Is it true that people in passive houses roast in summer?
Posted by Joanne Spurrell on 2 May 2017 at 2:45 pm
Passive House is a voluntary energy performance standard that results in buildings that are extremely airtight and have good levels of insulation. In such buildings, little energy is required for space heating and cooling so you can do away with conventional heating systems. But this begs the question: if your house is so airtight and energy efficient that you don’t need conventional heating in winter, won’t it overheat and be stifling during the summer?
To achieve the Passive House standard, different techniques are used in the building design, such as superinsulation. As these technologies have improved and the heat lost from buildings has reduced, the risk of overheating has increased. A lot of focus in the UK on Passive houses is on heating and retaining warmth, so what can you do to create a cooler environment in the summer?
If a material has a high thermal mass, like masonry, then it is good at storing heat. It is important for your home to have an internal thermal mass that is able to both absorb and release heat in time with the daily heating and cooling cycle. During the summer, buildings with good thermal mass will gradually absorb and store heat, then release it when the ambient air temperature drops at night. Not only will this will help stabilise the internal temperature of your home and reduce the risk of overheating, but it is largely self-regulating so you don’t have to worry about it.
The windows used in Passive houses are designed for low heat transfer, using triple-pane glazing with gases (such as argon) that have a low thermal conductivity. The challenge with glazing is providing a good amount of light and taking advantage of any spectacular views you may have, whilst considering the orientation of the windows and the heat that will ultimately be transmitted through them. South facing windows are great in the winter when you want to let as much heat and light into your home as possible, but this could be a huge problem in the summer. The solution to this is to use external solar shading, from fixed devices such as window eaves and awnings, to natural alternatives such as deciduous trees. These will still allow the heat and light in during winter, but offer shade from the high summer sun. External shading devices are significantly more effective at controlling passive solar gains than internal shading (such as blinds) and will reduce the need for cooling.
As Passive houses are so incredibly airtight, good ventilation is crucial in providing fresh air and comfortable temperatures. There are two types of ventilation: natural and mechanical. You can use natural ventilation when the outside temperature drops at night to help cool your house, by opening windows and allowing cool air inside. Opening windows at either end of your home to allow air right through is even more effective. Mechanical ventilation can also be used to cool your house if needed; perhaps you live somewhere with a lot of noise or air pollution, so opening windows isn’t an option. It may be more costly, but mechanical ventilation will also distribute the fresh, cool air more evenly. Either way, properly ventilating a Passive house provides much-needed cooling in order to prevent overheating in summer.
Your comfort is the top priority in designing a successful Passive house and overheating is a real concern, even in climates such as in the UK, but it can be avoided with careful planning, appropriate use of materials and correct ventilation.
Image: Rob Annable
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