Where do thermal bridges form around windows and how can you prevent them? Reveals, lintels, sills and installation joints solved at the design stage.
28.10.2026 Ivan Landa
Thermal bridges often form around windows, causing unnecessary heat loss, moisture condensation and, in extreme cases, mould.
Windows are among the most sensitive points of the entire building envelope. It is not just about the quality of the frame itself or the triple glazing. The way the window is installed, how the reveals are built, how the lintel is designed and how everything connects to the facade insulation also play a major role. It is precisely in these places that thermal bridges often form, causing unnecessary heat loss, moisture condensation and, in extreme cases, mould.
Many of these problems don't start during installation. Their cause usually goes back much further, to the design documentation. If the area around the windows is designed correctly, most thermal bridges can be prevented. Which details need attention, and what solutions do modern building materials offer today?
A thermal bridge is a point in a building structure where heat passes through more intensively than through the surrounding parts of the structure. In winter, heat escapes from the interior to the outside through it, and the inner surface of the structure cools down.
Windows are a special case because several different materials and structural elements meet here. The window frame, masonry, reinforced concrete lintel, installation joint, window sill and facade insulation must work together as one functional whole.
If any part is not sufficiently insulated, or the individual layers do not connect properly, a spot with increased heat flow is created.
The most common causes of thermal bridges around windows are:
On its own, each of these problems may seem like a minor flaw. Taken together, however, they can significantly impair the thermal insulation performance of the entire building.
When designing a window detail, you need to think about its entire perimeter. It is not enough to solve only the side reveals and assume that the rest of the structure will work automatically.
The reveals are the side surfaces of the wall opening into which the window is installed. Thermal bridges often form here due to insufficient insulation thickness or a poor connection between the facade insulation and the window frame.
A typical problem is when the facade insulation ends too far from the frame, leaving an exposed section of masonry between it and the window. Masonry generally has poorer thermal insulation properties than the surrounding insulation and lets heat pass through more quickly.
The solution is to design the reveal so that the facade insulation connects seamlessly to the window frame. It is also important to cover the installation joint properly and to choose a material that provides sufficient insulation even in a detail with limited space.
The lintel is a load-bearing structural element that carries the load of the masonry above the window opening. It is often made of reinforced concrete or another high-strength material, which, however, does not have such good thermal insulation properties.
The problem arises especially when external shading is planned above the window. Venetian blinds or screen blinds need sufficient space, which must then be properly connected to the facade's thermal insulation.
If the design does not account for this detail, there may not be enough room for the shading box, and the lintel insulation will have to give way. This creates a structurally weak point.
One solution is Purenit lintel boxes, which combine space for external shading with structural and thermal insulation properties. They allow the detail above the window to be prepared during construction and reduce thermal bridges in this sensitive area.
The bottom part of the window opening is often overlooked during design. Yet a significant thermal bridge can form right under the window frame, especially if the window is installed on an unsuitable base material.
The risk is even higher with large glazed areas, French windows, balcony doors or lift-and-slide (HS) doors. These structures are very heavy and require a solid, stable base.
Conventional base materials may not always offer the ideal combination of load-bearing capacity and thermal insulation. Purenit is therefore a suitable alternative: it can carry high loads while helping to reduce thermal bridges at the bottom of the frame.
The correct connection of the interior and exterior window sills, the waterproofing and the facade insulation is also important.
The space between the window frame and the wall opening is called the installation joint. How well it is executed has a major impact on thermal insulation, airtightness and protection against moisture.
Installation foam alone is not enough. The installation joint should be designed as a multi-layer structure:
If this detail is forgotten at the design stage, executing it correctly on site can become unnecessarily complicated. That is why it is advisable to decide in advance how the window will be installed and how the individual layers will connect.
One of the most common mistakes in building design is solving individual elements separately. The architect designs the windows, the structural engineer deals with the lintels, the shading supplier prepares the blinds, and the contractor then has to work out how to fit all the parts into one functional structure.
This can lead to unnecessary compromises. There is not enough space for external shading, the insulation has to be locally reduced, or complicated details arise that are difficult to build on site.
The right approach is the opposite. The following should already be resolved in the design:
A temperature field calculation for critical details can also be very valuable. It makes it possible to assess the temperature distribution in the structure, identify risk points and verify whether there is a risk of moisture condensing on the inner surface under the designed conditions.
Solving these details early is usually easier than fixing problems after the facade has been completed.
Purenit is a construction material based on rigid polyurethane foam. It stands out for its combination of thermal insulation properties, strength, dimensional stability and moisture resistance.
It is exactly this combination that makes it a suitable material for structural details around windows, where conventional insulation materials are often not enough.
Purenit can be used in several places:
They create a solid base for installing window frames. They help reduce thermal bridges at the bottom of the window and are also suitable for heavy window and sliding systems.
They prepare the sides of the window opening so that the facade insulation can be connected correctly, while also creating a solid structural point for anchoring shading systems.
They solve the space above the window where the lintel, insulation and external shading meet. Purenit elements make it possible to create a structural base for the box while also insulating this detail.
They create space for installing shading systems directly within the facade structure. Their design allows the connection to the external thermal insulation system and the necessary anchoring to be planned in advance.
The advantage of Purenit is that it is not just a soft insulation material. Thanks to its mechanical properties, it can also perform a load-bearing and anchoring function in the structure.
One of the biggest challenges when building window details is how the individual materials connect. If each part is prepared separately on site, the risk of inaccuracies, gaps and unsuitable structural solutions increases.
Prefabricated elements greatly simplify this. They are manufactured to the specific dimensions and requirements of the project and arrive on site ready for installation.
Their main advantages are:
With the right design, the individual elements can be combined to create a functional window detail that treats the window, the insulation and the external shading as one whole.
Before finalising the design documentation, we recommend going through a few basic points. They will help you uncover the most common weak spots before construction begins.
If any of these points remain unresolved, it is a good idea to revisit them before ordering materials and starting construction.
Thermal bridges around windows are not just a problem in older or poorly insulated houses. They can also occur in new builds if the design of individual structural details is underestimated.
What matters most is the correct connection between the windows, lintels, base elements, installation joints and facade insulation. That is why it makes sense to solve the entire window detail at the design stage, when individual elements can be adapted without unnecessary compromises.
At ISOparts, we specialise in Purenit structural and insulation elements for window details. We offer base profiles under windows, prefabricated insulated reveals and flush-mounted boxes for external shading. We will help you find the right solution for your specific project and construction requirements.
Planning a new build or renovation? Get in touch with us. We will be happy to go through the technical solution with you and help prepare window details that work properly from the very start.
