Three Superinsulated Houses in Vermont
Three Superinsulated Houses in Vermont
These three homes designed by Jean Terwilliger are all heated and cooled by ductless minisplits
Efficiency Vermont, a nonprofit agency that provides financial incentives for energy-efficiency improvements by homeowners, builders, and businesses in Vermont, has developed a certification program for new homes called the High Performance Certification. The aim of the certification program is to encourage the construction of new homes that are “ready … for net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. use, should you decide to add on-site renewable energy in the future.” Builders who enroll in the program are eligible for cash incentives of up to $2,000 per home.
Here is Efficiency Vermont’s list of minimum specifications for homes seeking their High Performance Certification:
- Foundation wall insulation: R-30.
- Slab-on-grade perimeter insulation: R-30.
- Horizontal insulation under a slab: R-20 for an unheated below-grade slab; R-30 for an unheated slab on grade; R-30 for all heated slabs.
- Floor assembly insulation (when exposed to the weather): R-40.
- Above-grade wall insulation: R-40.
- Ceiling insulation (sloped or flat): R-60.
- Insulation installation quality: Grade I.
- Blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. results: Less than or equal to 1 ach50.
- 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. : U-0.21 maximum.
- Door U-factor: U-0.25 maximum.
- Heating and cooling equipment: Must be Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. certified.
- Minimum AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. for boilers: 94%.
- Mechanical ventilation system: Must comply with ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant..
- Water heater: Must be Energy Star certified.
- Lighting: At least 95% of light bulbs must be Energy Star certified.
- Appliances: Must be Energy Star certified.
For cold-climate builders, hitting these targets should result in a comfortable house with very low energy bills.
At least one Vermont architect has jumped on the bandwagon
One architect who has eagerly embraced the specifications of Efficiency Vermont’s High Performance Certification program is Jean Terwilliger from Vermont Integrated Architecture in Middlebury, Vermont. According to Terwilliger, “Peter Schneider, who helped develop the specifications, said that if you stick to to this envelope package, it’s safe to just use minisplits for heating without a backup heating system.”
At the recent Better Buildings by Design conference in Burlington, Terwilliger gave a detail-packed presentation on some of her recent residential projects. Her presentation on February 1, 2017, was called “Three High-Performance Homes, Three Approaches.” (Terwilliger will be giving a reprise of her presentation at the NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland. www.nesea.org conference in Boston on March 9, 2017.)
Terwilliger’s co-presenters were two builders she’s worked with: Alex Carver of Northern Timbers Construction and Tom LeBoeuf of Northeast Craftsman Group. (Jared Moats, the builder of the third house in Terwilliger’s presentation, was unable to attend.)
All three homes mentioned in the title of her presentation are single-family detached homes:
- House #1 is a 1,254-square-foot house nicknamed “House in the Woods.”
- House #2 is a 3,756 square-foot house nicknamed “River View.”
- House #3 is a 1,790-square-foot house nicknamed “Lake Dunmore House.”
The three homes have HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index scores ranging from a high of 44 to a low of 12. (Low is better than high.)
Terwilliger is committed to the integrated designBuilding design in which different components of design, such as the building envelope, window placement and glazings, and mechanical systems are considered together. High-performance buildings and renovations can be created cost-effectively using integrated design, since higher costs one place can often be paid for through savings elsewhere, for example by improving the performance of the building envelope, the heating and cooling systems can be downsized, or even eliminated. approach. To begin the design process for the “House in the Woods” project, she invited the builder, Alex Carver, to visit the site with her. “I was looking at an open field with Alex Carver. I said, ‘How are we going to build this thing? What’s our wall system?’ I like to bring the builder on board early to have these discussions.”
THE KLINGENBERG WALL
The first American PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., the Smith House in Urbana, Illinois, had unusual exterior walls. The wall system specified for that house (and since promoted by the project architect, Katrin Klingenberg) includes a conventional stud wall — either a 2x4 wall or a 2x6 wall — sheathed with OSB or plywood. Vertical TJIs are installed on the exterior side of the wall 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. , either 16 inches o.c. or 24 inches o.c, in a manner similar to Larsen trusses. The TJIs are then sheathed with OSB, plywood, or some type of membrane, and the exterior cavities are insulated with blown-in insulation.
While GBAGreenBuildingAdvisor.com refers to this type of wall as a Klingenberg wall, Alex Carver prefers to call it an “Arctic wall” or a “Corson wall” (after Chris Corson, who used this approach for a house that Corson built in Knox, Maine after attending a presentation by Katrin Klingenberg at which she described her favorite wall system).
The first time that Alex Carver built a Klingenberg wall was at a Corson-designed house in Ripton, Vermont, called the Pike house. GBA reported on that project in an article titled Vermont House Uses Only Half a Cord of Firewood.
The Klingenberg wall
Alex Carver recommended a wall system he favors — namely, a variation on the Klingenberg wall. (See the sidebar at left: “The Klingenberg Wall.”)
At the “House in the Woods” project, Terwilliger and Carver specified 2x4 load-bearing walls sheathed with OSB (see Image #2, below). On the exterior side of the OSB sheathing, Carver installed vertical I-joists as studs (see Image #4).
The OSB in the middle of the wall assembly has taped seams; the OSB layer is the wall’s primary 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.. That approach allows the load-bearing 2x4 wall to be used as a service cavity.
At the Burlington conference, Carver told the audience, “On the exterior side of the OSB, TJIs are installed vertically. Here, we used 9 1/2-inch-deep TJIs, but on some projects we use TJIs that are 11 7/8 inches deep. The TJIs are screwed onto the studs with 4-inch screws. The TJIs are rock solid, and the TJIs are also used to tie the roof system to the wall.”
