When external venetian blinds are being planned for a house, most people focus mainly on their appearance, colour or control method. But in reality, the final result is often determined by something completely different — the detail above the window. That is exactly where the space for concealing the blinds is created, and if it is not resolved properly from the start, it often becomes unnecessarily complicated, expensive and visually unappealing later on.

That is exactly why PurBox was created. It is a smart and construction-friendly solution that prepares the space for external venetian blinds so that everything works correctly from a technical point of view, looks clean and does not create unnecessary heat loss.

What is PurBox and why use it for external venetian blinds

PurBox is a prepared box designed for integrating external venetian blinds into the façade. Put simply, it creates the space where the blinds are hidden once they are raised. At the same time, it is much more than just a “blind box”. A well-designed PurBox also helps insulate the space above the window and significantly reduces thermal bridging, which is a common issue in these kinds of details.

One major advantage is the material used. PurBox systems are made from purenit, or in combination with PIR insulation, which makes them lightweight, strong and dimensionally stable. They do not change under the influence of temperature, moisture or contact with construction chemicals. In practice, this means greater confidence that the detail above the window will function properly over the long term and will not deform over time or complicate further work.

The concealed plaster solution is also highly suitable from the perspective of insulation around lintels and window connection joints. The prepared boxes are easy to install within an ETICS façade insulation system and also work very well in modern low-energy and passive houses.

Which PurBox to choose: standalone vs. lintel PurBox

Choosing the right type of PurBox depends mainly on the stage your house is currently at. And this is exactly what makes the difference when deciding.

If your house has already been built, the windows are installed and you are preparing for external venetian blinds before the façade insulation is applied, a separate PurBox for external venetian blinds is usually the right choice. It is installed afterwards above the finished window and creates the necessary space for future concealed shading. It is a practical solution when the shell is already complete and you need to prepare the detail without complicated structural intervention.

A different situation arises when the house is still under construction and the structural openings are being prepared. In that case, a lintel PurBox makes excellent sense — a blind box that is installed during construction and replaces the lintel above the window. It is therefore not an additional element, but a full structural component that is planned from the very beginning.

Purenit blind box: PBI vs. PB and the differences between them

There are two basic box types in the range: PBI and PB. The main difference between them is whether the box also includes insulation.

PBI – purenit box with PIR insulation is the most commonly used and also the most efficient option. In this version, the purenit box is combined with PIR insulation approximately 30 to 60 mm thick, creating a very well-insulated detail above the window. This option is ideal wherever you want to create not only the space for the blinds in one step, but also a thermally efficient detail without thermal bridges.

PB – purenit box without insulation is made purely from purenit boards and is mainly used where the lintel insulation has already been solved separately. In that case, the box mainly serves to create the correct space for external venetian blinds within the ETICS façade insulation system.

How to measure PurBox correctly

One of the major advantages of PurBox systems is that they are among the easiest products to measure. That is exactly why they are also suitable for ordering via an online store and customers do not need to worry about them unnecessarily.

In most cases, all you need to know is the external width and height of the window frame. You can find these dimensions in the window manufacturing documentation, or simply measure them directly on site.

For a standalone PurBox, the required dimensions are:

• width according to the external width of the window frame,
• height according to the external height of the window frame without the sill profile.

The final box width then corresponds to the width of the window frame, while the box height is calculated according to the height of the window and the selected blind type. The box depth is then determined according to the slat type and the chosen solution.

For a lintel PurBox, the principle is similarly simple, except that the frame dimension is not used — instead, the clear structural opening is used. To place an order, it is therefore enough to know the external window dimension, or more precisely the minimum clear opening.

With lintel boxes in particular, it is important to determine the opening width correctly, because they are manufactured in predefined dimensions. So if, for example, the clear opening is 1,100 mm, the box is not selected to match this exact size, but rather the nearest larger option that will safely cover the opening. At the same time, you also need to allow enough space for seating the box into the structure on both sides.

In other words:

• for a standalone PurBox, you work from the dimensions of the window frame,
• for a lintel PurBox, you work from the clear structural opening.

That makes the selection and ordering process much simpler than it may seem at first glance.

In most cases, it also applies that standard purenit boxes with PIR insulation are supplied with 30 mm insulation on the rear side. The delivery usually also includes insulated reveals with installation profiles for the plaster-in guide rail, a starter mounting U-profile, a concealed plaster trim for the front face of the box, and mounting brackets. This gives the customer a ready-made solution that saves both time and labour on site.

The standard measuring method is suitable for window frames with a thickness of 40–100 mm. If the frame is smaller than 40 mm, it is better to send the window production documentation and photos of the installation on site so that an individual technical solution can be proposed.

How to choose the right PurBox size and depth

This is exactly where mistakes are often made. At first glance, it may seem enough to solve only the width of the window, but in reality the correct size and depth of the PurBox are what matter most. These depend not only on the size of the opening, but above all on the blind type, the blind dimensions and the façade build-up.

