What is a Passive House? by Trey Farmer

We are renovating The Theresa House to be a passive house certified project. So, what does that mean and why are we doing it?

Let’s start with the why and then dig into the how and the what.

We will be raising our family in this house and want it to be as healthy, comfortable, and durable as possible. We also want to live lightly and do our part to reduce our carbon footprint while being as be prepared for increasingly extreme weather and all that comes with it. Rather than build a new home out in the country, we bought an old home close to the highway and close to downtown and are breathing new life and fresh air into it. Like one of my architectural icons Malcolm Wells did 40 years ago, we are taking the worst and making it the best.

So, this all sounds good, but what does it actually mean in practice? The analogy I like to use is that a Passive House is like a Yeti Cooler with a nose on it: super insulated, air tight*, bear proof and with dedicated ventilation that filters and preconditions the fresh air coming in (like your nose!).

A Passive House

A Passive House

Passive House buildings can be houses, offices, fire stations, apartment complexes or schools. They can be tiny houses or skyscrapers. Yep, Passive Houses are not actually houses, they also aren’t passive! It’s not the best name but it is part of a lineage so it stuck around.

Their construction can be anything, there are no required techniques or materials. It is not a checklist or points based system like LEED or our local Austin version AEGB but instead is a series of low energy and air tightness targets tha must be met.

To further complicate things there are two sibling certifications with different organizations behind them. Without getting too deep into the family feud, the US based group Passive House Institute US (PHIUS) split off when it introduced climate specific standards to the varied climates of North America, while German based Passive House Institute (PHI) kept the standard static regardless of climate. We are certifying Theresa as a pilot project under PHIUS 2018+, the latest iteration of the US climate based standard. For something with such noble goals and a tiny market share, splitting hairs (and resources) seems crazy to me but such is life.

To build our house to the standard we First had to design our building and vet it through PHIUS and the energy modeling software they use - WUFI Passive. That process told us - based on the size and shape of our house, overhangs, window size and location, efficiency of our HVAC and appliances, and the climate of Austin - how much insulation we needed and where, how good our windows needed to be and how air tight our building had to be. The whole thing is a process - if we added more windows we might have to beef up the insulation or make things more air tight, if we deeper overhangs to shade those windows we might be able to be less airtight. We had all of these little levers to pull on that helped or hurt us in getting to the energy use targets required by the certification.

Insulation is pretty obvious - you add more of it and you lose thermal energy at a slower rate (and thus save money and have a more comfortable space). A couple caveats here though. First, continuous insulation is more effective than insulation with thermal bridging - putting insulation between steel studs can reduce its effectiveness by up to more than half making potentially it less valuable than the cost of installing it. Second, adding more insulation thickness gives diminishing returns - inches 1-3 may give you twice as much value as the 3-6, which will give you twice as much value as 6-9. In NH, where I grew up, the amount of hours below 20 degrees is roughly the same as the hours above 100 degrees in Austin. BUT if we want to keep our homes around 70 degrees, that is a 30 degree difference in Austin and a 50 degree difference in NH. In colder climates those extra 6-9 inches may be worth a lot more than in hot climates.

Windows also vary by climate. We always want them to be well insulated (a low U value) but in hot climates we want to shade them and keep the amount of solar heat gain to a minimum. To understand solar heat gain, think of your arm getting hot from the sun through a window as you drive down the road. In cold climates, we want to let some of that solar heat gain in to warm up the house in the winter, but shade it in the summer. Gratefully, winter sun comes in at a lower angle so this is easy to accommodate. In hot climates like Austin we are primarily focused on keeping direct sun off the glass and our solar heat gain coefficient down so that when sunlight does come through, it does not add to our air conditioning loads by heating up the inside of the home.

Air tightness is the weird one and the one that is hardest to understand - and the one we get most pushback on from builders. This is mainly because it is a “new” metric in the building industry and hard to price. So to start off with: houses do NOT need to breath - people do. We hear this a lot and am not really sure where it comes from. Houses have never needed to breath. A properly built house should strive to be as air-tight as possible. This reduces energy loss (all that air you spent hard earned money to heat/cool just leaking away through your old crappy windows), makes you more comfortable (drafts are gross!), and most importantly improves indoor air quality. You, average american, spend nearly 90% of your life indoors, 70% in your home. Where does your fresh air come from as you sleep? Take a minute to think about it. Hint: It’s not from your air conditioner. You get a tiny bit from opening and closing the front door and you might get some on those spring days when you open your windows. Otherwise, all the air you breathe comes via the thousands of little (or big) cracks in your walls, through your floorboards, down from your attic, through leaks in your duct work, in from your attached garage… These are really gross places that are full of dead pests and pesticides, insulation fibers, mold, dust and motor oil and all that Miracle Grow stored next to your WD-40 and snow tires. That is where the “fresh” air you and your kids breath comes from. If this isn’t freaking you out, read it again. Here is a link for some good air filters. Here is a link to Passive House designers throughout the US who can help you get good clean fresh air in to your house.

The good news is that many new houses are being built with dedicated ventilation, which means that they have a fan than brings fresh air directly from the outside (bypassing the squirrel family in your wall), filters it and supplies it to your living spaces. Passive Houses use an ERV or HRV for dedicated ventilation. These are magical boxes that supply filtered fresh air but they also temper it by pulling the heat energy out of exhausted stale air and putting it into the fresh air, reducing the energy penalty from bringing in all that fresh (but unconditioned) air. By making buildings super air-tight, and then using an ERV, Passive Houses ensure a constant supply of truly fresh, filtered air while being also incredibly energy efficient.

ERV Diagram

This is a very basic explanation of a Passive House building, but it scratches the surface of a lot of topics that much smarter people than myself have written on extensively. If you want to nerd out further, follow the links, listen to a podcast, buy a book, attend a conference. If you are in Austin come to a Passive House meet up or a Building Science Happy Hour to meet some good folks who are also trying to wrap our heads around how to make better, healthier buildings.