With double-stud walls, thick attic insulation, and triple-glazed windows, this home can be heated with just two ductless minisplit units

Before we met, my clients had spent considerable time researching how to build a net-zero home. They read product literature, studied the economics, understood the benefits, and had a pretty intelligent understanding of construction methods and materials. This body of knowledge became the starting point for our discussions and direction.

We took a “whole house” systems approach, refining the design to include building envelope details, floor plans, and elevations that would work in concert with the mechanical systems, site location, and renewable energy resources. The clients wanted a home that took advantage of natural light, was comfortable in all the New England seasons, and was free of toxic materials and mold.

Low-VOC materials and an ERV preserve indoor air quality

We planned between 2,100 and 2,400 square feet of living space — about the same as their previous home — as well as 1,642 square feet of unconditioned space (attic, garage, basement, and second-floor storage/bonus room).

We planned a double-stud wall assembly and engineered trusses for the roof framing. Other priority features included: high-performance triple-glazed tilt/turn windows, a quality ERV, non-toxic finishes, and low-maintenance exterior siding and trim. The prominent location — on a hill in an open field close to the road — influenced a style preference: local farmhouse vernacular.

A site with good solar exposure

My clients purchased a five-acre former cornfield in the town of Shelburne, Mass. The site affords wide views over the Pioneer Valley and excellent solar exposure; the photovoltaic system installers later confirmed that the site has a 98% solar window. The land slopes in various directions but the ideal house site is relatively flat, albeit constrained slightly by an eastern buffer zone from a nearby pond and property line corner to the west. Given the site conditions — lots of ledge — we opted for a small 10’x18’ basement and an insulated slab on grade with 4-foot frostwalls.

To collect roof rainwater we installed a “ground gutter” — a perforated drain pipe set in a 12-inch deep, poly-lined, stone-filled trench. The drain pipe runs along the perimeter of the house to a 50-gallon plastic storage barrel set up with an overflow pipe to daylight. The future plan is to install a float activated pump that will lift rain water to a larger tank on the hill to the west. Water from this tank can gravity-feed the gardens below.

The rain barrel is enclosed in a well tile with a lid for protection and access. (The volume of roof runoff is estimated at 450 gallons per inch of rain; if we capture 50% of this, the 42-inch annual rainfall for our area could yield approximately 9,753 gallons.)

A frost-wall-and-slab foundation

To create a thermal break at the edge of the floor slab, a band of XPS foam 4 inches wide and 8 inches high was cast in place during the frostwall pour. After the forms were stripped, a second band of 2-inch XPS was installed vertically down the inside of the frostwall.

After the initial interior backfill, a geothermal loop consisting of coiled polyethylene tubing was buried below grade. A glycol solution circulates through the tubing, which connects to a heat exchanger in the fresh air duct of the Zehnder ERV unit. The circulating glycol raises the temperature of the ventilation air during the winter, and lowers the temperature of the ventilation air during the summer.

The passive radon venting system includes a 4-inch perforated pipe, installed over compacted fill and covered with 1-inch stone. The pipe is connected to a roof vent. Over the stone we installed two layers of 2-inch-thick rigid XPS (for a total of R-20) covered with Tu-Tuff polyethylene.

The sill plate is a treated 2×12 except for exterior door locations where the foam is exposed. This was not a problem where we installed tile flooring; however in the main house with floating wood flooring this was something of a weak spot (see Image #17). Next time, I would extend the slab to the edge of the frost wall with only a 1-inch thermal break and provide a solid surface for the flooring.

Three bedrooms, a music room, and a yoga room

The first floor includes an open living and kitchen area, a music room, and an ADA accessible bathroom. The music room could double as a bedroom giving the owners the option to live comfortably on the first floor.

The two south-facing rooms are separated by a central stairway. The first-floor rooms connect to the garage, entry door, and west porch. (See the floor plan at Image #20.)

The second floor includes a master bedroom and bath, a study, a yoga room (which doubles as a spare room), and a guest room connected to another bathroom. The storage room over the garage is unheated and could be finished at a later date as additional living space.

