In September 2009 I completed a deep energy retrofit ("A Cost-Effective Energy Retrofit," 8/10) on a small house in Point Reyes Station, Calif., some 50 miles north of San Francisco. The owners were so pleased with the results they asked if I would build a new 750-square-foot rental unit in the back of their property, designed to meet the demanding Passive House energy-use standard. (Because affordable rental housing in California is in such short supply, the state encourages the construction of such second units - though they're limited to a maximum footprint of 750 square feet.)
I began work in February 2010, using a set of plans developed by architect James Bill. It was to be the first new PH project in California. There were no textbooks, elders, or old-timers to fall back on for advice or tips. My subs had no idea what they were getting into. My workers had worked on the retrofit and had the basic concepts of air-sealing, but all in all, we felt a bit like first-time explorers going to the moon.
The Passive House Standard
The Passive House standard was developed in about 1990 by German physicist Wolfgang Feist and Swedish professor Bo Adamson. It aims to provide people with buildings that use little energy, and to achieve this through efficient air-sealing, lots of insulation, and a minimum of thermal bridging (see "Passive House Seeks Broader Appeal," JLC Report, 2/11).
A unique feature of the PH standard is that it imposes an absolute energy budget on a house. The building can't use more than 120 kwh per square meter per year (38,000 Btu per square foot per year), of which no more than 15 kwh per square meter per year (4,800 Btu per square foot per year) can be used directly for heating or cooling.
Hiring a consultant. Meeting that goal is made possible through the use of a sophisticated energy modeling tool available through the Passive House Institute U.S. (PHIUS). That tool, called the Passive House Planning Package, or PHPP, requires considerable training to use correctly, and with rare exceptions is accessible only to Passive House consultants who have completed an intensive nine-day training program.
When I was considering this project, I realized that I had two options: I could either take the training course myself, or hire a certified Passive House consultant to develop the plans in conjunction with the project architect.
I didn't have to think about it for long. I'm a guy who likes to build things, not crunch numbers. So we hired Lowell Moulton, a local PH consultant, to work with our architect to make sure that the house we built would meet the standard. The resulting collaboration went smoothly.
But there was still plenty for me to do. Passive House construction requires a lead person on the job every day to make sure the workers are correctly addressing critical aspects of the shell. That person has to be fully versed in air-sealing and thermal bridging. More so than with any other building I have ever done, every member of the building and design team has to understand the jobs of all the rest. If one of the team fails, the entire outcome can fail.
The foundation we used was basically a carefully detailed, better-insulated version of the stem-wall foundations common in our area. After pouring the concrete and stripping the forms, we leveled out and tamped the interior crushed-rock bed and covered it with a layer of 6-mil poly. On top of the poly we laid a bed of 4-inch XPS, leaving a narrow gap between adjacent sheets, which we sealed with a Pageris foam gun and Touch 'n Seal 32-ounce canisters. We added a layer of 3-inch foam over the first layer, again sealing the joints, to bring the subslab insulation to R-35. We then covered the foam with another layer of 6-mil poly.
Insulated edges. To prevent a major thermal bypass at the edges of the slab, we cut strips of 2-inch foil-faced polyiso board and applied them to the exposed portion of the stem wall, finishing flush with the top. After placing the rebar grid and pouring and finishing the slab, we bedded the mudsills in a thick layer of asphalt roofing cement. The resulting reinforced slab (which would later be stained and serve as the finish floor) was completely enclosed in foam on all five sides and air-sealed to the framing that followed.