Farmhouse Style Meets Passive House
Climate Zone 4A: Steeles Tavern, VA
General Specs and Team
Location: Climate Zone 4A: Steeles Tavern, VA
Living Space : 2676 sqf
Cost (USD/sq. ft.): $85/sqf
(Note: Reported cost of $85/square foot does not include most of the labor costs.)
Designer: Promethean Homes (Daniel Ernst), with help from John Brooks
Builder: Promethean Homes (Daniel Ernst)
Energy Consultant: Think Little (John Semmelhack)
Insulation Contractor: Southland Insulators (Ivan Sandau)
Foundation: Sealed, insulated, and conditioned crawl space (CMUConcrete masonry unit. Precast concrete block used to build walls. CMUs have hollow cores that can be filled with concrete onsite for additional reinforcement. The use of stronger, more lightweight types of concrete such as autoclaved aerated concrete (AAC) is becoming increasingly popular in CMU manufacture. walls parged with surface-bonding cement)
Rat slab insulation: Horizontal layer of insulation under the slab: two staggered layers of Thermafiber VersaBoard 80 (2” thick mineral wool rigid board insulation) totaling R-16
Foundation wall insulation: Two staggered layers of Dow Thermax 2” thick polyisocyanurate with thermal ignition barrier (R-26)
Foundation 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.: 6 mil Dura Skrim and surface-bonding cement
Wall frame: Double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. construction; outer load-bearing 2x6 walls, framed 24" O.C.; interior 2x4 walls (non-structural) framed 24” O.C.
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. : Exterior rated 5/8” plywood
Wall insulation: Dense-packed GreenFiber 765LD all-borateBoron-containing chemical that provides fire resistance to materials such as cellulose insulation and provides decay and termite resistance to wood products. Borate is derived from the mineral borax and is benign, compared with most other wood treatments. cellulose (R-40)
Wall air barrier: Plywood sealed with Sto StoGuard (Gold Coat)
Siding: James Hardie HardiePlank pre-painted cement board siding, installed over rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. (3/4” thick furring strip) - Stables Construction Co.
Exterior trim: MiraTEC composite board trim
Windows: Accurate Dorwin ADC Series; triple pane, orientation-specific 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.; combination of fixed and casement windows (R-5)
Roof / ceiling: Ventilated attic space, 2x (sawn lumber) joist and rafter construction
Roof sheathing: Exterior-rated 3/4” plywood
Ceiling insulation: Fiberglass batts, loose fill cellulose (R-70)
Ceiling air barrier: Plywood sealed with StoGuard
Roofing: Follansbee TCSII (terne coated stainless steel), Senger’s Roofing
PHPP Specific Space Heat Demand: 4.71 kBTU1,000 Btus (ft2/yr)
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: 0.39 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 (average of pressurization and depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. multi-point tests)
Estimated Site Energy Use: 8,320 kWh/yr
Actual Energy Use: July 2012: 674 kWh; August 2012: 661 kWh
Heating/Cooling: (2) Mitsibishi MSZFE09NA / MUZFE09NA Mr. Slim ductless minisplit heat pumps, 9,000 BTU capacity, 10 HSPF, 26 SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive.
Ventilation: UltimateAir DX200 Energy Recovery Ventilator
Well pump: Gould Constant Pressure system (variable frequency drive)
Domestic hot water: AirGenerate AirTap ATI66 heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed.
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), 24” compact clothes washer and condensing dryer, induction range
Lighting: Philips AmbientLED (Edison base LED), tube fluorescent
• Low-flow plumbing fixtures
• Toto Vespin II 1.28 gpfGallons per flush. Measurement of water use in toilets. Since 1992, toilets sold in the United States have been restricted to 1.6 gpf or less. The standard for high-efficiency toilets (HETs) is 1.28 gpf. toilets
• Manablok water distribution manifold and PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. piping
Indoor Air Quality
• Low- and no-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 wood and 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
Green Materials and Resource Efficiency
Regionally sourced Southern Yellow Pine (SYP) used for framing, 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.
, furring strips, decking, and ceiling boards. Materials were certified through various agencies:
• Maple flooring sourced from Appalachian region, certified by AHMI
• Follansbee TSCII (terne coated stainless steel) roofing is designed for 100+ year lifespan, eliminating painting, replacement, recycling, and waste associated with reroofing
Modeled after the vernacular architecture of a Shenandoah Valley farmhouse, this multi-generational house was designed and built to meet the Passivhaus standard. I call it the Passive Bauernhaus.
