The Shell of a Passive House - Insulation and Air Tightness
When looking at a Passive House, one of the first things you will be aware of is the high standard of insulation. It’s like a thermos flask; the better the insulation, the lower the heating requirement and therefore the lower the energy consumption.
The most important part of a building in that respect is the thermal envelope. The best energy is that one you don’t need to consume.
To achieve this goal, the maximum U-values allowed in a Passive House are 0.15W/(m2K) for opaque elements like walls, ground floor and roof, and a maximum of 0.8W/(m2K) for windows and doors.
Air tightness plays also an important role (see heat recovery) as there is little point putting high levels of insulation into the house if there are lots of openings in the thermal envelope that let the heat escape.
With such a high grade of insulation, it is imperative that constructional precautions are taken to minimise overheating in summer. This can be achieved through clever shading design, utilising the MVHR system and through the high level of thermal mass which serves as a thermal buffer and reduces heat swings.
It is also very important to eliminate thermal bridging, not only because of the energy loss, but also to avoid condensation which could result in damage to the building fabric.
The image on the right hand side shows a thermographic picture which uses colours to illustrate the different temperatures. The darker the colour, the lower the temperature. The picture shows that the temperature in the corner of the ceiling is considerably lower than the rest of the room, meaning that condensation will occur which will weaken the fabric and could form mould (which is unhealthy for both the building and its inhabitants).
A Passive House is designed so that the maximum level of thermal bridging is only 0.01 W/mK.
The places that is essential to ensure that thermal bridging is minimized are the joints between the walls and the roof, walls to the floor, around the windows and doors and, if applicable, the join of a balcony to the building.
There are countless different insulation products available, many of which are natural, meaning that if someone is keen on “green” building materials (and wants to keep the environmental impact low at the building stage of the house), there is a range of materials that can be used.
