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====== Airtightness and windtightness ======

In the construction sector the terms 'windtightness' and 'airtightness' are used to describe different situations: 
Windtightness of a building protects the building components from air movement within the thermal insulation.  This mainly concerns the avoidance of air movement from the outside through the thermal insulation and then out of the thermal insulation again at another place, which reduces the insulative effect. \\
In contrast, airtightness avoids continuous air flow from the inside towards the outside through the building envelope ((or in the opposite direction as well)) due to which warm indoor air can flow out in winter((called 'exfiltration' 
))  and cool outside air can flow in((called 'infiltration')). A lack of airtightness can lead to substantial structural damage. \\
Disregarding both these measures results in high energy consumptions. \\
Both airtightness and windtightness in buildings are major quality characteristics of energy efficient construction. The building designer must however be aware that even in an 'ordinary' building quality assurance in this respect is essential for avoiding structural damage. \\

===== Quality characteristic airtightness =====

To summarise, it can be said that airtightness is a fundamental characteristic of buildings and therefore constitutes a quality feature. Airtight buildings have the following advantages: 
  * Avoidance of structural damage caused by moisture
  * Avoidance of draughts and cold feet 
  * Avoidance of high infiltration heat losses 
  * Basis for the use of a controllable demand-based ventilation system 
  * Basis for the functioning of thermal insulation 
  * Improvement of insulation against noise 
  * Improvement of indoor air quality 
By the way, "airtightness" is a rather unfortunate choice of term in building construction. Only a few building components in a construction are perfectly "leakproof" (for example a glass pane). Other than that, in a construction the only aim is to bring air leakage to a non-critical level, both across the surface and at the connections, that is low enough to achieve all seven advantages mentioned above. Even the best Passive House buildings are still quite non-airtight compared to a spacecraft or a submarine. Improving airtightness within this context can only bring advantages - and has no disadvantages; of course, air exchange must always remain at an adequate level (see the section on [[planning:building_services:ventilation:basics:types_of_ventilation|ventilation]]). 

===== Windtightness =====

Windtightness must not be confused with airtightness, as is often the case in everyday language. Windtightness of a building protects the building components from air movement within the thermal insulation, otherwise the insulating function will be disrupted thus resulting in increased energy consumption. The reasons for this are: 
  * Air flow through loose, uncovered fibrous insulation materials in areas with ventilation spaces behind them (such as under roof coverings or underlays, or behind rainscreens) 
  * Circulating flows between cold and warm sides of insulation layers in cavities 

These problems occur especially in the case of wooden assemblies and can be solved by means of so-called "full rafter insulation" (same as insulation between rafters but without an air layer). However, this point must be taken into account when planning all types of thermally insulated buildings. 
Windtightness is therefore a completely different thing from airtightness which is an important addition to the concept, especially for energy efficient buildings. 

===== See also =====

[[https://passipedia.de/planung/luftdichtheit|Overiew (German only) ]] of Passipedia article on "airtightness"