The DIY passive house
February 2, 2017
When Clayton Binkley and Adrienne James began designing a Scandinavian-summer-home-inspired residence in Seattle, they weren’t trying to build a passive house. Binkley, a structural engineer at Arup, and James, an architect, wanted an interesting home that was comfortable and energy efficient, had a strong connection to the outdoors, and used tactile, expressive materials. They wanted to use their experience to create a high-performance building, taking their time to experiment along the way.
Several months into the process, Adrienne completed her training to become a certified consultant for passive house design, a standard that’s common in Europe but just starting to make inroads in the United States. Unlike LEED, which awards points for a wide range of pro-environmental features, passive house is focused exclusively on energy usage.
With foundation construction already underway, the couple decided to take the plunge into passive house design.
Six years after embarking on the project, Binkley, James, and their 7-year-old son, Fergus, moved in in early September.
I caught up with them on-site during construction.
What have been the biggest challenges of this process?
James: The biggest hurdle is that the ideal passive house is a cube. This is clearly not a cube.
Binkley: A lot of the passive houses in the US were not terribly interesting design-wise, especially when we started. We took it as a challenge to preserve the architectural form and finishes that we had designed but dial up the energy performance.
The big construction challenge was definitely getting the house sealed tightly to avoid heat loss through air leakage and capitalize on the highly insulated walls. If you don’t seal the house properly, you waste energy and risk getting condensation at leakage points.
The rub is, of course, that you need fresh air to live and make the house feel comfortable, so you end up bringing in lots of cold outdoor air in the winter. Warming that air up becomes your biggest heating load. To counter this, you run your ventilation air through something called an HRV, which is a heat exchanger that recovers the heat from the exhaust air leaving the house and uses it to warm up the fresh air coming in.
James: The other part of passive house detailing is minimizing thermal bridges, which are basically anything that crosses from the inside to the outside that can conduct heat. Wrapping the whole thing in exterior insulation underneath our rainscreen cladding system eliminates a lot of those. We basically have two walls with an insulated cavity in between. We ended up with 11 inches of insulation in the wall, plus the exterior insulation layer.
How do windows work in a passive house?
James: The windows are what all passive house people get excited about because they represent the areas of most heat loss in your envelope. You need to overinsulate the frames on the outside to make them as good as walls in terms of heat escaping and minimize the thermal bridging. Ours were especially tricky because with our roof geometry none of the upstairs windows are rectangles and each one is unique.
Binkley: The roof’s a hyperbolic paraboloid, which basically means it twists. It’s high in the back and rolls down to the front, so it’s a doubly curved surface, but also doubly ruled so it can be framed with straight pieces of wood. We modeled the whole thing in Rhino and then wrote scripts to generate all the cut lists and compound miters for the roof joists.
We’re on this hill with a really tall house uphill to our north and a small house to our south, so we wanted to follow the hill down a bit with the roof so we don’t loom too much over our neighbor — that’s the practical reason for the geometry, aside from it being fun to build.
In plan, we pulled the building north and created a concave south edge to let light in.
The design of the doors was another interesting challenge. The guys who made our windows had a door really similar to what we wanted. The windows were expensive, but not out-of-control expensive, as windows go. The doors were off the chart, though, so we just built them ourselves.
It was a real learning experience to figure out all the air-sealing, thermal-bridging, and water-management details, as well as the hinging and multipoint lock construction on a 3-inch-thick door. It was more like product design than normal architecture.
Tell me about some of the other energy considerations you had to take into account.
James: Given that it’s an east–west lot, we were able to get quite a bit of solar gain passively, which helps even in Seattle’s gray winters.
We also thought a lot about controlling summer overheating, which is often overlooked in passive house design. People get so focused on minimizing the need for heating in the winter that they end up designing sweatboxes in the summer.
In our house, the shape of the roof allows for a fairly strong stack effect to move air from the cool underground basement up through the main floors. Also, we’re on a really good hill and the prevailing winds run north, so at night we can just open the windows and flush the house out.
We also decided to add low-temperature in-floor radiant heating under our wood floors. It is expensive when you pay someone to install it and build the pump and boiler panels, but actually very reasonable when you do it yourself. While the floor doesn’t get as warm to the touch as a radiant slab, you only need to heat one or two rooms and the whole place gets warm right away.
How did you measure energy efficiency during the design process?
James: In design and during the certification process, there’s a spreadsheet-based energy model that lets you assess your construction against the Passive House Institute US benchmark. You really look at every square inch of your construction — your insulation and the amount of framing, the window glass and frames, thermal bridges — and compare that against the heating demand for your particular climate.
James: We wanted this to be an experimental house that let us try out ideas, so then I can say to clients from experience what’s worth pursuing.
It’s funny — when people come over they get very excited about the house, not knowing anything about the passive elements. But there are no drafts, the light is nice, and it’s quiet. It just feels very calm. You need to have a high-performance envelope to meet passive house requirements, but that creates a quality-of-life benefit that you wouldn’t necessarily predict.