Guide blower door test

Nowadays, many new buildings are constructed to be completely airtight to ensure heat insulation. It prevents heating energy from escaping in an uncontrolled manner, resulting in a considerable protection of resources, environment and own finances. Whether the airtightness is given or if there was a fault in construction is determined by the blower-door test (BDT).

Average reading time: 9 min


Overview: Guidelines for airtightness

Blower door testing determines the tightness of the building envelope and helps indentify any leakage in the building fabric as well as energy-efficient renovation. The testing was first standardised by ISO 9972 and has been applied for about 20 years now. Meanwhile, the ISO is integrated into the new EN 13829 where the exact process of the single test phases is defined. Blower door tests as well as diagnosing air tightness is important for:

  • Energy-saving regulations
  • Heat insulation and air tightness of buildings
  • Determining air permeability of buildings

Installation of appliances concerning ventilation and air conditioning usually also includes measuring airtightness according to procedure A (description follows). The BDT should be done in all new constructions and renovated buildings to ensure compliance with standards.

The BDT can be done near the end of the construction phase (building envelope is finished) or after moving into the house. The time of the measuring is not as important as the preparation of the building in this context:

Procedure A: Examination of the current situation (after moving in, current state of old buildings)
Openings to the outside are closed just like in an everyday situation, however they are not sealed in a particular way.

Procedure B: Examination of the building envelope (usually done for newly erected buildings, quality control during construction phase)
A large part of noticeable openings, e.g. cat flaps or openings for the fireplace and fume hood are sealed before the test begins.

How does a blower door test work?

The machine consists of three parts: a fan, the actual measuring device with sensors inside and outside of the building (in the picture: right side on top of door) and finally the controlling software on the connected laptop.

A reinforcing metal frame around the blower is put up on-site by the professional and inserted into the exterior door frame or a big window. Possible gaps are sealed up with sheeting to ensure airtightness without any accidental air change. Equipped with sensors, the measuring instrument is also mounted to the door. The sensor system serves to register movements of wind and render them in real-time.

Before the test can begin all other windows and exterior doors are closed, other openings in the building envelope are sealed as well under procedure B. Atypical air passages like the chimney or waste water connection of shower and sink are masked with adhesive tape and film. Once the fan starts operating the data measured by the sensors is analysed electronically and shown on the laptop. The blower door test is done once with negative pressure and once with excess pressure. The target values can be compared to a moderate to fresh breeze (wind force 4-5), therefore it is necessary to stow away all necessary documents and light weight items for the duration of the test. The building itself can be used as usual during the test.

If the software indicates that a constant pressure of 50 Pascal is reached inside the building, the search for leakage begins.

The fan for the blower-door test is adjusted to the pressure value of 50 Pascal which is equal to five kilograms additional pressure per one square metre of the building envelope. Now the air change rate n is established. This value describes how of the indoor air must be exchanged for the previously adjusted pressure of 50 Pa. A good result for the n-50 value would be 0.3 in a building with heat insulation. That means the air volume is changed 0.3 times per hour (under this pressure) through small untight spots. A full 100% airtightness is actually not a desirable value at all, because that means air cannot escapte, mould will grow due to condensation of water vapour. The requirement for a low-energy house is a maximum n-50 value of 0.6, to name one example. Comparison: Non-renovated old buildings often have a n-50 value of 4 to 12 times per hour. These old buildings experience leaky joints and gaps inside the bricking, leasing to unpleasant draught and lost heat energy.