The ceiling of the second floor is insulated, separating the conditioned living space from the unheated attic above. The attic stair opening is covered with an insulated hatch (see Image #9) operated by a worm-drive winch. The winch opens or closes the hatch with a handle or (faster and more conveniently) with a cordless drill.

Where should the mechanical room be located?

Originally I considered including a 10’x10’ mechanical room in the attic, but we discovered through energy modeling that this would put the ratio of exterior walls to floor area over the desirable balance, so the mechanical room was taken out of the equation.

With the insulated room above the stairwell gone, the options were to insulate the second floor walls around the stairs and slope of the stringers or build a hatch above. The hatch seemed the most effective option from an energy standpoint and was subsequently confirmed during our blower door tests.

Exterior walls were framed with 2×6 studs 16 inches on center, sheathed with Zip sheathing. The interior side of the wall sheathing and the band joist were insulated with 2 inches of closed-cell spray foam, providing R-value as well as air sealing. The closed-cell foam was installed before any of the electrical, plumbing, or HVAC systems were roughed in, making it possible to heat the house during the coldest months of the winter to a comfortable 55 degrees with temporary electric heat — a testament to the effectiveness of just 2 inches of spray foam in the walls.

Connecting the wall air barrier with the ceiling air barrier

Before the attic roof trusses were installed, a 16-inch-wide strip of ¾-inch plywood was nailed and caulked to the top plate. Ceiling strapping was installed to the bottom chords of the roof trusses, and an interior polyethylene vapor barrier was stapled to the ceiling strapping. The polyethylene vapor barrier also acts as an air barrier; it was sealed to the plywood strip where it extends into the room. This technique allowed us to connect the horizontal plane of the ceiling air barrier to the vertical wall sheathing at the top plate (see detail at Image #18.)

The drywall was hung for the entire second-floor ceiling before framing any of the interior partition walls. Wiring penetrations through the top plates and electrical boxes were air sealed prior to installation of blown-in cellulose. Although it required us to install the drywall in two phases, it was much simpler to install one large flat ceiling with full sheets rather than fitting pieces around closets etc. For the flared window wells we added beveled ripped-down strips of 2x lumber to the edge of the openings for drywall nailers, creating sides with 30 degree angles, along with plywood for additional support across the sill.

Raised-heel trusses

The roof was framed with a combination of attic trusses and mono-pitch trusses with 28-inch and 23-inch heel heights. The attic trusses have an 18-inch bottom chord. The wide gable-end overhangs, which continue the line of the eaves, are also mono-pitch trusses.

To match the roof rakes with the eave overhang, we used dropped top chord gable-end trusses with 2×6 fly rafters. Cantilevered over the top of the dropped chord and connected to the inboard truss with joist hangers, the fly rafters make for a strong and relatively simple framing detail for wide overhangs.

The hangers were connected to the inboard truss on the ground, speeding up the installation of the fly rafters from the staging. I calculated the 21-inch width of the overhang using the angle of the sun at our latitude to provide shade in the summer and solar gain in the winter (see Image #19).

Tilt/turn windows with triple glazing

As the floor plans developed, helped by the energy modeling, we sized and located the fixed and operable windows. We found the ideal ratio of glazing to wall area while striking a balance between advantageous views, natural light, and solar gain.

The Wasco Geneo triple-glazed tilt/turn windows have sturdy, generous handles and are easier to operate than casement cranks. Using the “turn” option allows the sash to opens entirely into the room, which is ideal for cleaning. In “tilt” mode the top of the sash opens 5 inches from the top of the frame. This wedge-shaped opening improves fresh air convection into the rooms but protects it in the event of rain. The quality of the latching hardware provides a very tight seal when closed.

The entry doors are made of fiberglass and have a triple-point locking mechanism, providing a better air seal than a single latch. Three of the four exterior doors open onto enclosed spaces — the garage, mud room, and porch — for additional protection in winter.

Two types of siding

Wide corner boards, a wide water table, tall frieze boards, wide overhangs and rake trim were all details used to recreate the “folk Victorian” style that distinguishes the exterior.

The north wing has board-and-batten siding, finished red, to replicate the look of a traditional barn; these details create a deliberate contract with the main house. We accomplished this with a combination of stock fiber-cement trim materials, 2-by kiln-dried lumber wrapped with coil stock, and PVC window casing.

