Passivhaus on a Budget
Climate Zone 4A — Thaxton, VA
General Specs and Team
Location: Climate Zone 4A — Thaxton, VA
Living Space : 1808 sqf
Cost (USD/sq. ft.): $150/sqf
$150/sf includes septic, well, and some site work
Designer: The Structures Design/Build team
Builder: The Structures Design/Build team
Foundation: Modified “raft” foundation (monolithic slab)
Slab Insulation: Two staggered layers of horizontal 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. foam board (R-18) under entire slab; molded EPS forms at perimeter.
Wall frame: 2x4 studs, 16” on center, with solid lumber headers
Wall 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. : Zip System sheathing
Wall insulation: R-13 fiberglass batts between studs and 6-in.-thick EPS nailbase insulation (R-37) on exterior of Zip sheathing
Wall 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.: Zip System sheathing and tape
Siding: James Hardie HardiePanel fiber-cement siding installed over Home Slicker
Exterior trim: MiraTEC composite board trim
Windows: Klearwall windows with triple glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. (R-7)
Roof frame: Truss construction to create an unconditioned ventilated attic
Roof sheathing: 5/8” OSB
Ceiling insulation: Cellulose (R-60) on attic floor
Ceiling air barrier: OSB, sealed with duct mastic and tape
Roofing: Asphalt shingles
HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Rating: 38
Blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. results: 0.6 achACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations. @ 50 Pa
PHPP specific space heat remand: 3.16 kBTU1,000 Btus / (ft2•yr)
PHPP estimated site energy use: 5,483 kWh/year (457 kWh/month)
Actual energy use (Mar. - Nov. 2012): 442 kWh/month
Space heating and cooling: Mitsibishi Mr. Slim 9,000 BTU/h ductless 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. (MSZFE09NA / MUZFE09NA), supplemented by a coil in the ventilation ductwork that circulates fluid through a buried ground loop
Ventilation: UltimateAir DX200 energy-recovery ventilator(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.
Domestic Hot Water: Solar thermal system with tankless electric-resistance backup
Appliances: Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. rated appliances (where applicable), induction range
Lighting: Tube fluorescent and compact fluorescent
• Low-flow plumbing fixtures
• Toto 1.28 gpf toilets
Indoor Air Quality
• Low-VOCVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. paints
• Solid bamboo and ceramic tile flooring
• Balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). system provides fresh air to living spaces and bedrooms
This custom home, a PHIUS+ certified project, disproves the notion that energy-efficient houses are cost-prohibitive
When the time came for Jason and Stephanie Specht to find a builder, they started out with high ideals. They wanted a builder who wouldn’t skimp on the quality of construction and who wouldn't charge an exorbitant fee.
Since this was their first time building, there were a lot of unknowns and a lot of questions: As clients, would they be able to customize plans? Select materials and finishes? Could they realize their budgetary goals? Did the builder have a good reputation? Would the house be energy efficient?
Most of all, they were looking for a builder they could trust.
It was a difficult prospect, and their first efforts proved disappointing. After months of learning about the marketplace and figuring out the possibilities, they settled on Structures Design/Build. It was a business that offered custom plans and construction management, one that could take their project from concept to completion.
During their first consultation, Adam Cohen, a principal at Structures Design/Build, asked them if they had considered Passivhaus. (For more information on Adam Cohen, see Passivhaus Practitioners Share Their Success Stories.) Like many people, they had not yet heard of the Passivhaus standard. And, they assumed that energy efficiency was out of their reach — that it was just too expensive.
Reaching cost parity
Adam was quick to present a different point of view. On the contrary, he argued that constructing a Passivhaus wasn’t necessarily any more expensive than constructing a house to meet the existing energy codes. He told them that although their initial capital investment would certainly be larger, their monthly outlay would be the same. In short, Adam believed that by creating an optimized Passivhaus design, they could achieve cost parity with traditional construction.
What is cost parity? Literally, it means equal price. The phrase is often used to evaluate the performance of a new business (i.e. a new manufacturer). Economists say that a business achieves cost parity when they can produce a comparable product at a comparable price. In theory, it doesn’t matter if it is a commodity, a product, or service, the question is whether or not something is truly cost competitive.
In the context of Passivhaus construction, it is the idea that a builder can leverage their knowledge and experience to produce a Passivhaus at a price equal to standard construction, if the energy costs are included in the calculation. It is the idea that the additional capital costs of Passivhaus construction pay for themselves through savings in your monthly utility bill.
Research, research, and more research
Energy-efficient construction that was cost-competitive? What was not to like? Although the Spechts thought it was a great idea, they wanted to learn more about the Passivhaus concept. Jason started researching — and researching. He read the book, Recreating the American Home: 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. Approach, by Mary James. He studied the details of other residential Passivhaus projects, and spent countless hours on the Internet. In the process, he learned an entirely new vocabulary.
When the Spechts finally settled with the idea, they met with Structures Design/Build at the building site. And so began the process of designing a Passivhaus. Over the next few months they worked through multiple options: they finalized decisions on foundation type, building size, floor plan, and exterior style. The house would include an attached garage for vehicles, storage space for the canoe and other outdoor equipment, and provide a space where Jason could complete welding projects.
When the building process began, the research didn’t stop; it just took on a different dimension. Since Passivhaus projects often use new materials and techniques, Jason and Stephanie continued with their research and project involvement: they wanted to be sure they made informed decisions along the way.
