Laboratory investigation of timber frame walls with an exterior air barrier in a temperate climate

Recently, the requirements regarding global building airtightness to reduce the exfiltration losses became more severe as result of the trend towards very low energy buildings and Passive Houses. These very strict requirements regarding airtightness are currently achieved with an interior air barrier, which is labour intensive and consequently expensive. At the same time it is observed that new wind barrier solutions - to reduce windwashing of the insulation - can have a major contribution to the global airtightness of timber frame constructions. Consequently, it is questioned whether the labour intensive interior air barrier will still be necessary in practice when the global building airtightness can be guaranteed by an improved wind barrier only.
However, moving the air barrier from the interior to the exterior of the building envelope can imply an increased moisture load, and thus, higher risks for interstitial condensation against the exterior sheathing in cold and moderate climates.
The current paper presents the results of a laboratory experiment to study the hygrothermal behaviour of light weight timber walls with an exterior air barrier only. Four independent test walls (2.3m by 0.5m) are placed between a newly developed hot and cold box, operating at controlled temperatures, humidities and air pressures. All four walls are insulated with 30 cm of standard mineral wool to which OSB is applied as interior sheathing. The test walls differ from each other by the physical properties of applied exterior air barrier; airtightness, moisture buffer capacity, vapour permeability and thermal resistance.