Research leader Lars Gullbrekken shows how window testing takes place in the test chamber at SINTEF and NTNU in Trondheim.

Researchers have run over 1,000 tests on Norwegian windows: This is why rainwater leaks into homes

Researchers are contributing to the development of windows that can withstand heavier rainfall and stronger winds. Because that’s exactly what we can expect in the future.

Torrential rainstorms are becoming a major problem for our buildings. The combination of wind and rain forces rainwater into small cavities and openings and may result in leaks, causing significant damage to building materials. 

“This problem is exacerbated by climate change, which is expected to result in much more rain and wind than we are experiencing today. A more targeted focus on the climate change adaption of buildings is essential if we are to meet these challenges,” says Lars Gullbrekken. He is a researcher at SINTEF. 

Decades of testing windows

For many decades, researchers have been testing windows as part of the Norwegian annual product control programme, which in turn forms part of the voluntary NDVK compliance scheme for doors and windows. 

After two decades of testing, researchers at SINTEF and NTNU are now ready to present a summary of their results. Data from a total of 1,130 tests of the air tightness and rainproofing of different window types have led them to conclude that rain causes the biggest problems.

“Rainfall is the most significant cause of building damage in Norway. Many studies have highlighted that the frames mounted in building walls represent a weak point. But the window sash itself may also be vulnerable to defects and leakages. For this reason, there is a need for a quality assurance system for these products,” Gullbrekken says. 

Leaky joints

Data from 1,130 tests performed on a variety of windows demonstrate that the corners of a window are vulnerable points. As such, this represents the biggest challenge facing manufacturers trying to produce rainproof windows. 

Defects in joints on frames and sashes represent the most common leakage pathways, followed by gaps that allow water to penetrate between the sill and the frame. 

The solution to this problem is a thin rubber seal strip that is fitted along the opening edge of the sill and window frame. However, reducing the gap between the sill and the frame does not appear to improve rainproofing, Gullbrekken explains.

Has improved over time

The general trend is that the air tightness and rainproofing of windows has improved with time. Some types of window withstand rain and wind better than others. Swivel windows perform best, while the top hung varieties are the worst. 

Ultra-rainproof windows also exhibit the lowest levels of air penetration and can thus perform very well under harsh weather conditions. 

In contrast, larger and more complex window designs tend to fail the researchers’ rainfall tests more frequently.

Received help from a student

In order to produce the summary of many decades of work, the research team obtained help from former master’s student Therese Gransjøen, who joined the project as a summer job. 

“Therese has made a very good job of analysing and structuring these data. We’ve compiled all the knowledge that we’ve gathered over many years, and have learned a great deal from it,” says Professor Tore Kvande at NTNU's Department of Civil and Environmental Engineering.

Now it’s time for commercial actors to carry the work forward. 

“The results from our study may offer guidelines for the future development of weatherproof windows,” says Gullbrekken. 


Gullbrekken et al. Rain resistance of windows – Lessons learned from two decades of laboratory testingJournal of Physics: Conference Series, 2023. DOI: 10.1088/1742-6596/2654/1/012140


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About the study

This study has been funded by the Research Council of Norway as part of the research projects ‘Verktøykasse for klimatilpasning av boliger’ (Toolbox for the climate change adaptation of homes) and the innovation-driven CRI Climate 2050.

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