Heatwave? Get a Passive House and stay cool!

Richard Spencer, Certified Passive House Consultant

A well-designed Passive House in the UK climate not only requires little heat to stay warm in the cold weather but should also stay cool in the summer with no cooling energy input (in other words no air conditioning required).

Although not a piece of authoritative research, here’s the comparison in June 2018 for a Passive House in Leeds: the car outside is showing a temperature of 28.5 degrees Celsius; the living room in the house is at 22.5 degrees Celsius.

So how does the house do this.

Firstly, the house has thermal mass in the walls and floor that is connected to the room air: dense concrete blockwork with plaster on the walls and a concrete floor slab with bamboo laid on a low insulation underlay. This helps to smooth out peak temperatures by absorbing excess heat into the walls and floor.

Secondly, modelling in PHPP to check for overheating enables this to be estimated and helps to design effective shading elements on windows. If we want to understand the building’s performance further we can also carry out dynamic simulation of overheating. This approach is now being requested in many cases to demonstrate that designs are robust as an adaption to climate change. It is expected that extreme weather (including heatwaves) may be more common as the climate changes due to global warming. In our example house, there is a modest roof overhang on the South side of the house to shade the upstairs windows and deep reveals that partially shade the downstairs windows. These features keep much of the heat from the sun out of the house when the sun is high in the sky in the middle of the day. Shading of the west facing windows is less effective as the sun is lower and there is a little heating up in the evening due to this. This could be reduced by using moveable blinds or shutters, but it hasn’t been necessary to add these features at the moment.

Thirdly, a summer ventilation design. The MVHR is still running on heat exchange mode at this temperature (it doesn’t open the summer bypass until it is over 27 degrees Celsius inside). The heat exchanger is actually recovering cool, pre-cooling the warm air from outside using the extracted cool air from inside. We use window ventilation to purge heat out overnight with a carefully designed window ventilation scheme. The design incorporates stack ventilation (a low window coupled with a rooflight) in the hall way and cross flow ventilation (windows open on the North and South facades). Using inward opening tilt and turn windows in the tilt modes, this ventilation can be open all day and (importantly for summer cooling) all night. This enables any hot air that accumulates in the day to be purged out of the high-level windows and cool air at night time to be drawn in below.

Finally, low energy appliances and lighting are used throughout the house and the design of the heating and hot water systems minimises heat loss to the rooms. These are important in minimising the extra heat that these items contribute, which in the summer can lead to the overheating of the house.

Like any building without active cooling, a prolonged period of hot weather (and if it stays warm overnight) will tend to lead to the building warming up over the course of several days. Compared to a less well insulated building with no consideration of shading in the design, the Passive House should still be more comfortable through the period of hot weather.

One concern with heat waves is that more people will install air conditioning to make their poorly insulated houses comfortable, and this of course will increase energy use and (at least in the short term while power is fossil fuel generated) result in higher carbon emissions. Investing instead in insulation and carefully designed shading features is an alternative approach that should be able to successfully control overheating in many buildings without resorting to active cooling. The Passive House design method takes into account the risk of overheating and encourages the designer to build in features that reduce the proportion of the year that the building is uncomfortably warm.