By Daniel Ernst
When my family made the decision to move to Virginia, we agreed that somehow, some way, we would find a home that could comfortably accommodate seven people, across three generations. On a working farm. On a budget. Winding our way through an economic downturn, it made sense to consolidate our resources, minimize our family footprint, and maximize our efficiency.
Although we looked for months for a possible house renovation project, the dollars and cents of the market pushed us toward purchasing a site for new construction.
Evaluating site potential, deciding on appropriate architecture
The land that we purchased included a grove of open hardwoods, mature trees that added to the diversity and beauty of the property. We considered the land carefully before settling on a building site in a natural opening of the grove. Although we would have to cut down some of the oak trees for the site, the building would benefit from an ideal mix of sun and shade. Trees to the east and west would provide generous shade for Virginia’s hot and humid summers, while an open southern face would allow full passive solar potential in the colder months.
Having selected the site, we then discussed house designs. The Shenandoah Valley has an odd mixture of antebellum farmhouses, brick ranches from the ‘50s and ‘60s, and vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate).-clad subdivisions. I had an eye for the early architecture, built at a time when resource efficiency was taken for granted. Most of the older farmhouses started with a simple two-story rectangle and gable roof. Layered onto this basic geometry, succeeding generations often added one or two-story wings, creating the classic farmhouse style.
For a multi-generational house, this architecture offered my family the chance to balance our public and private needs. The main first floor would provide the meeting place, the second floor would give my family bed and bath, and the first floor wing would furnish the in-law quarters with some sense of privacy.
Starting with some principles, and some numbers
I met with an energy consultant, John Semmelhack of Think Little, before starting any detailed drawings. At that point, I only had a sense of the architecture and a rough idea of house size. John explained the various programs he used for energy modeling and a number of certifications available (EnergyStar for Homes, EarthCraft, and 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.). We discussed passive solar principles and building materials, local building techniques and new technologies. That discussion helped refine a list of ideas that we would use during the designing and building phases.
As I started drawing, I focused my efforts on designing an all-electric, net-zero-ready house, with the potential to certify the house to the Passivhaus standard. John easily persuaded me that the PHPP spreadsheet would provide the most accurate energy model. And although I was aware of the criticisms of the Passivhaus standard and PHIUS (for example, no feedback loop on cost-effectiveness), it was hard to argue with the level of detailed inputs and calculations found in the PHPP.
Compromise as a matter of course
In building, every choice demands a compromise. Few decisions are simple or straightforward. The designer and builder must balance a number of disparate perspectives in an ever-changing world. Durability vs. cost — aesthetic flourishes vs. simplicity — long-term operating costs vs. short term budget. Decisions may be driven by hard numbers, or emotional prejudice. There is no single answer. That is the joy and the burden of the architect and the builder.
During the design phase, I grappled with a number of these decisions. In the end, all of that wrestling produced a design that is architecturally similar to 19th-century farmhouses, but only at the surface level. Looking closer, there are some dramatic departures from the older houses:
- Sealed, insulated, and conditioned crawl space
- Double stud walls, dense packed with cellulose
- Airtight 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. , with liquid-applied WRB
- Triple-pane windows > R5
- Heat pumps for air conditioning and domestic hot water
- 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 (ERV(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.)
Novel crawl space construction
To use a witticism of John Brooks, I am not a Basementist. On the other hand, I am leery of using foam insulation surrounding a slab, at least in regions with moderate or heavy termite infestations. The thermal break between a footing and a slab is typically hidden beneath an exterior wall bottom plate — so it can never be inspected. Our oak grove had (has!) a remarkable abundance of termites, so I chose the oft-disputed middle ground — a crawl space.
The PHPP revealed a delicate balance for insulation levels on the crawl space floor, one that is unique to mixed climates. Too little, the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. increased significantly; too much, the cooling load crept higher. I chose to use 4” of rigid mineral wool insulation below a 2” thick rat slab. The mineral wool would not provide a nesting site for termites and ants. The rat slab would protect the insulation and vapor barrier, create a clean surface, and add thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. . Although I was not able to find an example of this technique, the technical department at ThermaFiber gave approval for using their 8-pound-per-cubic-foot density product in this application (and eliminated their minimum quantity surcharge to fill my order).
Achieving a high level of wall insulation
Based on his modeling experience with PHPP, John Semmelhack thought that R-30 would be a minimum starting point for wall insulation levels (in this climate). Given a less-than-optimal building geometry, that value climbed closer to the R-40 mark. By using a double stud wall, I could achieve this value in a cost-effective manner, simplify the flashing and trim around penetrations, and avoid the complications of exterior insulation.
