Author: Francis Bosenick
In cold-temperate climates, much of the focus surrounding Passive House buildings is on their low heating demand and high levels of comfort during the cold winter months. This is largely due to the fact that most of the energy is consumed in winter through heating and it is in winter where the energy efficiency measures show the most obvious return on investment. However, certified Passive House buildings are designed to be comfortable all year round and it is certainly worth describing how such buildings are designed to behave when temperatures rise and the risk of building overheating increases.
What is building overheating?
Building overheating is generally considered to be a period of time when indoor temperatures exceed 25 °C, a temperature where people start to feel quite warm and if temperatures continue to rise, start feeling uncomfortable. Indoor temperatures over 35 °C can pose a significant risk of heat stress and adverse health effects.
How do Passive House buildings avoid overheating?
Buildings constructed to the Passive House Standard must be designed and correctly balanced in PHPP to ensure temperatures do not exceed 25 °C for more than 10% of hours annually. In fact, it is advised that buildings are designed to achieve well below this limit and there are several ways correctly designed Passive House buildings can achieve this.
Although one does not always have complete control over the orientation of a building, especially in retrofit projects, it is still very important to consider which direction the building is facing and where the windows will be installed. Windows facing east or west are a particular risk, as the low altitude of the morning or afternoon sun makes shading difficult. South-facing windows (or north for those in the southern hemisphere) can be shaded during the summer by overhangs, whilst still taking advantage of most of the solar gains from the lower angle of the sun during winter.
In any building (Passive House or otherwise), shading (at least in some form) is used to reduce indoor temperatures, whether by closing fabric curtains or blinds, or using exterior shading elements or even trees. Without appropriate shading in the summer, sunlight can quickly increase indoor temperatures. Passive House buildings are designed to make use of the angle of the sun by installing correctly designed shading elements that reduce solar gains in summer but still allow for some solar gains when the angle of the sun is lower in winter.
Insulation and airtightness
Although the details vary in different climates, all Passive House buildings must be well insulated and have high levels of airtightness. There are many different insulation materials available on the market, all with their own pros and cons, and it is up to the building designer to choose the most appropriate materials for their project. It is often wrongly assumed that adding insulation to a building will increase the temperatures and warm up the building, however as Wolfgang Feist highlights in the Passipedia article The Passive House in summer, this is not the case:
“Insulation does not create any additional heat; it only reduces the heat exchange between systems with different temperatures. Therefore, it also protects a cool system from gaining heat from the surroundings”
This is where the common example comparing a Passive House building to a thermos flask comes in handy – it works just as well for a cold liquids as for a warm one.
Windows and Ventilation
We have windows for many reasons, to provide natural light, to see outside, to let fresh air in and in some cases, to get free solar gains during winter. Passive House windows in cold-temperate climates are triple glazed and normally have a low-e coating which helps to reduce the transfer of heat from the side where it originated, which in summer generally means the heat transfer from outside to inside is reduced.
Passive House buildings also need to be equipped with a Mechanical Ventilation system with Heat Recovery (MVHR) in order to keep a constant supply of fresh air running through the building. During the summer months, it is recommended that the system is set to summer bypass or extract only mode. This will stop the extract air transferring any extra heat to the incoming air. Only when ambient temperatures are well above 25 °C does the use of heat recovery become useful in summer. Summer comfort also benefits well from window ventilation, whether that be night time ventilation, cross ventilation or early morning ventilation (provided the temperatures drop to a suitable level). This is a much faster way of changing the air than having the ventilation system running and is, of course, free!
Minimising internal heat gains
It is also important to consider how a building is used during the summer and to be aware of indoor sources of heat from cooking, inefficient devices and water heating. All these indoor heat sources add up, and when not fully considered can end up adding considerable heat to the house.
With recent heatwaves clearly linked to climate change, the risk of future events and the associated building overheating is likely to increase. This means efficient mitigation strategies for both new and existing buildings should be considered and effectively implemented. Due to the rigorous design and planning requirements for Passive House buildings, such measures have already been considered for some time and Passive House buildings have proven to be much more resilient during heatwaves than conventionally built buildings.
References and further reading
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