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 vinyl-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 House). 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 sheathing, with liquid-applied WRB
• Triple-pane windows > R5
• Heat pumps for air conditioning and domestic hot water
• Balanced ventilation system (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 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 mass. 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 barrier 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 test before insulation and drywall (single-point depressurization). 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 temperature (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 Btu/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-build firm in Steele’s Tavern, Virginia. You can reach him at prometheanhomes(at)gmail.com_