These Superinsulated Homes Were Delivered By Truck
These Superinsulated Homes Were Delivered By Truck
Dartmouth College hires Unity Homes to assemble four single-family houses with panelized walls and prefab bathroom modules
Last fall, Dartmouth College realized that it needed to build four new single-family homes, pronto. Beginning this summer, the homes will be occupied by the “house professors” assigned to new “house communities” — the term that Dartmouth uses to describe the college’s dormitory clusters.
To build the four needed houses, Dartmouth hired Unity Homes, a company in Walpole, New Hampshire, that assembles homes from panelized walls and modular components. Founded by Tedd Benson, Unity Homes specializes in superinsulated buildings with very low rates of air leakage. (For more information on Unity Homes, see the links in the “Related Articles” box below.)
My son Noah, who’s a junior at Dartmouth, noticed the new homes under construction and tipped me off. So I drove to Hanover, New Hampshire, to check out the new buildings. Tim McNamara, Dartmouth’s associate director of Campus Services, graciously met me at one of the job sites. I was given a tour of the sites by three representatives from Unity Homes: Ryan Lawler, Justin Pouliot, and Brad Moore.
Panelized walls and modular bathrooms
Although Unity Homes is expanding, it only sold 12 homes last year, so it’s still a relatively small company. At its indoor manufacturing plant in Walpole, Unity Homes assembles building components into wall panels and bathroom modules. (Bathroom modules are shipped to the job site with the tub, vanity, sink, and wall-hung toilet already installed. Once these modules arrive on site, they are moved from the truck to their final location by crane.)
Unity Homes is convinced that its German-made CNC (computer numerically controlled) cutting machine and its focus on indoor assembly helps the company achieve a higher standard of construction quality.
Unity has developed a panelized wall system using 9½-inch I-joists as studs. The wall panels are sheathed with Zip System 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. on the exterior and ordinary OSB on the interior. The stud cavities are insulated with dense-packed cellulose. A 1½-inch-deep service cavity is created on the interior side of the walls with 2x3 studs installed flatways. (For more on this topic, see “Service Cavities for Wiring and Plumbing.”)
The wall panels are a little less than 10 feet high — they are sized for 9-foot ceilings — and are shipped in almost any length, as long as necessary for the plans being built.
Tim McNamara told me that Dartmouth’s decision to contract with Unity Homes was based on three factors: energy efficiency, speed of construction, and price. “These are solid, attractive, durable homes,” McNamara said.
Each 3-bedroom home has 2½ bathrooms
The four houses are located on three sites on the Dartmouth campus, all within walking distance of each other. The homes are similar two-story homes, each with 3 bedrooms and 2½ bathrooms. Two of the houses have 3,255 square feet of living space each. The other two houses are a little smaller (3,062 square feet each). All of the homes include an attached garage.
According to Ryan Lawler, these houses (including garages and porches, but excluding sitework) cost about $650,000 each.
Foundation. Combination of basement and slab-on-grade foundation. Basements have 9-ft. ceilings; slabs on grade are installed inside of frost walls.
Foundation insulation. Basement walls are insulated on the interior with 2½ inches of Thermax polyisocyanurate (R-16). Frost walls are insulated on the interior with 3 inches of vertical XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. (R-15), down to the footing. Under the slabs on grade and basement slabs there is a continuous horizontal layer of 3 inches of XPS (R-15).
Above-grade walls. Wall studs consist of 9½-inch I-joists. Exterior sheathing is Zip System OSB with taped seams. Interior sheathing is ordinary 7/16-inch OSB installed with construction adhesive. Walls include a 1½-inch-deep uninsulated service cavity for wiring. Ceiling height, 9 feet.
Wall insulation for above-grade walls. Stud cavities are insulated with dense-packed cellulose; final wall R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. is R-35.
Siding. Hardie fiber-cement lap siding installed over a 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. created with Coroplast strips (1/4 inch thick by 2 inches wide).
Roof construction. Raised-heel trusses create a vented unconditioned attic.
Roofing. Asphalt shingles.
Ceiling insulation. 16 inches of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. (R-60) on attic floor.