Although the double stud wall has been used for many years, and is still used by many builders of high-performance homes (see homes built by John Abrams of South Mountain Company, or Andy Shapiro in White Pine Co-Housing), building scientists continue to caution about the risks of this approach. To alleviate this concern, John evaluated the wall assembly using both Therm and WUFI software models, comparing it against the historical housing stock for this area.
Liquid-applied 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. and airtight sheathing
I used the airtight sheathing approach for the building’s air barrier. Since I planned to do most of the construction myself, I used Sto Corporation’s liquid applied air barrier system — StoGuard and Gold Coat. This product was costly (approximately twice the cost of 3M’s 8607 tape), but had several important benefits: it was permeable, durable, and rated for six months exposure.
To complete the barrier at the ceiling/attic level, I wrapped plywood sheathing over the ceiling joists, and applied the StoGuard joint treatment to these joints also (note that Sto does not approve using this product on horizontal surfaces). Having an attic floor simplified the roof framing process, creating a safe and secure platform for setting the ridge board and rafters.
John completed the first 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. before insulation and drywall (single-point depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home.). The results were impressive: 117 cfm50, or 0.33 ach50. Six months later, the final blower-door test essentially showed no change in the depressurization result.
Capping it off
I admit here that I like durable roofing materials. Initially, we had planned to use slate for the main roof, metal for the wing and lower sloped porch roofs. We had hoped to use Buckingham slate (the quarry is across the Blue Ridge mountains, so it was a local source); however, the cost was prohibitive — substantially more than Vermont slate, with which I had some experience. After weighing the options, we settled on standing-seam metal for the entire roof. Because the metal stock was Follansbee TCSII — formed from 304 stainless steel — it would last as long as a slate roof.
A local roofer, Bruce Senger, formed the pans from 1,000 ft. coils, then installed them using techniques that have existed for hundreds of years.
Heating and cooling strategies
Considering our mixed climate and design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year). (16°F), a heat pump made sense. Looking at the small heating and cooling loads, this was one decision that didn’t require a lot of debate (finally!). I chose the Mitsibishi Mr. Slim as the source; the only question that remained was the model and head location(s).
Having lived with a wood stove for much of my life, I was comfortable with point-source heating. However, I didn’t have any experience with point-source cooling. The PHPP showed that a single 9,000 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. /h head could handle the cooling load for the entire house (and nearly handle the peak heating load). But would distribution suffer?
To provide autonomy for the in-law quarters, I installed a single head minisplit to serve only the wing. For the main house, I installed a single head unit, high in the open staircase, to serve both the lower public area and upper story bedrooms. Although this decision introduced the risk of temperature stratification, I was aware of several other houses that had used this technique. During the hottest days of the summer (100°F), we measured a maximum 4°F temperature differential between the first and second floors — and the minisplit never operated above low speed. I will note here that temperature variation and comfort are extremely subjective, so this technique might not be satisfactory for some homeowners.
—Daniel Ernst is currently starting a design-buildCompany that handles house design and construction. Since both services are provided by the same firm, integrated design can often be more easily achieved. firm in Steele's Tavern, Virginia. You can reach him at prometheanhomes(at)gmail.com
We speak a different language
The fact remains that many in the residential construction industry (suppliers included), are not familiar with high-performance homes or techniques. Surround yourself with those that are knowledgeable.
Find your suppliers months in advance
Although I knew that certain supplies and vendors would be difficult to find, this proved one of the more frustrating aspects of construction. Just because you know about a product doesn’t necessarily mean you can source that product. Thinking about using foamglas or mineral wool insulation? Plan ahead.
Sealed crawl spaces need fast dry-in times
Due to my extended construction schedule, and a very rainy September 2011, mold developed on some of the crawl space framing. Resolved? Yes! Fun? No!
Orientation-specific 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. can be problematic
There are a lot of chances to make a mistake during manufacturing and installation. In my case, the glazing manufacturer's labels (from PPG) did not match the window manufacturer's labels (from Accurate Dorwin).
Preparing open-web joists for dense-pack cellulose is tedious
Vertical blocking in-line with the interior stud wall would have saved a lot of time.
Little things can hold you up
Most floor registers are unsuitable for balancing an ERV(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./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. : they have multi-angled fins and foot operated dials that cannot be locked into place. Lockable registers are special order, have long lead times, and are expensive.
- Daniel Ernst
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