A Tale of Two Houses
In building to the Passivhaus standard, one architect relies on rigid foam insulation while another designer avoids it like the plague
As difficult as it is to meet the requirements for Passivhaus certification, builders and designers have a great deal of leeway in how they approach it. There are just a few big hurdles to clear, including limits on how much energy the building can use and how airtight the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. must be.
Exactly how a builder accomplishes this is not spelled out. As long as the building meets the standard, it can win certification, either from the Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. Institute U.S. (PHIUS) or its European counterpart, the Passivhaus Institut (PHI).
At the North American Passive House Network conference last month in Portland, Maine, two designers with very different outlooks detailed their Passivhaus projects, demonstrating that many roads can lead to the same destination.
One of them, Andrew Michler, took the unusual step of avoiding foam insulation in the house he designed in the mountains of Colorado: none under the slab, none in the walls, none in the roof.
The other, New Mexico architect Vahid Mojarrab, chose components made with rigid foam as a way of achieving Passivhaus performance economically and reliably.
Is one better than the other? It all depends on how you look at it.
For Michler, a first-ever Passivhaus design
Michler has been living off the grid in the mountains outside of Fort Collins, Colorado, for 20 years. He’s done lots of retrofit work, and built a few houses, but about five years ago, during a construction dry spell, he veered into consulting and writing. He hopes to publish a coffee table book on high-performance building sometime next year.
A few years ago, Michler interviewed William McDonough, an influential architect and writer who published a book called Cradle to Cradle in 2002 in which he advocates benign, waste-free design. The interview was pivotal for Michler. When he began designing a guest house for his Colorado property, one of the most important considerations became the selection of the materials that would go into it.
Foam insulation manufactured with petrochemicals wasn’t on his list. Although it's commonly used in many parts of a high-performance house, Michler decided he didn't want it. “McDonough makes a fantastic case that we really need to look at what a resource is, in general, and not just think about attributes but think about the long stretch,” Michler said by phone a week after the conference.
Time running out to make the right choices
What troubled Michler was "the legacy of toxicity" that foam insulation represented. He objects to its high embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost., the global warming potential of its blowing agents, and the extremely slow payback of using large amounts of it in a building. Spray polyurethane foam has more than 100 times the greenhouse gas potential as cellulose, he told a conference panel; expanded polystyrene has as much as 18 times the embodied energy as cellulose.
“What are the chemical impacts of what we’re doing right now?” he asked.
“The industry has kind of baffled us with bullsh**, the foam industry,” he said by telephone, “where they can kind of say, ‘We’ve done this better,’ while totally ignoring all the other science and other issues with their product and then call it green because it’s less damaging. The whole idea with Cradle to Cradle is that you don’t create products that are less damaging; you create products that are more beneficial.
“If we’re looking at turning around the ship, so to speak, we don’t have 60 years; we don’t have 15 years. We really have to start talking about the impact of our building sector at this moment,” he continued. “In that sense, I don’t see the race as a marathon, as we often talk about it for payback periods, but as a sprint from start line to finish line.”
The answer: Cellulose, mineral wool, and a crawl space
Michler’s two-story, 1,200-square-foot house in Masonville, Colorado, is built with 2x4 structural walls insulated with mineral wool batts, taped plywood sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , and 14-inch-deep Larsen trusses insulated with cellulose and Roxul Drainboard, a rigid mineral wool insulation.
Exterior above-grade walls are rated at R-65 to R-85, depending on how thick they are. Their unusual depth made it impossible for Michler to find an insulation contractor willing to dense-pack them with cellulose. He ended up doing it himself, using weed mat to contain the cellulose between truss cavities, and OSB shelves to divide wall cavities into spaces no more than 7 feet tall to reduce settling.
The roof, framed with I-joists, is insulated with a combination of mineral wool batts and cellulose to R-75 (see the section drawing for more details).
Michler built the house over a crawl space. The concrete stem walls are insulated on both the inside and outside with Roxul Drainboard to R-40, and the floor consists of 16-inch-deep I-joists insulated with blown-in cellulose to R-58. OSB provides the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both..
“A lot of people gave me hell for having a crawl space,” Michler said, because it raised concerns about moisture and mold. Although the crawl space will have to be monitored, the design looks viable in the relatively dry climate where he lives.
- The windows are triple-glazed Intus units with a solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.51 and a U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.14.
- Whole-house ventilation is provided by an Air Pohoda heat-recovery ventilator connected to an earth tubeVentilation air intake tube, usually measuring 8 or more inches in diameter and buried 5 or more feet below grade. Earth tubes take advantage of relatively constant subterranean temperatures to pre-heat air in winter and pre-cool it in summer. In humid climates, some earth tubes develop significant amounts of condensation during the summer, potentially contributing to indoor air quality problems..
- A 2-kW photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array (off-grid).
- Domestic hot water from a solar-electric preheater tank and a 92% AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. Noritz on-demand propane heater.
