Dutch masters of steel since 1667
Dutch Masters of Steel since 1667

Thermally broken steel windows & doors

Published: May 23, 2022 | Author: Harry van Dalen

History of the steel frame profile

One of the most important characteristics of modernism in architecture was the use of slender steel frame profiles, such as in the renown Bauhaus Dessau building. At the beginning of the 20th century, new techniques made it possible to make larger facade openings in order to let in more light. In turn, giving a much larger audience with lower incomes, the possibility to have an abundance of light in their homes. These profiles were usually made from hot-rolled steel. The equal leg profile is perhaps the best known example of this. Working with solid steel resulted in very slim profiles. The welding of solid steel parts resulted in strong connections so that rotating parts could also be very slim. Moreover, it gave the architect a great deal of freedom to design his or her own frame detailing.

Cold bridge

But using solid steel also had a major disadvantage. Due to the high thermal conductivity of the material, large temperature differences between inside and outside caused problems with condensation on the steel, which in the end often led to corrosion problems. Not to mention the poor insulation the frames and single glass provided.

Profile-detail-SL30-ISO-PLUS EN

Thermal break

Nowadays, therefore, almost all metal exterior frames are fitted with a thermal break and insulated glass, instead of single glazing. But what is a thermal break? Essentially it is an insulation strip in between the steel inside and outside solid steel strips “breaking”  the thermal conductivity and stopping heat or cold from transferring, playing an important role in temperature preservation in the interior. It also enables modern steel windows to look almost identical to the windows used in the 20th century with the added benefit of energy efficiency.

What are the benefits of thermally broken steel windows & doors?

Thermal break steel windows & doors have excellent thermal insulation and sound insulation combined with the inherent strength of steel and slimness. This allows steel windows to comply with important environmental guidelines, such as BREEAM. Year round, the thermally broken windows and doors provide excellent insulation. For instance keeping the cold out in the winter and due to the large glass surfaces, allowing maximum light and warmth to enter. As the temperate of the inside steel strip is close to the room temperature, the possibility of indoor moisture condensing on the surface is avoided.

Thermal Insulation - heat transfer - SL30-ISO


What are the downsides of a thermally broken steel windows?

 The use of a thermal break means that these frames are built from separate parts and thereby lose their rigidity. By using increasingly thicker types of glass, the window and door frames are also becoming more and more bulky in order to be able to bear these large weights. Where design freedom initially led to great optimism among architects, it seems to be increasingly difficult to reach an agreement between the required functionality and the desired aesthetic. At least this was the case, until MHB invented the SL30-ISO® steel profile system.

Ultra-slim sharp-edged steel profiles

One of the most important goals that MHB has set for itself is to give architects the freedom to realize groundbreaking facade designs without compromising on the high technical & environmental requirements. The basis for this is laid with the invention of the SL30-ISO® & SL30-ISO-PLUS®: a solid steel window profile with an integrated insulator that forms such a solid whole that it has the same strength properties as the old uninsulated profile of solid steel. With the SL30-ISO®, architects can once again create ultra-slim frame profiles and design their own steel details. The strength properties of solid steel can again be used for what it was originally intended for: opening facades for more light and better visibility with a high degree of refinement in the material used.


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MHB Engineer
Harry van Dalen Engineer / Strength expert

Harry is intrigued by all the technical aspects of steel windows & doors and expert on various profile configurations.

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3d detail of the intersection of the MHB SL30-ISO-PLUS profile system

Profile Systems