Air tightness, friend or foe?

Industry standard calculations recognise airtightness as a positive characteristic when houses are designed to consume low levels of energy for heating or for cooling. In addition, airtightness will insulate the internal environment from externally generated contaminants such as particulates and volatile organic compounds (VOC’s). However, it will also contribute to the containment of internally generated contaminants to lower the quality of the internal environment. The overall impact of airtightness on the internal environment in New Zealand houses is not widely known and warrants further investigation. Air temperatures, relative humidity levels, dew points, particulate matter and VOC levels were monitored in the bedrooms of two, co-located houses, identical in layout and construction apart from details affecting their airtightness. Both spaces had controlled occupancy simulation that produced heat, moisture and contaminants from identical furnishings and decorations. The airtightness was found to have little impact on the internal thermal conditions and energy consumption. As expected, the vapour check, airtight house kept moisture levels above those seen in the conventional building but only slightly. PM10 levels in the conventional house exceeded guidelines for 41% of the time compared to 17% in the airtight house over the seven day period. The airtight test house reached VOC concentrations more than 300% above those in the conventional control house. This challenges conventional thinking on the contribution of airtightness to internal environmental conditions and warrants consideration in the review of building regulations