Limit value n50 (restoring buildings or new constructions)

Residential building, natural ventilation 3.0

Building with ventilation system 1.5

Passive house / Ultra-low energy house 0.6

Locating leakage

If a particularly high n50 value is measured, this can be an indicator for a leaky construction. For example, in case of a low energy house the value would be anything above 0.6. The indicated value of the volume flow shows the professional how much of the surface is leaky – the next step is to find out where the leaks are located. Each individual room is examined thoroughly with a thermal imaging camera. The process shows which spots in the house leak heat energy. Cool parts of the room are usually displayed in a blue colour on the camera, whereas red indicates heat and anything inbetween is rendered in yellow-green. Mostly windows, doors, outlets for electrical wiring and sometimes even the roofing is affected because they have the highest heat transfer (U values).
For instance, if the roof woodwork is rendered dark blue on the monitor that means there is a cold spot, a place with uncontrolled airflow. The examinant uses his hands to feel for the draught, human skin is sensitive to temperature difference. Another device is the thermal anemometer. It is held in front of the leak where it measures the airflow in m/s. This determines any draught inside the building. In order to determine all possible leakage it is important to check spaces that are not easily accessible as well. Any leaks that are found receive a makeshift repair with masking tape. If this measure decreases the n-50 value, it is a good sign that the construction defects can be rectified with minimum effort and alterations.

Give your neighbours a heads-up before using a fog machine – they might assume a fire has broken out!

Once the negative pressure has been measured the fan is reversed so that it sucks air from the outside of the building to the inside, generating a positive pressure. An efficient method is the measuring of the pressure difference: by adding colour to the indoor air with special fog machines or similar appliances it becomes clearly visible if and where the gas is forced out of the building. An assistant then carefully examines the building from the outside – wherever smoke escapes, there is leakage.

Typical leaky spots:

  • outlets for cables, wall sockets, switches
  • joints in building material
  • on guidance bars and pipe guides
  • fireplace, vent
  • doors, windows, glass inserts
  • building extensions such as balconies, bay window
  • top floor, gable
  • old buildings: material fatigue

Who conducts the test?

Licenced individuals include engineers, architects, and ventilation professionals with further education in energy consulting. Nowadays, construction companies usually also have a special department or at least one expert who is able to determine airtightness. A separate examinant will check the building for pressure difference before approving the building.

The duration of the test in a one-family house ranges from one and a half to three hours. The price calculation of your professional who is conducting sometimes already includes necessary sealing work. The locating of the leakage with a fog machine often charged in addition to the base price. Consult with your provider and let the professional explain exactly which services are included and how much they cost. However, keep in mind that the investment in a proper blower-door test now can save you trouble and extensive costs in the future. It is better to locate constructional defects right away instead of in a few years.


Draught inside the home is not only unpleasant but also causes increased expenses for heating and leads to preventable damage to the environment. During the construction phase of the building it is already possible to measure the pressure difference and locate leakage in the insulation and building materials, therefore the problem can be dealt with at an early stage. However, even existing buildings experience unintentional leakage and openings because building material simply wears out over the course of time. Silicone sealants on windows gets porous and loose, doors become distorted and the brickwork crumbles due to weather and temperature. Damage due to damp and mould also affects the fabric of a building.

In order to check the airtightness of a building envelope a powerful fan is positioned at the external door or window. In compliance with pre-defined parameters the blower first generates a low pressure, then a high pressure on the inside. Imaging methods now detect leakage caused by holes, gaps and weak spots in the building envelope. Thermal bridges, e.g. single glazed windows that let indoor heat escape to the outside become visible with an airtightness test. The determined leakage can be touched up and repaired afterwards. Oftentimes, unpleasant odours coming from adjecent flats are another reason why residents would want to investigate leaky spots.

The blowerdoor test is done by licenced craftsmen, construction firms, energy consultants or architects. The method can also be applied vice versa, for example if residents are under the impression that the building has been erected too airtight or the performance of the ventilation system is not enough to let out condensate and ensure sufficient air change.

Blower door test:

  • in newly-built houses and energy-efficienct reconstruction
  • actual condition of old buildings
  • recognising incorrect construction at an early stage
  • locating leakage and holes
  • detecting heat loss
  • saving heating costs
  • improving sound insulation
  • discovering odour nuisance from adjacent homes (smokers)
  • analysis of energy demand