For now, you’ll have to use your imagination to envision a new school on the wooded site a few miles north of Portland, Maine. There are only concrete stem walls outlining the shape of the building, and earth-moving equipment up in back shaping what will eventually become recreation fields.

But by next June, visitors should be able to see the new Friends School of Portland. The 15,000-square-foot building will be one of only a few Passivhaus school buildings in the country, and the largest Passivhaus structure in Maine. Architects also plan on making it a net-zero energy building.

The building will be the new home for the 90 students, from pre-kindergarten through the eighth grade, who now attend the Quaker day school in rented space in a 1950s cinderblock building a few miles away.

The Friends of Portland bought the 21-acre site two years ago in the town of Cumberland, and officially broke ground for the $5.5 million project this August. In a few years, they hope to launch a new capital campaign and construct an adjacent gymnasium.

At first, Passivhaus certification was not on the table

The building committee knew from the outset the school would have a superinsulated building envelope, but plans didn’t originally include Passivhaus certification because everyone assumed it would cost too much. Several things helped change that. First, the chairwoman of the building committee, Naomi Beal, is also director of PassivhausMAINE, the state affiliate of the North American Passive House Network.

Then there was there was an early decision to change architects. Gone was an architect who specialized in schools but not necessarily high-performance buildings. In came the new firm, Kaplan Thompson Architects of Portland, well-versed in Passivhaus and high-performance designs.

There were nearby Passivhaus buildings to tour, and there was the unavoidable influence of Beal’s year-long sabbatical in Germany where Passivhuas structures are common. Finally, the committee realized it would be possible to reach the Passivhaus goal without breaking the bank.

The building reflects Quaker beliefs

“We knew we wanted to be as responsible as we could to the earth,” said head of school Jenny Rowe. “As Friends, as Quakers, we really try to walk the talk. If one of the things we talk about with our students is taking care of the earth and being responsible stewards of it, then it makes sense to try to think of the most energy-efficient building we can.”

Getting the building certified as a Passivhaus building also will lend a certain cachet to the project that LEED certification or the mere “high performance” label just didn’t offer. “You can make anything LEED-certified,” Rowe said. “It doesn’t necessarily make mean it’s the most energy efficient way to do it.”

And low energy consumption is a very big motivator for the school. “For us to know that in every single year ahead of us we will have no energy costs, that’s pretty amazing for a small school where three kids difference in enrollment can make or break a budget,” Rowe said. “Having a predictable budget is very helpful.”

Seeking certification also was important to the builder, the Warren Construction Group of nearby South Freeport. Although very familiar with high-performance buildings, the company had never tackled a Passivhaus project before, Beal said. Nor has Porter Building Systems, the Gorham, Maine, based company that’s supplying the panelized walls for the building.

Kaplan Thompson has worked on a number of Passivhaus projects, but not project architect Richard Lo. That may seem like a lot of rookies, but Lo sees one advantage. “They have worked on low-energy projects and they are familiar with building well air-sealed projects,” he said of the key players. “They also see this as an interesting and exciting challenge, exactly because they haven’t been down the full road to Passivhaus certification. That’s actually been part of the interesting background to the project team — they’re very enthusiastic about it.”

In fact, all the players on the project, including the architects, chipped in some money to pay the $6,000 or $7,000 fee required for pre-certification.

Ventilation was a major hurdle

With so many people packed into the building, ventilation was key. When it came to choosing the equipment, Lo said, there were limited options available, and in the end the architects chose a RenewAire energy-recovery ventilator that was both relatively inexpensive and relatively simple. The only catch: It wasn’t certified by the Passivhaus Institut and runs at an efficiency of between 60 percent and 77 percent. That, Lo says, is well below the 90 percent efficiency that certified equipment would typically have.

By beefing up sub-slab insulation, choosing windows with a high solar heat gain coefficient (SHGC) and bulking up on roof insulation, Lo said, it looks like the building will “just sneak in” for certification.

Some of the other features in the building:

• Walls: Walls will be framed with 2x6s, sheathed with Zip System OSB, insulated with dense-packed cellulose between the studs and an additional 4 inches of polyisocyanurate rigid insulation on the exterior (for a total R-value of about 47).
• Roofs: There are two types of roof, depending on their location on the building. One will be a ventilated, truss-framed structure with 26 inches of loose cellulose (R-91); the other will be framed with I-joists and insulated with dense-packed cellulose and 4 inches of rigid polyiso on the exterior (for an R-value of about 79).
• Foundation: Slab-on-grade construction with 8-inch stem walls insulated with 5 inches of expanded polystyrene insulation on the outside and 2 inches of EPS on the inside (R-28). Beneath most of the slab will be 12 inches of EPS (R-48), although that drops to 4 inches where concrete is thickened for load-bearing walls.
• Heating and cooling: Space heating and cooling are provided by Daikin minisplit air-source heat pumps. Some are wall-mounted ductless units, and others are ducted, depending on where they are, with heat output per head ranging from 5,804 Btu/hour to 34,000 Btu/hour.
• Windows: Intus Eforte triple-glazed uPVC units with a SHGC of 0.62 and a (glass only) U-factor of 0.106.
• Doors: Schuco ADS aluminum units with the same efficiency ratings as the windows.
• Renewable energy: Roof-mounted photovoltaic panels with a rated capacity of 36 kW.
• Hot water: On-demand electric resistance.
• Air-tightness: The goal is 0.5 air changes per hour at a pressure difference of 50 pascals.