Should Passive House designers be doing more to design buildings that avoid summer overheating? This article from mainstream business magazine Forbes considers Bahnstadt, a Passive House district in Heidelberg, Germany. The story quotes residents who were struggling to stay comfortable as summer temps soared in 2023.
The two residents have different motivations for living there and different experiences of their Passive House dwellings. It highlights our responsibility as designers to ensure our design fits the client and that the two will work together.
Take the family of five living in Bahnstadt who have to retreat from parts of their home in the hottest days of summer. The father complains the designers should have thought more about the impact of climate change and done more to reduce overheating. They are contemplating installing air-conditioning.
A older single woman in the same development has changed her habits to help keep comfortable during summer peaks, opening windows in the evening and using window blinds to block sun entry. Many blinds across Bahnstadt are drawn shut on the sunny day when the journalist visits.
When undertaking Passive House design, it’s vital to draw out information from the clients about how involved (or not) they want to be in moderating temperature extremes. There’s a spectrum of willingness and our job is to design to fit. The best design is a team sport.
I can make one more comment about Bahnstadt, based on my visit in 2014. It was an interesting time to be there as lots of building was underway but a large variation in the quality of construction was apparent as I wandered about. Poor flashing details around the windows were the most obvious problem; reverse lapping is a common mistake but unfortunate. At least here the use of concrete and external insulation will minimise or delay the effect of water not being shed away from the building.
I also expect the extent of overheating will vary a lot, even by apartments within the same building, because of differences in window orientation and local shading. Compared to ensuring thermal comfort in winter, overheating is a very site-specific problem. The best architects design by facade—for instance, using a particular window construction on all western facades—but contractors on the other hand will want the same windows used everywhere in the building for reasons of efficiency.
The City of Heidelberg set up its own development company in order to build Bahnstadt above building code legal requirements. Every building was designed to meet the Passive House standard but most are not certified. The developer established its own quality assurance process instead. Maybe the flashing issues I saw were identified and rectified. Regardless, this was an impressively ambitious project that strove for sustainability in multiple ways and is clearly hugely successful. Bahnstadt uses at least 80% less energy for heating than a comparable city district.
For the record, I will state again that my advice to clients is always to install a reverse-cycle heat pump in a Passive House home, in order to provide cooling on the hottest days. As the Forbes article points out, the cooling load is much lower in a Passive House building; it will be a very small heat pump compared to a conventional building the same size. And where air conditioning is powered by PV, it’s a cost effective and responsible way to ensure comfort.