Passive House, Net Zero, or just a Pretty Good House?

Here’s the lowdown on our current favourite approaches to achieve our project’s energy performance goals. We’re more interested in the principles than checking boxes or getting a plaque, and want to pick and choose what will work best for us. (And yes, there are other important goals like low water consumption, healthy and local materials, and creating something beautiful, but today we’re talking energy.)

1. Passive House
2. Net Zero House
3. Pretty Good House

Passive House:

The Passive House standard has 3 key performance requirements:

• Annual heating demand <= 15 kWh/M2/year. This is a 70-90% reduction over what most houses use for heating.
• Total primary energy demand <= 120 kWh/M2/year (this is a measure of total energy consumption, including the energy required to generate and transport the energy from the source, using a single source energy factor for all of North America)
• Air tightness, as Air Changes per Hour (ACH) <= 0.6 ACH50, as measured by a blower door test. This is about 4 times more airtight than a typical new house.

Here’s a snappy video that explains the fundamentals of the Passive House approach:

The basic concept of the Passive House standard is to focus first on the structure itself: highly insulated foundation, exterior walls, roof; airtight enclosure, and high performing windows – and then supplement with mechanical heating and/or cooling systems. So we reduce demand as much as possible first before looking at efficient ways to provide the remaining energy we need. Mechanical ventilation is also required to maintain air quality.

The requirements are performance based (focused on the end result rather than prescribing certain systems or building elements), but they do lead us to certain choices that we must make in order to meet the requirements, such as triple pane windows, very high performing heat recovery ventilators (HRVs), and thermal bridge-free construction. Equipment and windows must also be tested specifically to Passive House specifications, which can limit the available choices of certain products like HRVs.

Passive House does not require site-generated renewable energy, although it does provide a credit toward primary energy demand for on-site solar PV.

Passive House performance is modeled using PHPP, which is a giant spreadsheet that does a lot of backend calculating based on heat loss, heat gains, and local monthly average climate data.

For smaller buildings like ours, we will have difficulty meeting the standard without good unobstructed southern orientation (although there are a couple of new certification options that offer some hope – post to come). The south side yard is especially challenging because butting up against our desire to maximize south-facing windows are perfectly reasonable zoning requirements intended to protect the privacy of our neighbhours. Depending on the distance of the house to the property line, we are limited in the amount of glazing and the rooms in which we can have glazing facing the side yard.

What we do have going for us is a simple shape: both houses will basically be rectangles, which makes the detailing for no thermal bridging and a continuous air barrier much simpler (the less corners the better).

Regardless of whether we meet or seek official Passive House certification, I think the passive approach is common sense for any building anywhere, and this will form the foundation of our design approach.

Net Zero

Net zero energy means that, on an annual average basis, our project produces as much energy as we consume. Net zero carbon emissions means that we either purchase carbon offsets, or we avoid the use of fossil fuel based energy.

Can we produce on-site as much energy as we consume in a year? This will depend on three key things: the design of our enclosure to minimize energy loads, our behaviour as energy consumers, and the amount of solar PV capacity we can fit on the roof. In our case, BC Hydro allows net metering, so we can feed excess generated energy back into the grid, and draw it from the grid when we are in deficit.

Passive House does not require the use of on-site renewable energy, but it makes net zero energy consumption viable because it significantly reduces demand.

We’ll have to do some modeling to look at the solar potential of our roof in our specific location, as well as take a stab at estimating our energy demand to get a better sense of where we’re at.

I think this is a goal worth shooting for. We will design for an all-electric project with solar PV, since BC produces much of its electricity using “carbon-neutral” hydro electricity.

The Pretty Good House

The Pretty Good House is not so much a standard as a practical discussion happening on Green Building Advisor. It’s about making realistic choices based on the best bang for your buck in your climate, and still, by the way, ending up with something way better than code minimum. So not going quite as far as Passive House, but, say, 80% of the way there.

We’ve also seen a big variation in the actual energy consumption of certified Passive Houses, which reminds us of the critical importance of occupant behaviour. How does the impact of behaviour compare with the impact of a well designed home?

I like the Pretty Good House approach – and I would argue that it can lead us to a Really Good House – but it does require a sound understanding of the fundamentals. This is why programs like Passive House are great, because they work as learning tools.

So in summary, we will be applying passive house principles, striving for net zero energy consumption, and hopefully ending up with a couple of Really Good Houses. I am fortunate to work with a bunch of building science geniuses at RDH, who will help us get the details right.

Our project gives us a unique opportunity to compare an existing house retrofit to a new build on the same site, which is very exciting. There is also a new, slightly relaxed Passive House standard that was just released, designed for small houses on constrained lots like ours, so that might prove a viable option for us.

We will document our before and after energy consumption and share the results here. Stay tuned!