Published 2026-01-29

The airtightness of a building affects everything from indoor environment and ventilation to moisture safety and energy use. Yet the importance of airtightness is often underestimated in house construction and renovation. To check airtightness, new buildings are usually tested with a Blower Door – a method that provides a reliable measure of the building’s total air leakage.

– “Airtightness is essential for a building to function as intended,” says Lars Tobin. He has worked with testing since the 1990s, including at SP (now RISE), and teaches airtightness and leak detection. In an interview with us, Lars explains how small leaks can have major consequences and why systematic measurements are crucial for a sustainable construction project.

Why is airtightness so important?

– A leaky house is not comfortable to live in. It often leads to draughts that reduce comfort and make it harder to keep energy use down. Comfort and energy go hand in hand. If you don’t have comfort, you don’t have good energy management,” says Lars.

But there is more to it than just energy optimization. Airtightness also affects moisture safety, fire safety and the quality of the indoor environment.

– Many people forget that it’s about fire safety, for example how easily a fire can spread between apartments. And about creating a clean indoor environment – you don’t want radon or other unwanted emissions getting in, or cigarette smoke from the neighbor. Airtightness is about human health and safety,” he explains.

A tight air barrier is essential to stop moist air from leaking into the structure and prevent condensation in cold parts of the house, which otherwise risks leading to moisture damage. During his many years in the construction industry, Lars has come across countless leaky buildings that suffered from moisture damage.

– Much of the moisture damage we see today could be prevented with the right knowledge and tools. Ensuring good airtightness avoids problems both during the construction process and in the long term. At the same time, airtightness affects how well the ventilation system works. In an airtight building, a larger proportion of the air enters through leaks instead of through the vents, making the ventilation flow difficult to control, especially in F-systems and to some extent in FT-systems.

When should the blower door test be conducted?

Air leakage is not always visible to the naked eye and without the right measurement method, critical leaks can easily be missed. For new construction or major renovations, the building is therefore tested with a Blower Door.

– It’s important to test the house at an early stage so that any shortcomings can be rectified. If you only discover problems after the internal walls are in place and everything has been plastered, it can be an expensive story to correct,” says Lars.

If deficiencies are found, it is always advisable to follow up with a new test after the measures to ensure that the desired result has been achieved. A Blower Door measurement shows the overall airtightness of the building, but to find exactly where the air is leaking, the test needs to be supplemented with leak detection. This can be done with a thermal imaging camera or smoke, depending on the season and prevailing conditions. A thermal imaging camera is useful when there are temperature differences between the indoor and outdoor environment. Smoke works all year round, but is a more time-consuming method.

How to get reliable results

– Many people think that experience is enough to know when a house is airtight enough. But airtightness must be checked, no matter how confident you are in your own ability, says Lars. He recommends the following structured approach:

1. Check drawings in advance – can you build airtight from the plans?
2. Educate staff on site – awareness of all construction workers is crucial.
3. Control during the construction process – have someone other than the builder carry out visual checks.
4. Early-stage pressure testing – address any issues and retest to verify the results.

While conducting a blower door test is relatively easy, attention to detail is what ensures reliable results.

– Sealing all leaks that are not included in the pressure test is essential. It is important to pay attention to all the details: are the windows closed? Is it tight around the penetrations? Is there water in the floor drains? And if it’s too windy outside, the curves can be uneven and the measurement results uncertain. It requires accuracy and some experience not to miss anything,” says Lars.

During our one-day course in airtightness testing, Lars goes through practical testing with Blower Door, common sources of error and what is required to get reliable results. The course is included when you buy a Blower Door from us.

Airtightness testing as quality assurance

There are no rules on how airtight a building must be – only that it should be “as airtight as possible”. Lars therefore emphasizes that developers should set clear requirements for airtightness already at the planning stage, so that there are concrete benchmarks to follow throughout the construction process.

For him, airtightness testing provides more than a measurement. It is a tool to control the quality of the work and an integral part of the quality assurance of the construction project.

– Pressure tests serve as a verification of work quality. They help contractors to see if the building is of high standard, while ensuring it is safe to occupy and sustainable in the long term. The energy savings provided by an airtight building are just a bonus – the most important thing is people’s health and safety.