In general, the following indicative depths are used for standard blinds:

• PurBox 180 mm is suitable for standard Cetta 80 and Zetta 90 slats when using 30 mm PIR insulation,
• for narrower Cetta 65 and Zetta 70 slats, PurBox 160 mm is used,
• PurBox 160 mm is suitable for standard Cetta 80 and Zetta 90 slats,
• for narrower Cetta 65 and Zetta 70 slats, PurBox 150 mm is used.

Choosing the right depth should always be based not only on the blind type, but also on the planned insulation thickness.

What if the box is longer or corner-shaped?

For larger windows or assembled sets, it is also necessary to consider that the box may not always be supplied in one piece.

If the PurBox is longer than 2,400 mm, it is supplied as standard in two parts. The connection is then made using a joining kit, i.e. connectors or brackets. If required, the division can also be designed individually according to the specific site situation.

Corner assemblies can also be solved in the same way, where it is necessary to provide the external dimensions of the entire set and ideally add a simple exterior-facing sketch. This makes it possible to prepare a solution that exactly matches the specific building.

What insulation thickness should be chosen for PurBox?

When designing a PurBox, it is also worth thinking about how thick the ETICS façade insulation will be. This has a direct impact on how elegantly the box can be concealed within the façade and how well the entire detail will perform.

From a long-term perspective, it is generally recommended to choose insulation ideally around 180 to 200 mm, because insufficient insulation may later lead to greater heat loss, window condensation or reduced thermal comfort.

For easier orientation, the following common options can be used:

• With 150 mm insulation, blinds with Cetta 65, Zetta 70 or Setta 65 slats and a PB 150 mm box are usually suitable.
• With 160 mm insulation, Cetta 65 and 80, Zetta 70 and 90, Setta 65 and 90 blinds and a PBI 160 mm box can be used.
• With 180 mm insulation and above, practically any standard type of external venetian blind and a PBI 180 mm or larger box can be used.

System reveals for PurBox: types and installation method

Just as important as the space above the window is the detail at the sides. And this is where system reveals come into play, helping to resolve the junction between structures and the concealed guidance of the blinds.

The result is a cleaner appearance, easier installation and less improvisation on site. The reveals can also be easily adjusted to the required dimensions, which means the work proceeds quickly and accurately. These details have a major influence not only on appearance, but also on functionality and the long-term durability of the whole solution.

In practice, you will most often come across three options:

System insulated reveal

The system insulated reveal is suitable wherever you want to resolve the detail around external venetian blinds in a simple and clean way while also maintaining good thermal insulation performance.

It is made from a stabilised thermal insulation foam polystyrene board with high compressive strength requirements. This makes it an ideal solution especially for polystyrene façades, where it integrates well with the complete insulation system.

A major advantage is that it is designed for full-surface bonding, so installation is usually quick, clear and technically clean. That is exactly why it is a very popular option wherever the goal is a simple and reliable solution without unnecessary complications. It can be used both with a purenit box and with a sheet-metal box.

PET reveal

PET reveal is a highly versatile option, suitable wherever you need greater flexibility in both installation and application.

The material is rigid, lightweight and holds anchoring hardware well, which is a major practical advantage on site. At the same time, it is resistant to construction chemicals and moisture, so it also performs well in more demanding conditions.

PET reveal is suitable for both polystyrene façades and ventilated façades. It can be used for both mechanical fixing to brackets and full-surface bonding, which makes it a very practical and universal solution. Another advantage is the quick and easy installation, thanks to which it adapts well to different types of construction details.

Purenit reveal

Purenit reveal is a robust and highly durable option that works well with the entire PurBox system. It is made as a sandwich of purenit and XPS, specifically 10 mm purenit as the carrier board and 20 mm XPS as insulation.

This option is especially suitable for mineral wool façades and ventilated façades, where a stronger and more technically resistant solution is needed. Unlike the previous options, it is usually not bonded, but mechanically installed to the window frame using brackets, although it can also be bonded to a solid substrate.

Its major advantages are easy installation, high durability and at the same time excellent insulation performance, which helps solve the detail around the windows and blind guides effectively.

That is why the choice of a suitable reveal always depends on the specific building detail, the façade type and the installation method that is most suitable for the given project.

How PurBox installation works

One of the main advantages of PurBox is the speed and simplicity of installation. This helps avoid delays on site and allows the individual stages of work to follow on from each other more smoothly.

After measuring, the appropriate box type is selected, installed into the opening and connected to the surrounding structures. Thanks to its low weight, handling is easy, and for lintel versions even heavy equipment is not required. The façade can also be applied directly onto the PurBox after suitable priming without unnecessary complications, which speeds up the entire process even further.

The external venetian blinds themselves do not have to be dealt with immediately. You can order them together with the PurBoxes, or later once the building is ready. This gives greater flexibility in both planning and budgeting.

Why choose PurBox

PurBox is not just a technical accessory. It is a solution that helps avoid common problems when preparing for external venetian blinds, simplifies work on site and at the same time delivers a cleaner façade finish.

Not sure which PurBox is right for your house? Contact us and we will be happy to help you with the selection, measuring and the right solution for your specific project.

Complete range of Purboxes