Passivhaus Fundamentals

An essential part of every Passive House is an air-tight building envelope (see the requirement in the certification criteria). This ensures that there are only a very limited amount of gaps and cracks within your envelope, giving you full control over your internal environment and significantly improving thermal comfort – no more draughts.

Sufficient insulation is what’s needed within the building’s envelope, providing enough thermal separation between the heated or cooled conditioned inside environment and the outdoors. This improves thermal comfort and reduces the risk of condensation, no more cold internal surfaces in winter.

Now this doesn’t mean that you can’t open your windows! The incorporation of a mechanical ventilation unit means that you simply don’t need to rely on opening them to achieve good indoor air quality. The unit effectively recovers heat and coolth that would otherwise be wasted whilst also filtering the air that’s coming into the building. This leads to fewer pollutants in the air and a lower risk of condensation meaning a healthier indoors.

It’s not just the solid areas of your building envelope that need to have good levels of insulation but your windows too. No more single glazing, but instead low-emissivity double or triple glazing with thermally broken or non-metal frames. The size of the windows should be appropriate to each orientation, to allow solar radiation to penetrate during the winter months (free heating!) but not result in too much solar radiation during the summer. Watch out for how well they’re sealed too, as leaky windows just won’t do.

The insulation not only needs to be sufficient in thickness but also needs to be continuous. This means keeping penetrations through the insulation to an absolute minimum, and if not avoidable then using materials that are less conductive to heat (i.e. timber in place of metal) and/or incorporating thermal breaks (whereby a material that doesn’t conduct heat well separates the two conductive elements). Otherwise your wonderfully insulated building will have a number of thermal highways that will cause increased energy consumption and increased condensation risk whilst impacting thermal comfort.

Indoor temperature - Maximum of 20°C with no more than 10% of the hours in a given year being over 25°C. Allowances may be made for extreme climates.

Heating and Cooling - Maximum heating demand of 15kWh/m2/yr or a heating load of 10W/m2. Maximum cooling demand of 15kWh/m2/yr or a cooling load of 10W/m2 (if installed). Allowances may be made for humid climates.

Humidity - Humidity estimates not exceeding 12g/kg for more than 20% of the year. This works out to approximately 60% relative humidity at 25°C.

Airtightness - Maximum reading of 0.6ACH50 with on-site verification.

Energy Demand - Maximum Primary Energy Renewable (PER) demand of 60kWh annually per square metre of living space.