Airtight building envelope – step 4
Airtightness is central to the Passivhaus concept and the standards required go far beyond Part L (2010) requirements. A detailed air tightness strategy should be specified by the designers and indicated on all production drawings. The airtight layer is normally located on the warm side of the insulation layer and should not be confused with a wind barrier (vapour diffuse) membrane on the outside of a timber frame building.
In order to ensure that compliance with the designers intentions are maintained throughout the build an ‘airtightness champion’ should be appointed. Ideally this person would be a site operative who will be present throughout the build; someone who thoroughly understands and can communicate the designer’s air tight strategy.
In some cases the airtight barrier will have to be installed or partially installed during early stages of the build when it would not normally be necessary to address these issues. For example, to ensure continuity of an internal air tight barrier across the intermediate floor to wall junction in a timber frame, a section of the air barrier will need to be laid over the top of the ground floor wall construction before the first floor joists (or cassettes) are added.
The air barrier will then need to be returned around the outer ends of the joists or floor cassettes so it can continue up the inner side of the first floor
wall. Failure to install the air barrier at this stage would jeopardise the final air tightness as the alternative solutions are far less robust.
In masonry construction air tightness is likely to be achieved using wet plaster, it is necessary to carefully parge walls before intermediate floors and partition walls are installed, and before any service runs hinder access to walls and prevent the installation of a continuous barrier.
Junctions between masonry and joinery require special attention as proprietary airtight tapes will be needed to ensure a continuous air tight seal at these locations. Plastering lathes and mesh should be used to mechanically fix and seal any tapes back to the parge coat.
Ideally the air tight layer should be located behind the services void, and on no accounts should plasterboard be used as the airtight layer. Any departures from or changes to the air tightness strategy shown on the drawings needs to be communicated to the Passivhaus consultant or designer before they are made.
Read about steps one, two, three, four and five in our next articles.
Excerpt from “Passivhaus primer: Contractor’s guide. So you’ve been asked to build a Passivhaus?” by BRE.