Stopping air is the second most important job of a building enclosure next to rain. The external envelope of a building should be as airtight as possible and this doesn't only apply to Passive Houses.
Uncontrolled air movement in and out of the walls of a building causes problems:
Figure 1: Due to insufficiently airtight building components, warm and moist air can flow from the inside towards the outside and thus lead to condensation on colder parts of the construction. Most building damage occurs in this way. This can be remedied by careful airtight construction.
Source: http://passipedia.passiv.de/ leakage_problems
Houses should breathe - but only through designed holes. These holes come in all shapes, sizes and designs (e.g., windows). Every building needs to ventilate, but not infiltrate2.
Ventilation is the intentional movement of air from the outside to the inside of a building. When people or animals are present in buildings, ventilation air is necessary to dilute odors and limit the concentration of carbon dioxide and airborne pollutants such as respirable suspended particles (RSPs) and volatile organic compounds (VOCs).
Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope. Normally, infiltration is minimized to reduce dust, increase thermal comfort and decrease energy consumption
Traditionalists can point to old houses and claim the only reason they’re still standing is because air leaks amount to natural ventilation that dries everything out and keeps the house healthy plus you can always put a jersey on. In reality, air leaks mean you’ve lost control of air movement. Air and moisture can be forced into wall and ceiling cavities where water vapor condenses and fosters the growth of mold. Interestingly as we put more insulation into wall cavities, without warming the frame, we increase the risk of condensation forming in these houses.
The ARIDON® SMART WALL forms an effective rigid air barrier and underlay to dramatically reduce the amount of air flow into the wall. The moulded V seals within the SMART WALL interlocking joints embed into the overlapping panels and create an extremely effective air seal for the life of the building with no reliance on tapes or sealants. In addition, the physical properties of the SMART WALL panels mean that a seal is formed along all timber framing lines thereby minimising air movement into and between adjacent wall framing cavities. Therefore an air leak in one wall cavity has negligible impact on adjacent cavities.
Note: The standard installation method for the SMART WALL system was not designed to provide a 100% airtight envelope (it is breathable in a controlled manner), however - it is far superior air control layer than traditional construction methods. If an airtight SMART WALL system is required for a truly passive building with ventilation, then this can be achieved via an alternative installation method. Please contact the ARIDON team to discuss.
THE SMART WALL IS A RIGID AIR BARRIER (UP TO AND EXCEEDING EXTRA HIGH WIND ZONES)
The rigid high density construction, thickness and dimensional stability of the SMART WALL panels ensure a very strong and robust rigid air barrier. BRANZ has tested the ARIDON® SMART WALL system in their B1/VM1 wind chamber. The testing involved ARIDON® SMART WALL panels fixed to a 2.4m x 2.4m panels of 90mm x 45mm timber framing. The result = The wall framing broke at the same time as the SMART WALL panels broke, at a failure load of 4.4kPa. The SMART WALL panels were as strong as the wall itself!
Specifiers can therefore have confidence in specifying the ARIDON® SMART WALL system in wind zones up to and exceeding Extra High.
The ARIDON® SMART WALL system is just that – “A SYSTEM”. That means that all the componentry and practices required to ensure that a first class Rigid Air Barrier is achieved have been thought through. Because ARIDON® SMART WALL can only be installed by an ARIDON® approved installer and with an ARIDON® PS3 Producer Statement to back it up, this is one Rigid Air Barrier you can rely on to deliver in practice what it promises on paper.
What is the difference between an air barrier and a rigid underlay?
The primary function of air barriers is to moderate airflows at junctions and inside the wall cavity. Airflows in certain weather conditions encourage significant amounts of water to move along their path, and it is therefore important to manage airflow in cavity walls with barriers and air seals. An air barrier can be in the form of internal linings in many situations, however for Extra High wind zones, E2/AS1 of the New Zealand Building code require air barriers to be in the form of a Rigid Underlay. External air pressures in higher wind zones can transfer to interior linings, and exceed recommended loadings prescribed by some lining manufacturers. Rigid underlays therefore protect linings from undue air pressure loadings, and help ensure drainage cavity depths are maintained for the proper functioning of the drained cavity behind the claddings. ARIDON® SMART WALL panels have been BRANZ Appraised as a rigid wall underlay.
Does my project need a Rigid Underlay?
If the site is located in an Extra High wind zone then E2/AS1 (External Moisture) requires that a rigid underlay be used. Even if a Rigid Underlay is not required under E2/AS1, ARIDON® SMART WALL provides peace of mind that there is not going to be unwanted air movement into the wall cavity.
How does air get into my building?
The three most important forces affecting air movement in homes are equipment fans, wind, and the stack effect.
Figure 2: Air Movement in Homes (Source: www.greenbuildingadvisor.com/air-leaks)
What is a Stack Effect?
Stack effect is the movement of air into and out of buildings resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. Like wind, the stack effect can move large volumes of air through a building envelope. In the winter, the warm air in a heated building is lighter (less dense) and rises up and out (either via gaps in the ceiling or wall). The flow of air leaving the top of the building draws cold air into cracks at the bottom. The reverse happens in summer when hot air outside of an air-conditioned house can push cooler indoor air down from the ceiling and out of cracks in the walls or floor. But the differences in temperature and pressure aren’t as great during the summer as they are during the winter. When it’s cold outside, the pressure created by the stack effect are 4 Pascal’s per story of height; when it’s hot, about 1.5 Pascal’s per story of height3.