What makes a Passive House?

It’s not the insulation

By Rachel Rose for Ethos Homes

What defines a certified Passive House?

  1. Mechanical ventilation

  2. An air-tight building wrap

  3. High levels of insulation

  4. None of the above

Surprised that the correct answer is D? The Passive House building standard is much better known now compared to a couple of years ago. More people associate it with the components above, or know that Passive House homes use very little energy to heat and cool.

However, the most important distinguishing feature of a Passive House building is … energy modelling at the design stage. It is done using PHPP, special software that qualified Passive House designers are trained to use. Many different data points are entered: the insulation values of the roof, walls and windows, the orientation to the sun, presence of shading etc. PHPP will then accurately predict how the building will perform, showing how much energy will be required to maintain comfortable year-round temperatures and whether that will meet the Passive House standard.

Certified Passive House homes are by design greater than the sum of all their parts. They work because of how all those parts interact with each other. When you walk into a Passive House, you can open cupboards and find the MVHR, you can see the high-quality joinery … but the PHPP energy modelling is invisible. Yet you experience the result, because it has guided every aspect of that home’s design and construction.

Energy modelling is why the Passive House standard is both so flexible and predictably reliable. Passive House buildings can accommodate local conditions, all manner of site constraints and not least, your personal preferences. What if you want your new Christchurch home to have a wall of glass to a private south-facing courtyard? Or if your access to winter sun is blocked by neighbours? PHPP makes it easy to work out how to compensate and by how much. For example, insulation levels in the slab or roof could be increased, or higher-performing windows/doors used.

I spend a lot of my work life writing about Passive House. I’m also having a new home built that is targeting Passive House Plus certification. Because it’s been modelled in PHPP by an experienced Passive House architect, I can be confident that I’ll get the house I pay for. If it’s built according to the design, then it is certain to perform as the model predicted. Without PHPP energy modelling, it’s just a lottery.

Getting it wrong can be costly in multiple ways. In New Zealand, there have been unfortunate examples of projects where high-performance components were specified by architects or designers without understanding their interactions and combined effects. I know of at least one high-performance home that overheats so badly in summer it is virtually uninhabitable. I also know some clients are paying far more than they need to, because homes are being over-insulated for instance. This is especially a risk in warmer climates like Auckland. The lesson? Your high-performance project needs to be energy modelled by someone who is trained in using PHPP.

Ethos Homes’ projects are certified by Sustainable Engineering Ltd. Its principal is Jason Quinn and he’s a very experienced (and New Zealand’s first) Passive House certifier. He made a very good point in his book, Passive House for New Zealand: “When we do the maths first, we can modify the design and immediately see how that impacts performance in PHPP. [Vary] the insulation, factor in landscaping that provides summer shade, take into account occupant behaviour … modelling different scenarios is so cheap and simple compared to having builders re-do work on site or leaving disappointed clients to install enormous air-conditioning units in order to survive the summer.”

If you’re up for some non-fiction reading this summer, you can read Jason’s book online or download it for free.