Partitions and Interior Finish
We didn't frame any of the interior partitions until the exterior walls were fully drywalled, taped and mudded, and insulated. This approach complicated the schedule somewhat but offered two benefits: First, it eliminated difficult-to-seal inside corners where the drywall on the exterior walls was interrupted at a partition. And second, partition walls can now be moved without introducing air leaks. That's not important in the short term, but when the interior is remodeled somewhere down the road, our method makes it much more likely that the air-sealed interior will maintain its integrity.
Mechanicals and final blower-door test. We built a drop ceiling over the kitchen and mechanical-room area to provide a space for the ERV and associated ductwork, and furred a 3-inch wall along most of the north side to accommodate the plumbing and electrical for the kitchen, mechanical room, laundry, and bath, and piping for the code-required sprinkler system. All this was completely within the air-sealed shell, making further sealing unnecessary. Our final blower-door test gave us a figure of 0.36 ACH - comfortably within the 0.6 ACH required by the Passive House standard.
Solar Thermal and ERV
The heating demand of a Passive House is so low that incidental heat from appliances, lights, and cooking - even body heat - can supply a large part of what's needed. The heating system for this project relies on an active solar-thermal system with an electric resistance backup, with heat distributed by the ERV.
Rooftop solar. Four rooftop-mounted solar thermal panels from Vaillant (858/205-9089, vaillantsolarsystems.com) are sized to meet about 80 percent of the home's expected demand for both space heating and domestic hot water, even during cloudy winter weather. The hot water from the panels is stored in a pair of 80-gallon superinsulated storage tanks. The plumbing was laid out as a drainback system, meaning that the water in the panels flows back to storage if there's any threat of overheating or freezing. A small 20-gallon electric water heater holds a backup supply of domestic hot water.
When the thermostat calls for added space heating, a small inline pump circulates water from the solar storage tank through an inline heat exchanger mounted in the supply duct of the RecoupAerator ERV (800/535-3448, ultimateair.com). If the storage tanks have been drained of heat by a prolonged cloudy period, an inline resistance heating coil picks up the slack.
Fresh air. To provide consistently high air quality, the ERV runs continuously, recovering up to 80 percent of the heat from the exhaust stream and transferring it to the incoming makeup air. Three ducts pull stale air from the utility room, kitchen, and bath. The normal rate is 12 cfm from the utility room, 56 cfm from the kitchen, and 24 cfm from the bath, but both the kitchen and the bathroom have manually controlled booster switches to bump up ventilation when needed.
Balanced supply ducts are located in the bedroom and the living/dining room. A night cooling cycle controlled by a sensor in the incoming duct brings in cool air when appropriate.
There was a steep learning curve on this project, and I'd do a few things differently next time. For example, we probably spent more time than we needed to devising our own methods of sealing around pipes and wires. My next Passive House would use more out-of-the-box tapes to seal seams and penetrations.
There have also been some commissioning problems. Energy monitoring equipment installed soon after the house was completed revealed that it was using far more electricity than expected. We eventually determined that this was due to a defect that caused the solar-thermal pump to run around the clock for nearly six months. Given the small amount of heat needed, I now think the heating system is more complicated than it needs to be. Next time, I would consider using one or more wall-mounted towel warmers in place of the inline electric resistance heater in the ERV duct line.
Cost and performance. I deliberately bid a low price for this job, because I was interested in Passive Houses and saw it as a great learning opportunity. Some of the work was done by volunteers or workers who charged a reduced rate, and we had the benefit of some discounted materials from suppliers. As a result, it's impossible to give a firm cost figure for the job. Based on my experience as a builder in this area, I'd say that it cost about 10 percent more than a comparable structure built to conform with Title 24, California's current energy-conservation code. In Europe, where Passive House has a well-established track record, the PH cost premium over that of conventional construction is said to be as little as 5 percent.
The tenants now living in the house - which is owned by a local land trust - have been amazed at its ability to self-regulate and remain comfortable regardless of the weather outdoors. To me, that underscores an often-overlooked benefit of this approach to building: Because there's little need to fiddle with a thermostat or compensate for other shortcomings in the building envelope, a Passive House is both user-friendly and tolerant of occupant error.
Terry Nordbye is a building contractor in Point Reyes Station, Calif.