Appropriate materials for air barrier in passivhaus
It is crucial to specify the appropriate materials that will be used to form the air barrier. An air barrier must be impermeable or virtually impermeable (i.e. not allow air to pass through them at 50 Pascals). Typical air barrier materials include:
- Vapour control layer membranes (used in timber frame construction)
- Concrete (but not unparged concrete blocks)
- Orientated Strand Board (used for SIPS panels and sheathing in timber frame)
- Cross laminated timber plate (used as structural panels)
- Parging coat (applied directly to masonry)
No permeable material for air barrier in passivhaus
Do not use materials that are permeable such as fibrous insulation material. It does not matter how hard a permeable material is packed into a gap, air will still be able to pass through it. Oriented Strand Board (OSB) class 3 (external grade) has been successfully used as an air barrier on a large number of Passivhaus projects however research shows that there is substantial variance (<0.001 m3/m2.h.Pa – 0.01 m3/m2.h.Pa) in the air permeability of OSB boards being supplied to the Western European market. As a result of detailed investigation it has been shown that OSB used as an airtight layer in Passivhaus construction should be quality controlled to a maximum air permeability ≤0.0018 m3/m2.h.Pa (Langmans et al.,2010). In response to these findings the Passive House Institute have advised that an elemental limit of q50≤0.1 m3/(m2h) should be used when specifying OSB as the airtightness layer *(Peper et al, 2014).
OSB as air barrier in passivhaus?
Designers should be aware that at the time of writing this level of quality control is not mandated in Europe. It is therefore advisable to specify a minimum grade of 18 mm OSB-3 and to either ask for evidence that substantiates the q50 or to test a sample batch of any boards prior to procurement to ensure that the air permeability complies with the above recommendation. Improved airtightness levels are likely if OSB class 4 is specified and or thicker boards (22mm) are used however there will be a cost implication associated with the decision to specify a higher grade material. Other alternatives include options to use modified OSB sheet products, available from some manufacturers, incorporating a laminated airtight cellulose based layer. Given that the suppliers of these boards often have no access to information relating to the quality of airtightness of the OSBs they distribute, it is imperative that mandatory information is provided by manufacturers regarding airtightness. This information should feature on product agrément certificates and be imprinted on the boards.
Excerpt from “Passivhaus primer: Airtightness Guide” by BRE