Design stage – identifying the air barrier strategy
The key principle for achieving airtightness is to create a single, continuous and robust airtight layer (or air barrier). This layer surrounds the heated volume of the building and in general it should be located on the warm side of the insulation, therefore also fulfilling the requirements of the vapour control layer (VCL). It is important to define a single continuous air tight barrier – either it’s a complete barrier or it’s not!
Having a secondary layer is often confusing and is unnecessary.
It is helpful to identify the location of the air barrier in the building’s fabric by using the ‘red line’ method. To do this, simply mark the line of the air barrier using a red pencil onto a section through the building. It should be possible to trace the air barrier around the whole building envelope without any breaks in the line. If you need to lift your pencil from the paper then you must mark this area as a break in the air barrier, and provide a clear detail showing how the gap is to be connected and sealed. For example a gap in the barrier caused by a penetrating services pipe or window unit will need a robust detail to ensure an effective seal is achieved. For this reason it is recommended that multiple sections through the building fabric as well as each floor plan are assessed (including all service penetrations) during the initial design stages.
Passivhaus airtightness documentation
At the detailed design stage all complex junctions and air tightness details should be documented by large scale drawings (1:10) highlighting airtight barriers in a bright colour. It is important to note that the air-tight barrier may not always be a specialist membrane; at times it will be a rubber seal or a pane of glass or possibly a laminated timber board. When drawing connecting details it is important to understand precisely where the air-tight line is and how the various connecting elements will be joined together.
Excerpt from “Passivhaus primer: Airtightness Guide” by BRE