He concludes with advice for construction consultants and contractors:

– Consider airtightness as part of a systems approach. Set a clear requirement for airtightness, verify it with testing and leak detection, and see the connection between construction, ventilation and airtightness. This will result in a safe and sustainable building.

Get started with airtightness testing

Published 2023-03-13

With powerful fans in door panels – so-called Blower Doors – Olle Syrén of AEM Byggkonsult checks the airtightness of residential and commercial buildings. Firstly, to confirm developers’ energy calculations and secondly, to detect any air leakage.
– “We see a continued increase in demand for density measurements,” says Olle.

Energy-efficient buildings have long been on the agenda. That is, houses that, thanks to smart design, require minimal energy and therefore have low running costs. Airtightness is an important parameter to consider in order to avoid unnecessary energy consumption. When it is cold and windy outside, a building with poor airtightness lets in large amounts of extra air that needs to be heated. Air leakage and internal overpressure also increase the risk of moisture damage to the building envelope.

This is where Olle Syrén comes in. Olle previously worked as a carpenter, supervisor and project manager on construction sites before he was attracted to AEM Byggkonsult to do construction inspections and density measurements.

– Density measurements started as a ‘fun thing’. We saw a clear need for such measurements. Not least for housing companies that needed to carry out density measurements to verify their own energy calculations in connection with new buildings,” says Olle.

Measures of house tightness
In addition to confirming the energy declarations, these measurements also show that new constructions comply with the Planning and Building Act and are in line with the building law. This, in turn, means that the building authority can issue a final decision, after which buildings are considered completed and can be put into use.

How does a Blower Door tightness measurement work? Well, in short, it’s like this: A large fan is installed in a front door using a frame and textile panel that fits tightly around the door frame.

All interior doors should be open while other exterior doors and windows should be closed and fans and ventilation systems should be turned off. First, a certain overpressure (e.g. 50 Pa) is created in the building. The airflow is then reversed to generate a corresponding negative pressure. The average of the airflows required to create these pressures provides a measure of the density of the building.

“Simple and easy”
Olle Syrén talks about the three Blower Doors he uses for different missions.

– The Retrotec 300 fits in an ordinary car. It is simple and straightforward. Ideal for pressure testing in small spaces in apartments and small houses. Then I have a Retrotec 5000. It is the Blower Door that I use the most. Especially in many villas. It is very versatile and can handle small jobs as well as quite large measurements. Our latest purchase is the Retrotec 6310 powerhouse. It has triple fans and I use it mainly for measurements in industrial halls because in these cases it involves moving very large volumes of air.

It is the variety of challenges that makes the job exciting and fun, says Olle Syrén. Mostly Olle test prints newly built houses, but recently he has also been commissioned for industrial properties. One example is the construction company Ylab’s 90 000 square meter transshipment center in Stigamo outside Jönköping.

Advocating early measurements
A measurement usually takes 5-10 minutes. The measurement in this large warehouse took almost an hour. Even with three large fans running at full speed, it takes quite some time to create the right pressure in such large spaces. And in order to get the safest possible test, it sometimes takes some manual effort to control fans and get the pressure you want. All this takes time, of course.

It is increasingly common for construction companies to order density measurements during construction, which Olle also advocates.

– This is when you have the best chance of addressing any air leaks. When everything is painted and finished, you have to demolish walls to fix the defects.

What common errors do you find during airtightness measurements?
– I sometimes find that electricians, plumbers and other tradesmen have been negligent in sealing jobs. In some cases I have observed concrete slabs with leaking sills and poorly operated windows, to give a few examples.

What kind of measurements do you do most?
– The majority of the jobs involve measurements of completed building envelopes. But from time to time, other assignments come up as well. For example, I once assisted in an investigation into the smell of cigarettes between two apartments in an apartment building. Using a blower door, I discovered a leak through the insulation in the foundation of the house.

Is there anything in particular that makes the job challenging?
– No two jobs are the same. There is always some special detail to consider in order to get a good measurement. During the actual driving, you need to make sure that the measurement is done as it should be. In many cases, you need to have door guards so that people do not enter the spaces to be measured. To go to the toilet or “take a breath of fresh air” (laughter). ■