Blower Door Test: How airtight is a house?

The Blower Door Test is a differential pressure measurement method that provides information on how airtight a building is. The test makes it possible to determine the air exchange rate inside the building and detect leaks in the building envelope.

A blower door test thus helps to optimize the energy efficiency of the house. At the same time, causes for subsequent damage to the building structure are eliminated during the construction process. A sensitive issue here are in particular leaks in the thermal insulation. Leaky spots in the insulation layer lead to so-called thermal bridges. Possible consequences may be the formation of condensation, moisture damage and mildew. In addition, depending on the test results, the internal climate and thus the living comfort of a building can be optimized right from the start.

Table 1: Areas of application and optimization possibilities through a blower door test

Blower door testApplication / Opimierungsmöglichkeiten
1.Avoidance of energy losses
2.Prevent mold growth
3.Avoidance of drafts

How does a building become airtight?

The airtightness of older buildings usually leaves much to be desired. Cracks in the masonry and leaking joints cause a great deal of heat energy escapes from the house at these points and the residents especially in the cold season additionally suffer from drafts. The most airtight constructions are a prerequisite and at the same time the result of a professional thermal insulation.

The thermal insulation simultaneously acts as airtightness and wind protection layer

In the Energy Saving Ordinance (EnEV) 2014, a complete insulation of the building envelope is required for new buildings as well as for older houses with energy-saving measures. Exceptions apply only to roof and basement: If the attic is not used for residential purposes, insulation of the top floor ceiling can be made instead of a complete roof insulation. For a EnEV-compatible basement insulation in the context of renovations is sufficient for a not intensively used and unheated basement cellar ceiling insulation. The insulating layer usually takes over the task of airtightness and wind protection layer at the same time.

Airtightness is not identical with vapor-tightness

The main argument of the insulation opponents is that a house must "breathe" in order to create a pleasant living environment and to protect the building fabric. This means, however, that many constructions require the diffusion-open nature of the thermal insulation - ie the possibility of active moisture regulation. Improper execution of the insulation, incorrectly installed vapor barriers or insulating materials that block the moisture exchange can result in serious building damage. However, airtightness and vapor-tightness are not identical. A diffusion-open thermal insulation can (and must) still be airtight. The degree of their diffusion openness, however, depends significantly on a possibly introduced vapor barrier film and especially on the respective insulating material.

Relevance of thermal bridges

Lack of airtightness results in a well-insulated house, especially from the presence of thermal bridges. These are areas from which the heat is released to the outside faster than in the neighboring areas. Thermal bridges are particularly common in window and door openings, rafters and other beam constructions with external wall contact, as well as in utility connections. If such sites are not sealed, large amounts of energy can escape from them unused. At the same time, the surface temperature on the room side of the wall decreases in these areas. This results in the risk that condensation and long-term moisture damage or mold.

Tips & Tricks

The most complete possible air sealing is important for the protection of the building fabric. Mold damage due to thermal bridges can also occur without condensation water failure. If the humidity on the wall surface is 70 to 80 percent due to low surface temperatures, mold can already form.

Requirements for airtightness of buildings

The aim of thermal insulation is to achieve the best possible living comfort with the least possible expenditure of energy. The national DIN standard 4108 prescribes for new buildings and energetic renovations the creation of an air and wind sealing layer over the entire surface. With increasing insulation thicknesses, this is becoming increasingly important: Although transmission losses of heat energy are minimized by the insulating layer, it loses its efficiency when the energy supplied is lost through leaks - that is, through convection.

Determination of airtightness via the air exchange rate

The airtightness of a house is determined by the air exchange rate. The air exchange rate n is a measure of the supply air volume flow in relation to the respective building volume of the building. The reported value indicates in each case a multiple of the room volume.

The rate of change of air results from the air volume flow within one hour, if a pressure difference of 50 Pa (Pascal) is maintained to the outside air - it is therefore indicated by a value n50. The air volume flow is divided by the room or building volume. An air exchange rate n = 15 / h means that within one hour, the air volume within a building will be completely replaced 15 times over an hour. The smaller the value, the more airtight the building is.

Statutory regulations for the air exchange rate

The air change rates in residential buildings are also regulated by the DIN 4108 standard, which specifies limits for different types of buildings. For buildings without ventilation systems an air exchange rate of n50 = 3.0 is required, for passive houses a value of n50 = 1.5 is required.

