Passivhaus is a low-energy performance standard that can be applied to many types of buildings, from homes, care homes, schools, hotels, and supermarkets. This aims to reduce room heating and cooling energy demand while providing excellent indoor comfort levels. This is accomplished mainly by following a “fabric first” approach to construction, with very high airtightness, improved insulation rates and decreased thermal bridging, and the use of mechanical ventilation with heat recovery.
Airtightness is an integral part of building energy-efficient and a Building Regulations necessity. According to NHBC, home energy usage accounts for nearly 27% of UK carbon dioxide emissions. A 2002 BRE report found that air leakage can account for up to 40% of building heat loss. Buildings designed to meet the Passivhaus specification achieve a 75% reduction in space heating requirements compared to standard practise for new buildings in the UK. Therefore, the Passivhaus model provides a reliable approach to help the construction industry achieve the government’s goal of reducing carbon emissions by 80% by 2050.
Airtightness with Passivhaus
The key principle for achieving airtightness is to create a single, continuous, durable airtight layer covering the building’s heated area. It is typically located on the insulation’s warm side, thus also fulfilling the vapour control layer requirements.
To obtain Passivhaus certification, under test conditions, a building must achieve airtightness of less or equal to 0.6 air changes per hour. This is expressed as n50≤0.6h-1 @50 Pa, where n50 is defined as the number of air changes per hour at a 50 Pa reference pressure difference. The result is measured using the building’s internal air volume (m3) instead of its surface area (m2), thus the units are expressed as m3/m3.h, simplified to h-1. This must be the average pressure and depressurization conducted using a blower door sample. Meeting this level of airtightness is difficult but achievable with a clear design plan, with the final result being sensitive to on-site workmanship quality
Current Building Regulations require air permeability at 50Pa to be 7-10m3/h/m2, depending on whether a project is in England, Wales, Scotland or Northern Ireland, so the Passivhaus limit is about five times lower than the maximum allowed. A uniform comparison between air-permeability values and n50 air-change rate values is not feasible because there is no direct relationship and each uses different test and measurement protocols. But to put this in context, the airtightness rate of n50 ≤0.6 h-1 @ 50 Pa is roughly equivalent to a crack in the building’s envelope that is less than a 5p piece per 5m2. For contrast, a building that meets the restricting airtightness requirement for Part L (2013) of the Building Regulations (Section 6 of Building Standards, Scotland and Building Regulations Part F, Northern Ireland) would have a 20p piece gap per 1m2 of envelope.