A central part of the Passivhaus principle is to make use of solar gains in winter to reduce the heating demand. This can mean that there is a potential for overheating in the summer, particularly where south facing glazing has been maximised. Therefore it may be necessary to incorporate some external shading to reduce the amount of direct solar gains in the summer. Correct positioning of fixed shading devices will allow maximum use of direct solar gains from the lower angled winter sun when it is most needed.
Requirement for passivhaus certification
It is a requirement for Passivhaus certification that temperatures exceeding 25°C cannot occur in a building for more than 10% of the occupied year. For a dwelling the occupied year is considered to be 365 days a year but for a school this period might be much shorter. In the light of climate change predictions designers are recommended to achieve a figure of 5% overheating frequency or less (using current day data) and to make provision for additional seasonal shading devices to combat future overheating risks.
Strategies to prevent overheating
A number of further strategies are available to reduce overheating risk in Passivhaus design. These include the use of conventional cross ventilation and night purge ventilation. Mechanical options include using the Heat Recovery Ventilation system in by pass mode with or without additional ground or brine loop pre-cooling options. Thermal mass may also be used where appropriate to attenuate some of the diurnal temperature variations induced by unwanted solar gains however attention should be given avoid over reliance on this concept since it may be contra indicated during periods of prolonged overheating.
Source: “Passivhaus primer: Designer’s guide: A guide for the design team and local authorities” by BRE