Airtightness

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The airtightness of buildings is determined using a differential pressure test ( blower door test ).

Blower door test

A fan embedded in a building envelope (usually a door or window) creates and maintains a constant overpressure and underpressure of (for example) 50 Pascal inside the building  . The amount of air escaping through building leaks must be forced into the building by the fan and is measured. The so-called n50 value (unit: 1 / h) indicates how often the interior volume is converted per hour.

Guideline values ​​for the n50 value have been included in Part 7 of DIN  4108 since November 1996 . The energy saving regulations that have been in force since 2002 also contain limit values. The measurement is carried out according to DIN 13829.

The importance of the airtight building envelope in terms of building physics

When checking the airtightness of new buildings without a ventilation system, it is more a matter of avoiding structural damage through moisture penetration of the outer walls or roof surfaces than of minimizing the exchange of air.
Leaks that cause a relevant loss of heat are generally detected in the winter even without a differential pressure test due to drafts.
In the case of low-energy and passive houses, however, it can make sense to check the leak rate to prevent an uncontrolled exchange of air, since the air exchange required for hygienic reasons is generally ensured by a mechanical ventilation system.
In buildings without a ventilation system, however, a certain degree of leakage is necessary in order to ensure a minimal exchange of air even if the user fails to remove the water vapor that is always generated in inhabited rooms through occasional shock ventilation . Outside air must also be able to flow in to enable the operation of extractor hoods, wood or coal stoves and gas appliances such as stoves, instantaneous water heaters and heating devices.

In contrast to historical buildings, the building elements and materials used today no longer allow sufficient water vapor to escape to the outside in winter. Building materials such as synthetic resin plaster, emulsion paints , concrete , styrofoam , plywood - and, to an even greater extent, OSB boards and vapor barrier films, only allow moisture to pass through to a small extent in the form of water vapor. Traditional mineral building materials such as brick, lime and clay, as well as natural, fiber-containing materials such as wood, hemp, coconut, cork and wool, are able to remove significantly larger amounts of moisture to the outside through sorption processes and condensation of the water vapor with subsequent capillary transport , if they are not in it Resin-containing coatings or foils are prevented.

In addition, the air temperature in centrally heated rooms is now generally higher than it used to be, where often only kitchens and rooms were heated at all.
Since warm air can absorb more moisture than cold air, there is significantly more water vapor in the air today.
The inner surfaces of the outer walls of a modern, thermally insulated building have a higher temperature, so that the air humidity no longer condenses over large areas on the outer wall or on the glass surfaces of the windows. In the past, condensation water formed on the single-glazed window panes, which dripped into the channel provided for this purpose in the window sill and was occasionally wiped up there. The relatively leaky window sashes and frames constantly let a small amount of moist indoor air flow outside and dry outside air flow inside.

If the water vapor is not discharged to the outside through disciplined ventilation, it therefore remains in the room air until it either condenses at the coldest point on the wall ( cold bridge ) or diffuses into the outer wall (where it can also condense if the water vapor diffusion resistance is unfavorable ) flows to the outside through leaks in the building envelope and usually cools down to such an extent that it condenses inside the wall and leads to strong moisture penetration at certain points.
Since the first two problem areas can be avoided structurally, it is mainly leaks in the inner wall closure that often lead to structural damage. Carelessness when installing vapor barriers typically leads to flaws in pipe or cable penetrations, connections to walls, ceilings, windows and doors as well as in angled places in loft conversions, which are no longer noticeable after the wall has been clad.
Solidly manufactured and plastered exterior walls rarely lead to problems, as there are hardly any movement joints with this type of construction. If, in exceptional cases, continuous joints in the masonry allow a flow, the damage caused by the condensation is usually manageable.
The inner cavities of drywall constructions, on the other hand, offer hardly any resistance to air flows if leaks occur in the inner wall closure. Even making sockets for electrical installation in the wall paneling windproof requires special measures.
Soaked mineral wool is hardly able to dissipate moisture again. Natural insulation materials distribute moisture better, but light, flexible insulation materials are generally not able to dissipate large amounts of moisture because there are too few through-going capillaries. This can partly be compensated by planking the walls with wood fiber or clay building boards , which absorb, distribute and evaporate moisture better than OSB or plasterboard .

The frequency and extent of moisture damage in "light" construction methods justify the effort required for the "blower door test", which should be carried out after the interior plaster has been applied, but before the cladding of those surfaces that are only planked in dry construction. so that any leaks in the vapor barrier film can be repaired beforehand.

The subject of airtightness from a legal point of view

On February 1, 2002, the so-called Energy Saving Ordinance came into force in Germany, which suspended the heat protection and heating systems ordinance that had previously been in force.

Important components of this new regulation are the following:

  • Tightness and minimum air exchange, regulated in § 6 of the ordinance. In detail, § 6 regulates that buildings to be constructed are to be designed in such a way that the outer shell including the joints must be permanently airtight in accordance with the state of the art. The joint permeability of exterior windows, French doors and skylights must comply with Annex 4 No. 1. With regard to tightness, reference is made to Annex 4 No. 2.
  • The following applies to the minimum air change:
Buildings to be constructed must be designed in such a way that the minimum air exchange required for health and heating purposes is ensured. If no windows are used for ventilation, Appendix 4 No. 3 must be observed.

With an internal pressure that is 50 Pa above the external pressure, it must take at least 20 minutes (n 50 <3 / h) until a complete air exchange has taken place through the imperfections in the building [for ventilation systems at least 40 minutes (n 50 <1, 5 / h)].

  • The airtightness is measured with the blower door test . This allows the basic determination of compliance with the guidelines described. However, the contractor is still not completely on the safe side even with a positive blower door test. As stated there, the method allows a basic statement, but it is not able to detect concentrated leaks in the walls. The blower door knife is required to carefully examine such leaks found, but this does not guarantee the finding itself. If places are found which, despite a positive blower door test, are below the specified standards, there are still defects in the Legal sense. These are to be documented by individual measurements.

Web links and sources

Individual evidence

  1. ↑ a b " Necessity of airtightness " by Jochen Ebel, Ebel engineering office.