Passive House, passive solar: what’s the difference?

by Pete Bielski, Ethos Homes managing director

The Ethos Homes team enjoyed hosting Open Days at Bushland Park in November. We had a great turnout of people who were well-informed and keen to learn more. There were some really good questions put to us.

Some of those questions highlighted the confusion between the Passive House standard and passive solar.

This home in Wanaka is a certified Passive House Plus—and also uses many of the natural materials like strawbale walls, natural plasters and earth floors that are often found in passive solar homes. The best of both worlds, perhaps? Design by Hiberna, photo Jessica Eyres, used with permission.

These are two different but overlapping approaches to creating comfortable, energy-efficient homes. Passive solar originated in the US in the 1970s, while the Passive House standard was developed in Europe, beginning in the late 80s. 

“Passive” is used as the opposite of “active” in both cases, but it means slightly different things. A passive solar home will face north for maximum sun, concentrates glazing on the sunny side and stores energy in high-mass materials. Features like exposed concrete floors or rammed earth walls soak up the sun’s energy to release it later as temperatures drop at night. The home is being heated passively by the sun’s rays rather than being actively heated by a fireplace or heat pump.

A home that meets the Passive House standard is passive in the sense that occupants don’t need to be actively involved in keeping the environment comfortable and healthy. The house takes care of it for you. Mechanical ventilation systems mean the indoor air quality is superior: damp, stale air is quietly removed and fresh filtered air is constantly introduced. It doesn’t matter if you’re not home to open windows. High levels of insulation keep the indoor temperature stable at a comfortable 18-22 degrees (no coming home to a freezing house in winter and trying to get the fire started and cook dinner at the same time).

Passive solar can be great in some circumstances. It suits new builds on larger blocks of land, where a building can face north without shading. It also suits people who don’t mind or enjoy actively managing their home environment. Sometimes the average temperature in a passive solar house can hide big swings in temperature over the course of a day. Shades may need to be drawn in summer to avoid the house overheating and windows opened to get a cross-breeze or dump heat at night. And of course if you live somewhere like Christchurch where we sometimes don’t see the sun for a week in winter (or even in summer like early December), a house that relies on sunshine for warmth is going to get cold.

Passive House on the other hand, is very flexible and it fits modern circumstances well. In many families, both adults are at work all day and the house is closed up tight for security reasons. Sections are getting smaller and houses are getting higher, especially in those suburbs zoned for higher density. It may not be possible to orient your house to the north, or the north may be a busy road with no privacy. 

Passive House homes can deal with this because they are by nature really flexible (features that perform less well can be compensated for). A certified Passive House building uses solar gain if it is available but can compensate if it isn’t by increasing insulation values for instance. Crucially, that solar gain is part of the careful calculations done during the energy modelling. If it will overheat on summer days, we can find solutions to prevent that. 

The certified Passive House standard takes the good concepts from passive solar and uses it where it’s possible and makes sense. It doesn’t rely on thermal mass or solar gain, these are just options in the tool kit. 

What really sets Passive House apart is that it is an international, measurable standard that has been proven over 30 years on every continent except the Arctic. As a homeowner, you can be confident of how the house will perform, before it’s built. There are objective targets to meet, they are independently verified—and indoor temperatures will be stable in a very narrow range that is super comfortable. All the time, 24/7, in every part of the house. You know what you’ll get and you’ll get what you paid for.