The so-called hygienic air exchange rate is n50 = 0.3. It is the minimum level for air exchange in buildings. If it is undercut, it may cause odor problems, high levels of dust and microorganism exposure, and excessive radon levels.
The air exchange rate can be measured by a blower door test.

Table 2: Limits for air exchange rates in conventional buildings and passive houses and "hygienic air exchange rate"

RequirementAir exchange rate n50
Building without ventilation system3,0
passive house1,5
Hygienic air exchange rate0,3

How does a Blower Door Test work?

On the one hand, the blower door test serves to detect leaks in the building envelope, and the actual air exchange rate is determined by the measuring procedure. In this case, air is forced into the building by a fan or, on the contrary, it is sucked out. The regulation of the air volume flow or its adaptation to the building density is carried out by means of differently sized measuring orifices. During the measurement, a pressure difference of 50 Pa is created between the interiors and the external environment of the building. Factors such as wind load that impact the outside walls of the building at the time of measurement also have an influence on the real pressure differences. For this reason, a calibration of the meter must be done in each case.

Application of the measuring fan

The fan for the Blower Door Test has an adjustable metal frame, which is equipped with rubber seals and an air-impermeable tarpaulin. It is used airtight in a door or window frame. The measurement in a doorway gave the test procedure its name - the German translation of the term is "fan door test". The metering orifices create an overpressure inside the fan. The integrated measuring instruments determine:

  • The pressure difference between outdoor and indoor spaces
  • The pressure in the fan itself: This value determines the size of the air flow that is transported through the meter.

The speed of the device is controlled so that a pressure difference of 50 Pa is achieved. In the vacuum measurement as much air is discharged to the outside as penetrates through leaks in the interior of the building. The air exchange rate is calculated on the basis of the values ​​determined by the test.

Information about the location and effects of leaks

A blower door test provides qualitative and quantitative information. It allows:

  • The location of leaks and the determination of their strength.
  • The determination of the air flow, which flows at a test pressure of 50 Pa through the entirety of the existing leakages.
  • The calculation of the air exchange rate n50.

Sequence of a blower door test

The sequence of a blower door test takes place in the following work steps:

  • Tour of the building, recording the spatial conditions.
  • Temporary sealing of function openings: These include, for example, extractor hoods, fireplaces, ventilation openings in bathrooms and toilets or empty drainage pipes.
  • Construction of the measuring fan.
  • Performing the measurement and leakage location.

When testing the airtightness of the roof insulation, artificial smoke is often used during the test. It is advisable to inform the fire brigade and the neighbors about the intended test in advance.

Final report and recommendations for action

The client for a blower door test receives a report on the test results, which has the status of a report. For example, the performance of the test and the submission of measurement results are prerequisites for the application for public funding for the construction of a passive house. Where appropriate, the report will include a photographic documentation and recommendations for the client to carry out necessary sealing measures.

Time and costs for a blower door test

Blower door tests are carried out by specialist companies specialized in this field. The costs and time required for the test depend on the size of the building, the required preliminary work and the workload for identifying and documenting leaks. The measurement itself takes about two hours.

Time required for a single-family home: at least half a working day

A complete blower door test takes at least half a working day, including the construction of the measuring device and the sealing of function openings for an average single-family dwelling. Financial support for the costs of a blower door test by KfW or regional funding programs is possible.

The cost of the test includes a few other factors. These include, for example, basic prices and hourly rates, expenses as well as the effort for documentation production. As a guideline for a single-family home, costs between 350 and 600 euros can apply.
Recommended before a commission, however, the comparison of several offers. With the same scope of services, the prices of the providers can be quite different.

Blower Door Test - quality inspection for buildings

A blower door test simultaneously checks the quality of the building. For new buildings, it can already be part of the planning and then serves to check whether a complete air insulation of roof and exterior walls including window and door openings and connection points has been made. In old buildings, the test serves on the one hand to check the quality of the remediation measures that have taken place. On the other hand, a blower door test can be used to evaluate the condition of the building fabric before the completion of the construction work. The test can be carried out in single or multi-family houses, but also in individual apartments.

Tips & Tricks

A blower door test is not mandatory for new buildings or for refurbishments, but is recommended for quality control of construction and insulation measures. For passive houses, an air exchange rate of n50 = 1.5 is required by law. A blower door test and the corresponding report belong here to the application documents for a public funding.

Video Board: Blower Door Directed Air Sealing in a Zero Energy Home (ZEH)