- Heat comes from domestic hot water system, which feeds both a radiant loop in a wall separating the living room from the bathroom in the middle of the house, and a heat coil in the supply side of the heat-recovery ventilator. The HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. runs continuously, drawing 30 watts of electricity to move 62 cfm.
- A phase-change material called BioPCmat M51 is installed on two walls to help even out spikes caused by solar heat gain.
The blower door test measured air-tightness at 0.45 air changes per hour at a pressure difference of 50 pascals (ach50). Michler built the house for $220 per square foot.
In New Mexico, the aim is to compete in a spec market
If Michler had the luxury of working on a guest house on his own property, and the time to feel his way through some of the building details, Vahid Mojarrab faced a completely different set of challenges: How to build to the Passivhaus standard with a labor pool familiar only with conventional techniques, and do so quickly and efficiently enough to stay competitive on price.
Mojarrab knows the production housing market well. He's built more than 1,200 houses in Colorado, New Mexico, and California but was first introduced to the Passivhaus standard five years ago in a training program in Champaign, Illinois. "It just made sense to me to be able to take advantage of the airtightness and some of the basic science to achieve a high-performance building," he said by telephone a week after the Portland conference. "When I came back to Santa Fe, we started investigating how we could incorporate that.
"Obviously, the real estate market was in a downturn, so it was really hard to be able to go into the market with something so different and ask more money for it when people where looking around with really depreciated real estate all around the country, especially in our area."
Mojarrab's plan was to stick as closely as possible to familiar building practices so crews wouldn't have to learn an entirely new way of construction. He had two other advantages: an investor named Bob Schneck who was wanted to collaborate with Mojarrab to advance Passivhaus building in the region, and energy modeler and "Passivhaus optimizer" Graham Irwin of Essential Habitat in California.
Start with typical local practice and work with that
Because of building requirements in Santa Fe, builders were already used to adding a layer of rigid foam insulation on exterior walls. "What we did," Mojarrab said, "was to take that detail and just bumped it up."
Instead of adding 2 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., they added 10 inches of EPS in the form of a panel faced on one side with OSB — half of a structural insulated panel. The 2x6 exterior walls were air-sealed by taping the Zip panels before the SIPs went up, creating an air barrier where it would stay protected from plumbers, electricians, and other subs. With blown-in cellulose in cavities, and the 10 inches of EPS over the sheathing, total wall R-values are 61.
"It has to be taped; it has to be taken care of a little bit," he said of the Zip wall system. "The construction approach has to adjust a little bit, but it wasn't really a drastic measure. Nobody looked at our details and said, 'We cannot build this. Oh, it's so expensive.'"
Mojarrab placed 4 inches of EPS insulation beneath the slab. In the roof, he used another 10-inch thick, one-sided SIP in addition to blown-in cellulose in the I-joist framing for a total R-value of 97.
Mojarrab's VolksHouse 1.0 (meaning "People's House") came in 6.5% below normal construction costs and was appraised for 35% more than similarly sized houses in the same area.
Some of the construction details:
- Domestic hot water: Solar thermal with an electric backup and an 80-gallon tank.
- Heating and cooling: A Mitsubishi Mr. Slim ducted minisplit heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. rated at 8,100 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour for cooling and 10,900 Btu/hour for heating.
- Whole-house ventilation: UltimateAir energy-recovery ventilator.
- Windows: Optiwin. Southern exposure, solar heat gain coefficient of 0.63 and a U-factor of 0.11; east, north and west elevations, solar heat gain coefficient of 0.53 and a U-factor of 0.11.
The blower door test measured air tightness at 0.3 ach50.
Mojarrab built the 1,700-square-foot, three-bedroom, two-story house for $165 per square foot. It is certified by PHI and also won a New Mexico "Emerald" rating, which Mojarrab says is similar to a LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -Platinum rating.
Foam was an important building component
Mojarrab's decision to make rigid foam insulation integral to his design was an easy one. "The reason that we used the EPS for the performance of the building was that it was an easy way to get the performance very quickly," he said. "We are dealing with production housing, so the people who are building the houses are not very sophisticated. They just put this stuff together."
Using the half-SIPs on exterior walls was very familiar to the crews he worked with. "It's the detail they are used to," he said. "They're not unfamiliar with it, so it can be executed again and again with great success."
Mojarrab continues to tinker with his designs and reduced the amount of rigid foam insulation in subsequent designs. But he thinks it will be difficult to eliminate foam entirely, especially beneath the slab and in the roof assembly.
"As we are proceeding, we are really refining our walls," he said. "I can't argue with Michler or anyone else when they say that foam doesn't have any place in their buildings, but at the same time I feel like I'd rather use it for insulation than burn it in my car. It's available to us. Our builders get it. It makes a better building. Why not use it until we come up with a better solution?"
Then, too, there are the pressures of the production housing market in which Mojarrab is working. "What we try to do in all of our projects is to optimize it, to make every dollar count," he said. "That's production building. Every dollar counts in overall construction costs."
- Images #1 through #5: Andrew Michler
- Image #7 through #11: Vahid Mojarrab, WAMO Studio
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