Securing a loan was easier than expected
Jason worked at a local credit union. And even though he had a solid understanding of residential mortgages, and how to secure a loan at a good rate, he worried that it would be a troublesome process. He attempted to find a bank that offered EEMs (Energy Efficient Mortgages), and prepared to argue the merits of the Passivhaus concept. What he found was disappointing: loan officers were unaware of EEMs, even if their company offered them in their mortgage service portfolio.
In the end, he met a loan officer at a local bank that spoke his language, someone who valued energy efficiency and conservation. And all that worrying was for naught. The building plans were appraised at a value that was equal to the construction contract price — even without figuring in the utility bill savings. This was a pleasant surprise, and confirmed the idea that energy-efficient construction could be cost-competitive.
Now they were ready to break ground and start building.
Build your house on solid foam
Structures Design/Build specified an insulated slab — sometimes called a raft foundation. (For more information on raft foundations, see “Foam Under Footings.”) But unlike most raft foundations, this design incorporated a thickened edge. This had several benefits. One, the only proprietary piece 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. foam was the integral footer; the majority of the slab could be insulated with standard sheets of EPS foam (readily available in most markets). Two, the slab could be poured at a standard thickness, reducing the amount of concrete required for the foundation.
It was a win-win situation, with lower costs, less shipping, and lower 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..
Rigid air barriers rule
Zip 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 tape created the wall air barrier, and standard OSB completed the assembly on the second floor ceiling. The Zip System is gaining traction in the U.S. marketplace and is now commonplace in many locations. It doesn’t take long for a framing crew to install and detail this system to create an excellent air barrier.
Adam wanted to perform blower-door tests at various stages in the construction process. He conducted the first test when the airtight sheathing was first sealed, but before the framing crew cut the window and door openings. This multistage testing protocol helped him quantify and isolate the leakage paths, and later, determine the effectiveness of other air barrier components (i.e. window frames and utility penetrations).
Down come those thermal bridges
The framing crew built a fairly common structure — 2x4 studs, 16 inches on center, with solid lumber headers. They used open-web floor trusses for the second floor (providing a cavity for plumbing and ductwork) and engineered trusses for the roof.
So at first glance, it didn’t look like a Passivhaus structure. However, they later installed 6-inch-thick nailbase insulation over the entire wall, including the gable ends. This brought the nominal R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. up to R-37 and created a wall that was free of thermal bridges.
They later installed a deep layer of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. above the second floor ceiling to finish the thermal enclosure.
Small heating and cooling loads
Mechanical ventilation is provided by an energy-recovery ventilator (ERV). The ERV is equipped with a heat-exchange coil on the fresh air inlet; this water-to-air coil is tied into a ground source loop (a plastic pipe buried in a horizontal trench). It is designed to temper the incoming airstream and reduce the temperature difference between exhaust and supply.
A ductless 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. covers the peak summer and winter loads; the unit’s indoor head is located high in the stairwell and services both levels.
Transfer grilles help reduce pressure differences between the bedrooms and adjacent common areas, and encourage room-to-room mixing when the doors are shut. (For more information on transfer grilles, see Return-Air Problems.) But according to the homeowners, an open door definitely reduces heat build-up during the hottest summer nights; it also provides the greatest comfort level.
What you don’t see is what you get
Whether the impression is accurate or not, in the U.S. there is a pervasive idea that a green home requires weird architecture, recycled materials, and lots of high-tech gadgetry. Jason and Stephanie often found that it was difficult to explain the Passivhaus concept to friends, family, or site visitors. People touring the house wanted to see all of the innovative features — but in fact, there wasn’t much to show: most of the innovation was hidden behind the siding or the drywall.
Yes, the solar thermal system was visible out front, but the water heater looked entirely ordinary. There was a small black box in the utility room (the ERV) — but that was nothing to get excited about. And visitors could open the windows, admire the depth and heft of the triple-pane windows ... but there wasn’t much else to see.
Perhaps it is a cultural artifact, or just human nature, but most people don’t get enthusiastic about air barriers, thick insulation, or low energy bills. The Specht family does. And they are even more excited about the fact that they built a high-quality house—an ultra-low-energy house — without blowing their budget.
• Get involved! Although they were building a very 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., the Spechts were very active in the design and build process. This gave them a sense of ownership and control, and contributed to the overall success of the project.
• Material choices had a bigger financial impact than the decision to build to the Passivhaus standard. Fiber-cement siding, hardwood and tile floors, solid surface countertops, and quality fixtures all cost more, period.
• Consider how volunteer organizations can help. A local club, the Roanoke Renewable Energy Electric Vehicle Association (REEVA), installed the solar thermal system — without charge. That’s community activism at its best.
• There is a learning curve with new products and technologies. It took some trial and error to learn how to best operate the 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.: during the cooling season they found that letting the unit work longer at low speed provided greater comfort. Also, the Klearwall windows required adjustment by a factory representative. (I should note that these were among the first Klearwall windows shipped to the U.S. market).
• The solar thermal system generates a significant amount of heat inside the laundry room. On a warm and sunny day, the closed loop pump becomes a miniature radiator (i.e. loop temperatures reach 150°F). This is undesirable in the summertime. Placing the water heater in the garage would resolve this issue.
— Daniel Ernst is currently starting a design/build firm, Promethean Homes, in Steele's Tavern, Virginia. He posts regular blogs on his company's web site.
- Daniel Ernst
- Adam Cohen
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