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. details. Siga Fentrim tape is used to seal the seam between the bottom of the exterior wall sheathing and the concrete foundation. A layer of 10-mil Stego polyethylene is installed under slabs; the Stego continues up the interior side of basement walls, over the top of the foundation, and is sealed with tape to the sheathing of the above-grade wall. Siga Majpell membrane is installed under the bottom chord of the roof trusses; the membrane is held in place by 2x4 strapping.
Windows. A mixture of triple-glazed Integrity windows and triple-glazed Marvin windows. Marvin windows have a whole-window U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.25 and a whole-window SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of 0.23.
Design heat load. According to Manual J calculations, design heat loads for these four homes range from 32,600 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 to 35,100 BTU/h.
Space heating and cooling. Mitsubishi ducted minisplits systems. One outdoor unit (#MXZ-5C42NAHC) per house. Multiple indoor units in each house; typical indoor unit is #SEZ-KD18NA4(G).
Domestic hot water. Rheem electric resistance tank-style water heater.
Mechanical ventilation. Zehnder ComfoAir 550 heat-recovery ventilator with dedicated ventilation ducts.
An admirable focus on airtightness
Unity Homes strives for a blower door test result of 0.6 ach50 or less for every home it builds. During my visit to these job sites, I saw plenty of evidence that workers take airtightness seriously. Everywhere I looked, I saw Siga tape.
Unity Homes employees had ready answers to all of my technical questions — and that's rare on a job site. Many GBAGreenBuildingAdvisor.com readers have probably experienced the frustrations that come from talking to clueless contractors. If you've experienced those frustrations, you can imagine what a relief it is to finally find a contractor who gets it. Unity Homes is such a contractor.
Unity Homes is building some of the best single-family homes in New England, and every employee I met is concerned with building a high-performance home. If Unity Homes represents the future of home building, we're all in luck.
A job-site photo album
This photo shows the floor joists as seen from the basement. The joist hangers are fastened to a beam supported by a steel angle. By hanging the joists this way, the top of the subfloor is co-planar with the top of the concrete foundation. This detail makes it easier to run flooring between rooms that have a slab-on-grade foundation and adjacent rooms that are over a full basement.
The seams of the Zip sheathing are carefully taped. These homes have a mixture of Integrity windows (left) and Marvin windows (right).
Siga Fentrim tape is used to seal the joint between the bottom of the wall panels and the concrete foundation.
Yellow Stego Wrap (a durable polyethylene vapor barrier) is installed under the basement slab. The Stego Wrap continues up along the interior side of the basement wall and over the top of the foundation, before being taped to the exterior Zip sheathing of the above-grade wall. As the photo shows, Thermax brand polyiso insulation is being installed on the interior side of the Stego Wrap.
Second-floor joists are supported by joist hangers. The rim joist is on the interior side of 9½ inches of cellulose wall insulation.
The windows are mounted to the rough jambs with steel clips.
The ceiling air barrier is created with Siga Majpell membrane. The membrane is secured with 2x4 strapping. The Majpell membrane is strong enough to support 16 inches of cellulose.
Busted! Did Unity Homes workers really use Siga Sicrall tape, which retails for $48 a roll, to hold down temporary polyethylene installed to protect the stairs? Yes, they did — for the simple reason that there are plenty of rolls of Siga Sicrall sitting around on a Unity Homes job site. (Shhh ... don't tell the boss.)
Unity Homes specifies raised-heel trusses for their homes with unconditioned attics. The attic insulation (about 16 inches of cellulose) will be blown on top of this Majpell membrane. The membrane is supported by 2x4 strapping. AccuVent baffles ensure that ventilation air from the soffit vents reaches the attic.
The outdoor unit of a ductless minisplit is usually attached to a wall with brackets, a mounting method that keeps the unit above the expected snow depth. According to Ryan Lawler of Unity Homes, the best location for the outdoor unit of a minisplit system is on a garage wall — preferably on the gable end, where less snow accumulates. These compressors can be noisy, and wall mounting (though useful to keep the units above the snow line) has a potential drawback: it can enhance the vibrations and noise. Garage walls are obviously much better than bedroom walls for mounting this type of equipment. (Unity Homes hires Bergeron Mechanical of Dublin, New Hampshire, to install all of their minisplit systems.)
