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Green Homes

A Net-Zero Home in Massachusetts

Most of the area of the south-facing roof of this new home in Shelburne, Massachusetts, is covered by a large photovoltaic array. The two panels on the right are solar thermal collectors that produce hot water.
View Gallery 20 images
The frostwall was insulated on the interior with extruded polystyrene (XPS) foam.
Image Credit: Charles Bado
A glycol solution will circulate through these buried coils of plastic tubing. With the help of a pump, the fluid will circulate to a heat-exchange coil located in the supply-air duct system connected to the Zehnder energy recovery ventilator (ERV). The temperature of the glycol solution is expected to be warmer than the outdoor air during the winter and cooler than the outdoor air during the summer.
Image Credit: Charles Bado
This loop of perforated pipe is part of the passive radon mitigation system. The horizontal loop will be buried in a layer of crushed stone. The vertical riser will be connected to a vent pipe that exits through the roof.
Image Credit: Charles Bado
A crane was used to set the roof trusses. Note that the seams of the Zip System sheathing installed on the walls have been sealed with Zip System tape.
Image Credit: Charles Bado
The double-stud walls created deep interior window stools. The window rough openings were splayed so that the natural light isn't restricted.
Image Credit: Charles Bado
The Zehnder ERV (on the left) supplies the house with fresh ventilation air while exhausting stale air from the bathrooms. The box on the right houses the heat exchange coil filled with a glycol solution that is circulated to buried loops of plastic tubing.
Image Credit: Charles Bado
Zehnder sells plastic ventilation ductwork designed to work with the company's ERVs and ceiling grilles.
Image Credit: Charles Bado
This insulated hinged attic hatch seals the top of the stairway leading to the attic.
Image Credit: Charles Bado
This view of the west side of the house shows the entry porch and the small roof protecting the two heat-pump condensers.
Image Credit: Charles Bado
The post supporting the entry roof is a black locust 6x6.
Image Credit: Charles Bado
Exterior view from the southwest.
Image Credit: Charles Bado
The first-foor sunroom faces east.
Image Credit: Charles Bado
The European-style tilt/turn windows open inwards. The windows can be opened one of two ways: using the bottom-mounted hinges or the side-mounted hinges. Note the splayed window jambs and the wide stools.
Image Credit: Charles Bado
A view of the kitchen, looking north.
Image Credit: Charles Bado
This eating area is tucked into the northeast corner of the kitchen.
Image Credit: Charles Bado
This detail shows the configuration of the vertical XPS insulation on the interior side of the concrete frost wall and the horizontal XPS insulation under the slab.
Image Credit: Charles Bado
A strip of plywood was attached above the top plates of the double-stud walls. The plywood is sealed to the top plates with caulk and to the Tu-Tuff polethylene air barrier at the ceiling. This ensures continuity between the ceiling air barrier and the wall air barrier.
Image Credit: Charles Bado
This drawing shows how the roof overhang shades the window in June, but allows the sun to enter the house in January.
Image Credit: Charles Bado
First floor plan: the living room and music room are on the sunny side of the house, while the attached garage is on the north side.
Image Credit: Charles Bado
Second floor plan: part of the guest bedroom is above the garage.
Image Credit: Charles Bado

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.

Lessons Learned

• Open-web floor joists would have allowed for easier installation of ductwork than the I-joists used on this project.

• Ensure adequate spacing (a minimum of 10 inches) between the inside of the gable end walls and the inside face of first joist to allow better access to the band joist.

• It’s best to spray foam the exterior stud bays before building the interior 2x4 stud walls.

• The house ended up with thermal bridging at the perimeter of the slab, especially under exterior door thresholds. A better detail would have included an extension of the slab at door openings and a better thermal break using rigid foam.

• We should have insulated the walls of the basement mechanical room rather than the ceiling.

• We should have performed energy modeling early on during the design phase of the project. Energy modeling wasn’t performed until one month after we had already broken ground; at that point, the plans were mostly fixed. If energy modeling had been performed three months earlier, we would have foreseen the problems associated with locating the mechanical room in the attic.

General Specs and Team

Location: Shelburne, MA
Bedrooms: 3
Bathrooms: 3
Living Space: 2400
Cost: 234
Additional Notes: Cost does not include land.

Designer: Omnibus Designs (Charles Bado) Builder: Omnibus Designs Energy consultants: Mike Duclos (DEAP Energy Group) and Peter Talmage, P.E. Insulation contractor: Bryan Hobbs

Construction

Foundation: Frost wall and slab on grade

Slab insulation: 4 in. horizontal XPS (R-20) under slab and 4 in. vertical XPS at slab perimeter

Frostwall insulation: 2 in. XPS (R-10) on interior of frostwall

Wall construction: Double-stud walls with load-bearing 2x6s, 16 in. o.c., on the exterior and 2x4s, 16 in. o.c., on the interior

Above-grade wall insulation: 2" closed-cell foam spray foam (R-12) on interior side of wall sheathing plus 10" of dense-packed cellulose (R-34) for a total of R-46.

Wall sheathing and air barrier: ½-in. Zip sheathing with all seams taped

Siding: CertainTeed Weatherboard fiber-cement 5 1/2 in. plank siding

Exterior trim: A combination of fiber-cement trim boards, aluminum coilstock, and PVC.

