a thermal break)

Again, you can tell if your chosen system has a thermal break within it by looking at the section drawings and investigating the thermal performance data given.

steel framed doors to extension
thermally broken steel pivot doors

Look Out for Misleading Information

In order to disguise that a steel window or door is not thermally broken a glazier or fabricator may quote the Ug value as the ‘U value’ for the system. The ‘U value’ of a double-glazed unit may be 1.0 W/m2K but unless it is installed into a thermally efficient framing system your overall thermal performance (the Uw value) will be no where near that.

This can be misleading to architects and specifiers who see the ‘U value’ and assume this is the overall thermal performance for the system when in fact it is not.

You should always look for your glazier to be specific about what type of U value they are talking about. It should be the Uf value (thermal performance of the frame), Ug value (thermal performance of the glass) or Uw value (overall weighted thermal performance of the glass installation).

If you wanted to ask any questions about thermal break technology, or if you wanted to check which systems of ours are thermally broken, just get in touch with the team at IQ.

Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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