One of the Mitsubishi ducted minisplit units before the installation of the ductwork. This unit is mounted in the basement, suspended from the joists.
Lots of refrigerant tubing! This type of distribution box is needed when your minisplit system has one outdoor unit and three or four indoor units.
This ducted minisplit unit was installed near the ceiling of a walk-in closet on the second floor. The unit will later be hidden by a suspended ceiling with removable panels.
Mechanical ventilation is provided by a fully ducted HRV system. The Zehnder ComfoAir 550 is located in the basement.
Fresh air enters this Zehnder distribution box through the large diameter duct on the left. The ventilation air is distributed to various rooms through the small diameter ducts on the right.
A ceiling-mounted supply ventilation register.
This duct chase includes small-diameter plastic ventilation ducts (Zehnder ComfoTubes) as well as rectangular galvanized ducts that deliver warm air from the ducted minisplit system.
This soffit allows heating ducts and sprinkler pipes to be located within the home's thermal envelope. Note the fresh-air delivery box on the wall that requires three ComfoTubes to deliver the needed ventilation airflow.
All of the bathroom fixtures, including this wall-hung toilet, are installed in the bathroom module before the module leaves the factory in Walpole, N.H. Once the bathroom module is delivered to the job site, it is lifted from the truck and installed in its final location with a crane.
Exterior walls include a 1½-inch-deep service cavity for wiring. This service cavity includes a baseboard chase; the idea is that homeowners can remove the baseboard if they need to run communications cable.
This is what the baseboard chase looks like after the walls have been Sheetrocked.
While the service chase is handy for running wiring, the fact that the chase is so shallow comes with disadvantages. Unless modifications are made to the wall, it's impossible to use electrical boxes deeper than 1½ inch. Moreover, to make sure that drywall screws don't cause any short circuits, electricians sometimes need to install steel protection plates. (Siga Rissan tape was used to seal the hole where this cable penetrates the interior OSB.)
If it's necessary to install an electrical box deeper than 1½ inch, workers have to cut a rectangular hole in the interior OSB to create an airtight recess to accommodate the box. The seams of these recessed nooks are sealed with Siga Sicrall tape.
With enough planning, would it be possible to anticipate wiring runs accurately enough to create all of the necessary notches and holes in the manufacturing plant where wall panels are assembled? Perhaps. But in the real world, electricians still find it necessary to notch and drill framing members in order to run their cables.
When an electrical cable needs to penetrate an exterior wall, it passes through a plastic sleeve. This photo was taken from the exterior of the home. The penetration will be carefully sealed after the electrical box is installed. The seams of the recessed nook that accommodates the electrical box are sealed with Siga Wigluv tape.
Here's a photo of a similar exterior electrical box, after the box has been installed.
Because the Siga Majpell membrane is flexible, it's easy for the electrician to slip cables between the Majpell and the 2x4 strapping. This wouldn't be possible if Unity Homes used OSB as the ceiling air barrier.
The furring strips used to create a rainscreen gap are made of Coroplast, a brand of corrugated plastic often used to make political signs. The Coroplast strips are 1/4 inch thick and 2 inches wide.
The Coroplast extends behind the window trim as well.
This photo shows the fiber-cement water table trim, capped with Z-flashing. Note that the Coroplast rainscreen strips extend behind the water table trim.
When window rough openings are flashed, a "skirt" flashing (the flexible black membrane in the photo) is installed at the rough sill. Unity Homes uses synthetic roofing underlayment (Grace Tri-Flex) for this purpose. Later, when workers install the lap siding, the Tri-Flex is woven into the upper half of one of the siding courses. When the next course of lap siding is installed, the Tri-Flex becomes invisible. In this photo, Justin Pouliot holds up two pieces of siding to demonstrate the principle.
At the fourth job site I visited, work was more advanced than at the other three sites. Workers there had already installed asphalt shingle roofing and fiber-cement lap siding.
Unity Homes does a handsome job with the exterior window trim. The exterior casing is fiber-cement.
Martin Holladay’s previous blog: “Are New Homes Getting Better?”
- All photos: Martin Holladay
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