Windows: Wasco Geneo triple-glazed fixed and tilt/turn units, U-0.16, SHGC 0.39 on east & south, SHGC 0.17 on west.

Roof framing: Roof trusses (creating a vented unconditioned attic)

Roof sheathing: 5/8 in. Zip sheathing with all seams taped

Ceiling insulation: 17 in. blown-in cellulose (R-56)

Ceiling air barrier: Tu-tuff polyethylene vapor barrier and continuous ½ in. drywall

Roofing: Englert 24 ga. standing-seam steel.

Energy

Space heat and cooling: 2 Fujitsu 12RLS2 ductless minisplit heat pumps; heating capacity 16,000 Btu/h; cooling capacity 12,000 Btu/h; supplemented by several 400-wall electric resistance heating panels.

Mechanical ventilation: Zehnder ComfoAir 350 ERV with ComfoFond-L 350 geothermal heat-exchange loop buried under the slab

Domestic hot water: 2 Stiebel Eltron SOL 27 solar thermal collectors connected to a Superstor SSU-80SE 80-gal. storage tank; 4,500-watt electric resistance backup heater.

Energy Specs

Blower-door test: 1.96 ach50

Annual energy use: 6.6 MMBTU (modeled, not measured)

Water Efficiency

Rainwater collection system

Second-floor bathroom has a hot water recirculating pump on a timer to reduce the amount of water wasted waiting for hot water to reach the faucets

Green Materials and Resource Efficiency

Locally milled black locust lumber was supplied by Blue Sky Farms, Colrain, Mass.

All of the contractors and vendors used for the construction of the home live and have businesses in Franklin County.

Alternate Energy Utilization

PV system: 30 roof-mounted Bosch C-S- M60 PV modules totaling 7.65 kW; estimated annual output 8,896 kwh; system includes battery backup.

13 Comments

  1. User avater
    Dana Dorsett | | #1

    What is purpose of the glycol loop slinky heat exchanger?
    Is it ONLY to temper the ventilation air? If yes, did any body do the math/modeling on the pumping power to see if it so much as breaks-even on an energy-use basis?

    Heating and cooling with a pair of RLS2s I would expect the pumping power per BTU-saved on the ventilation tempering to have a worse COP than the seasonal averages for the mini-splits.

  2. Dwight Harris | | #2

    ACH
    Great write-up. Thanks for sharing!

    I'm surprised that with the combination of zip system, closed cell foam and dense pack cellulose you didn't get a lower ACH value. What do you suspect were the weak spots?

    Like Dana, I'd also like to see the math of the geo loop dedicated to the ERV.

  3. Barry Stephens | | #3

    COP of ground loop
    A reply to Dana's question on the efficiency of using the ground loop pre-heater. We have seen the COP on these systems during winter months of upwards of 20. We are gathering data, and will be able to publish results for projects in VT, MA, CO and AK shortly. We also used this system in a HVI -25C testing of the CA 550 unit with the ComfoFond-L ground-source unit. We achieved a 99% Apparent Sensible Recovery rate, and a 91% Sensible Recovery Efficiency. That with a very efficient pump that is normally using 8-10 watts of power while running. We also get cooling and dehumidification in the summer, so overall, this system is quite impressive. Of course, I sell them, so take that as you will, until the full set of third-party data is out.

  4. User avater
    Dana Dorsett | | #4

    I look forward to the published results!
    An analysis of the lifecycle energy cost of the system compared to ductless heat pump lifecycle would be useful too, but for now a credible set of data for the average annulized COP would be interesting enough on it's own.

  5. Peter L | | #5

    $234 / square foot?
    That's $561,600 for the home and that did not include the cost of the land. That seems awfully expensive for this type of home. The 1.96 ACH50 blower door results were higher than what I would expect on this type of build. Passive House is < 0.60 and I thought for sure this home would be in the < 1.00 range. The 0.39 SHGC on the south windows seems low. I figured it would calculate out to have 0.45 or higher on the SHGC on the south windows.

    Just being blunt here but these types of builds will not do much to further motivate the green energy building movement. People are not going to want to buy a home that is $234 / ft² and ends up looking like this.

  6. Michael M | | #6

    and ends up looking like this
    +1. At any price per sq ft, these homes *must* be more handsome to succeed in anything more than niche. The design is way behind the function. This will not happen without professional help.

  7. User avater
    Stephen Sheehy | | #7

    rain water collection
    Why spend money for rainwater collection when living with 42 inches of annual rainfall?

  8. T. Barker | | #8

    I love the idea of the splayed window jams! But probably significantly more cost to build and finish I would think.

    1. Malcolm Taylor | | #9

      They used drywall returns. The additional cost of nailing up two cant strips on the jambs shouldn't be much.

      1. T. Barker | | #10

        Mmmm... and a little more thought and work for the framing underneath. And the increased complexity for the drywallers, including an extra corner, and the drywall detail at the "upside down top sill" at the top of the window. And the angled wooden window sill itself. Times 30 windows.

        Maybe not a huge amount, but I suspect the bids went up another notch.

        1. Malcolm Taylor | | #11

          If you really want to get elaborate, here's what Patkau Architects did in a house near me:
          https://patkau.ca/projects/pyrch-house/

          1. T. Barker | | #12

            Awesome! Architects sure like to spend other people's money though.

  9. Malcolm Taylor | | #13

    I've been in it. A really beautiful series of spaces and views. Almost impossible to imagine living in it though.

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