Principles of design
To help obtain the high energy standards necessary for Passivhaus, it is crucial that the design of a building allows it to be buildable. The essential principles, both in the design and the build processes, to be considered by the whole team throughout the whole process in delivering a Passivhaus are:
Continuity of super insulation
There has to be a consistency of ‘wrapping’ the whole building, with particular attention to the junctions of the different fabric elements.
Minimising thermal bridging
The need for structural integrity, and the need to allow light and access into a building leads inevitably to the use of different materials with different thermal properties. Good design of each of these details to minimise the thermal loss is crucial both to keep heating demand down and to avoid cold spots where condensation might form.
Passivhaus buildings need to achieve 0.6 air changes/ hour @ 50 Pascals, approximately 15 times more airtight than the present UK Building Regulation requirements.
The airtightness strategy needs to include a continuous layer joining each of the fabric components, using an appropriate material robust enough to last the lifetime of a building. Not only does an effective airtightness layer stop draughts and reduces energy consumption (by limiting heat loss), it also protects the fabric of a building by preventing internal warm humid air from passing into the fabric, where it could form interstitial condensation.
To minimise this risk, a rule of thumb is for the airtightness layer to be within the first third of the insulation build-up from the warm side. Beyond that third, it is advisable for the detail to be ‘modelled’ (eg, in WUFI hygrothermal modelling software) to ascertain the risk.
Minimising thermal bypass
A thermal bypass is the wicking of heat from a building by air movement around and through its fabric. A breather membrane layer on the cold side of the building’s fabric will help avoid some of the more common examples.