The main house has fiber-cement lap siding and flat fiber-cement panels on the gable ends. We used a similar 4’x8’ panel installed vertically with 1×2 trim strips to create the board-and-batten look for the barn-like north wing.

One concession to natural wood: the west porch has a 6×6 post supporting a roof beam and entry door trim made from naturally rot-resistant black locust (see Image #11).

Paying attention to VOCs

Walls and ceilings are finished with ½-inch drywall painted with zero-VOC paint. The floors of the second-floor rooms are finished with floating cork tile, while the bathroom floors, tub surrounds, and shower surrounds are finished with ceramic tile.

The first-floor living and kitchen areas have engineered yellow birch flooring. Since we installed the birch flooring over a concrete slab, and since we were trying to avoid high-VOC glues, neither nailing nor gluing were options. We opted for a self-adhesive 1/8-inch-thick underlayment called Elastilon, which also adds a cushion over the slab.

After installing a couple of courses of the birch flooring panels, the protective film is removed, allowing the flooring to adhere to the upper surface of the underlayment. This proved to be an excellent option as it is nontoxic and odorless.

An ERV and ductless minisplits

A Zehnder energy-recovery ventilator (ERV) introduces a continuous stream of fresh air to ventilate the house. By allowing moisture to be transferred between the incoming and outgoing air streams, the ERV helps control indoor humidity levels. According to Zehnder, the high-quality ERV filters drastically reduces some airborne pollutants.

Since the ERV pulls stale air from the bathrooms and kitchen, there was no need to install separate bath exhaust fans or a kitchen range hood. Zehnder claims that the unit’s heat-recovery efficiency is 95%. We also installed a geothermal loop to temper the outdoor air before it is distributed. The glycol-filled loop is buried about 16 inches below the double layer of 2-inch XPS foam installed below the slab.

Our HVAC contractor originally proposed installing an 18.2 SEER, 24,000 Btu/h air-source heat pump (for space heating and cooling) with a forced-air distribution system. We concluded that this system, efficient as might be, was oversized for the home’s heating and cooling load and cost more than right-sized equipment.

We decided that two Fujitsu air-source heat pumps were a better fit. A small shed roof was built over the compressors to provide protection from excessive snow which adversely affects the units’ performance. One minisplit head was installed on each floor, centrally located and mounted 30 inches above the floor. (Peter Talmage, one of our energy consultants, recommended this approach, which is closer to the floor than usual, for several reasons. Mounted near the floor, the unit pulls in cooler intake air than it would if it were mounted near the ceiling. The warmed air is blown out across the floor, stirring the cold air at floor level. The warmed air isn’t blowing directly on occupants, reducing comfort complaints from those who are sensitive to moving air. Finally, locating the units near the floor makes it easy to access the air filter for cleaning.)

Ceiling-mounted electric-resistance radiant panels (manufactured by Enerjoy) were installed in the guest and downstairs baths to augment the heat pumps in the coldest months of winter.

Over the course of designing and building this house, I came to appreciate some of the science and economics underpinning zero-energy construction methods. In particular, I concluded that the extra investment in a better building envelope should be offset by savings in HVAC equipment. (For this house, the minisplit units and electric-resistance heaters that we installed cost $14,610 less than the system first proposed by our HVAC contractor.)

Blower-door tests and thermal imaging

We hired Mike Duclos from DEAP Energy Group to evaluate our design and to provide a HERS rating. Mike also helped us participate in the Energy Star Tier 3 program, which provided us with a $7,000 cash rebate.

Mike’s initial blower-door test and thermal imaging survey helped us identify most of the air leaks and potential weak spots in the envelope before the drywall was installed. In addition to his testing and inspections, Mike was an invaluable source of technical advice. He could not stress enough the need for effective air sealing.

As a builder who traditionally focused on the R-value of walls and roofs, I have long overlooked the significance of air sealing. (There’s nothing like seeing blower-door data to drive this point home.) Before the job started, I bought a foam applicator gun and used it every time I found a cool spot, a drilled hole, or a leaking seam that needed sealing. The $32 turned out to be an excellent investment.