Passive solar and Passivhaus
Ever chatted to someone for a while and then realised the conversation train has somehow run off the rails? You finally realise you’re not actually talking about the same concept. This can happen when people interested in sustainable homes talk about passive design. Be very careful! Is the topic passive solar design, Passive House (Passivhaus) design or some mixture of both?
Our homes use passive solar design.
Passive solar design principles minimise the energy required to stay cool in Summer and warm in Winter.
As explained by Your Home, passive solar design is based on several key features:
- Locate living areas on the north side of the house, exposed to the northern sun
- Heat in Winter using direct sunlight through large north facing windows
- Cool in Summer using eaves to shade windows and natural air circulation to vent warm air
- Prevent heat gain in Summer or heat loss in Winter with good insulation and double glazing
- Maintain a stable indoor temperature using thermal mass (eg concrete floor)
The Master Builders Association of Victoria released this informative video showing how to apply these principles when designing a new home.
After living here during summer, we know this style of construction works for hot weather.
Passivhaus (Passive House) is not the same as passive solar.
Back in the late 80’s, a couple of German blokes decided there had to be a way to build homes that didn’t gobble energy like there was no tomorrow. Going back to basics produced the Passivhaus (Passive House) philosophy based on five core principles.
- Thermal insulation – insulate floors, walls and ceilings to minimise heat transfer
- Windows and doors – double (or triple) glaze along with insulated frames
- Eliminate thermal bridging – stop sneaky heat transfer through items like steel beams or more conductive building materials
- Airtightness – minimal air flow in or out of the house to keep heat (or coolth) inside
- Heat recovery ventilation (HRV) – an essential component which serves two purposes. Firstly, HRV controls the flow of fresh air in and stale air out. Secondly, it exchanges heat between the two air flows. In Winter, warm air going out is cooled by heating the cold air coming in.
We’re talking precision German engineering here people. You can’t just say here is a Passive House I built. When it’s finished, a crew rolls up, test equipment and clipboards in hand. They run all sorts of checks to see if the house meets requirements. One test uses a fan blower to raise the internal air pressure which shows how much air leaks out. A house that passes the full examination becomes a certified Passive House, one of about 50,000 on the planet. If it doesn’t meet the requirements, the best that can be said is the design uses Passive House principles.
What’s the difference?
One obvious difference between a Passive House and a passive solar design is the control of air flowing through the house.
In a pure-bred Passive House, that air flow is tightly locked down. Very few air molecules enter without exchanging their heat (or coolth) with the ones leaving the building. The HRV takes care of this chore with little involvement from the occupants. Heat exchange ensures indoors remains at a consistent, comfortable temperature year round.
For passive solar design, air flow control has a less rigorous approach. Occupants need to be more actively involved in the process. They get the best results by watching indoor/outdoor temperatures to decide when to open/close windows. Natural air circulation relies on humans clambering off the couch on a regular basis to make the best of prevailing weather conditions. Also, a passive solar house doesn’t leak like a sieve, but it’s unlikely to meet the air tightness requirements for a Passive House.
Another point of difference is thermal mass. A tightly sealed and controlled Passive House can be a very light structure while a passive solar design includes thermal mass like a thick layer of concrete or heavy, rammed earth walls. However, that’s not always the case. Phase change materials (PCMs) are a 21st Century alternative to traditional thermal mass materials.
There is certainly common ground. The two approaches are similar with their requirements for good insulation, reduced heat transfer through double glazed windows and avoiding thermal bridges.
Passive House in Australia.
Local architects and designers are finding the Passive House principles can be applied to Australian climate zones, although the number of certified homes is quite small at this time.
In true Aussie multi-cultural style, some designers add a dash of passive solar to their Passive House plans. Why run a HRV in Spring when a cooling breeze will do the same job? Would northern facing windows shaded by eaves help with heating in Winter and reduce the thermal load in Summer?
Passive House is an option for inner city locations where there are more restrictions on access to sunlight and breezes. Not being able to find a site that ticks the passive solar boxes needn’t be an obstacle for a sustainable design.
Which one is better?
Oh no you don’t! That’s one of those trick questions like being asked, “Can you guess my age?”
There are so many factors to take into account before deciding on the design method. What works for your land, house and budget is the correct answer. It’s not a competition as each philosophy has something to offer in making our homes more sustainable.
Don’t forget that it’s possible to mix and match with the two approaches. Blending features from both could make sense. There may be Passive House design principles that could be justified without going all the way to full certification.
What’s my takeaway?
Researching Passive House design principles broadened my horizons. I now understand how the two design approaches differ, as well as ways they can complement each other.
Note to self – Remember to say “passive solar design” whenever talking about the sustainability features of our houses.