Carver said, “The WRB [Solitex Mento] is put on the exterior side of the TJIs. There is no exterior sheathing on the TJIs — just the WRB and strapping. We double-strapped the exterior to prepare for vertical siding.” He also noted, “The window bucks are made out of 2-by lumber.”
These double-strapped walls have plenty of ventilation air between the siding and the WRB. Looking at the cross-hatched strapping, Carver realized that the space was deep enough to be used to run refrigerant tubing (linesets) for the ductless minisplits (see Image #6, below).
Carver insulated the TJI cavities with dense-packed cellulose; the 2x4 stud bays were insulated with mineral wool batts.
Carver said, “I’ve heard people say, ‘That’s an expensive wall.’ And I thought that using TJIs would probably cost more than the double-stud approach. So I compared costs.” (See Image #11, below.)
Carver continued, “What we found is the cost of the 2x6 wall used for double-stud walls is significant. Just looking at the materials, the costs for the two systems are very close. And I think there is less labor on the Arctic wall. There is no need for a double top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate..”
Two of the homes have double-stud walls
The second builder at the Burlington presentation, Tom LeBoeuf, worked on House #2 (“River View”), a house with double-stud walls (see Image #12).
LeBoeuf explained that the 2x6 exterior studs are load-bearing, while the 2x4 interior studs don’t bear any loads. “There is a 3-inch space between the stud walls,” LeBoeuf said. “We use gussets made from 1/2-inch plywood to tie the walls together — we line up the studs so we can gusset them. The gussets prevent the 2x4 walls from bowing inward.”
When designing the River View house, Terwilliger kept in mind the fact that not every window needs to be operable. “Fixed glass is less expensive than operable windows,” she noted. Most of the windows at the River View House are triple-glazed vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). windows from Koltech. The large sliding glass door is from Loewen. According to Terwilliger, the Loewen unit is a “very nice quality triple-glazed unit and less expensive than the European PVC tilt/slide.”
House #3, a.k.a. “Lake Dunmore House,” is also framed with double-stud walls (see Image #16, below). Moisture monitoring probes were built into the walls. Terwilliger reported, “We’re getting moisture content readings ranging from 8% to 16% according to the monitoring equipment embedded in the walls.”
All three homes discussed by Tewilliger used Intello membrane as the ceiling air barrier (see Images #8, #15, and #21).
Every house has at least two ductless minisplit heads
Terwilliger specified ductless minisplits for heating and cooling at all three homes. All three homes also have at least one supplemental heat source. House #1 includes a wood stove for backup; House #2 has a wood stove and electric-resistance wires under both bathroom floors; and House #3 has a propane fireplace and an electric-resistance towel warmer in one of the bathrooms.
Terwilliger explained how the River View house ended up with five minisplit heads. “There is a minisplit head in each of the bedrooms, which I think is overkill,” she told me in a phone interview. “It was the contractor’s issue. The homeowner wouldn’t have minded fewer heads, but the contractor wanted a separate head in each bedroom. The homeowners have used them for cooling, but they haven’t really used them for heating. During the winter, they prefer a cool bedroom.”
Deciding how many minisplit heads to install, and where to put them, can be tricky. “In one of the other houses, I wasn’t as involved during construction, and I didn’t have any say about where the minisplit heads went,” Terwilliger said. “There was one up and one down. The upstairs head went in the master bedroom, which may have been a mistake. The other bedroom and the hall are suffering because they don’t have a heat source.”
I asked, “Have the homeowners accepted the situation?” Terwilliger responded, “They are basically living with it. In general, we usually advise putting in — or at least considering the possibility of — some electric resistance heat in each bedroom, and definitely in bathrooms.”
Some homeowners can be unhappy with an unheated bathroom on a slab. “In the Lake Dunmore house, there is an electric towel warmer in the upstairs bathroom, but there are issues with the first-floor bathroom, which is on a slab. The bathroom is adjacent to the mechanical room, which has a heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. . We should have put some heat in the bathroom.”
Heat-pump water heaters pull heat out of the indoor air, cooling the room where the water heater is located. “At a couple of these houses, the owners have learned how to switch their heat-pump water heaters to electric resistance during cold weather,” Terwilliger explained. “We’ve found that with bathrooms that are on a slab, even if you have R-30 insulation under slab, the slab is still pulling heat out of your bare feet.”
I asked Terwilliger whether the minisplits work well in cold weather. “Some people want to set the thermostat back at night. If they do that, the minisplit can have trouble on a cold morning.”
Mechanical ventilation systems
In House #2 (the River View house), Terwilliger specified a Venmar heat-recovery ventilator(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. . In the other two houses, Terwilliger tried something different: Lunos fans. House #1 has two pairs of Lunos fans, while House #3 has three pairs. (For more information on Lunos fans, see European Products for Building Tight Homes.)
“We chose to use the Lunos fans because of cost — not having to run ductwork. The Lunos approach works when the house is small enough.” I responded that some people think that Lunos fans are pricey. She answered, “Running ductwork ends up being really expensive. At the River View house, two guys spent a full week putting in ductwork.”
I said, “Good point. I’ve heard of some Zehnder systems that end up costing $8,000.”
Terwilliger responded, “Eight thousand dollars? They can cost more than that. We’ve seen Zehnder systems that cost $10,000 or even $12,000. That’s why we’ll use a couple of pairs of the Lunos fans for a small house.”
An attentive builder is essential
When I asked Terwilliger for any final thoughts, she told me, “A successful high-performance project takes builders who are willing to pay attention to the details. That’s how you get the air sealing done.”
She continued, “On the ‘lessons learned’ front, I'd say that the envelope part is getting much easier, with more builders with tight-building knowledge, so getting the mechanicals right is now one of the larger struggles. Even if the mechanicals are ‘design/build’ by contractor, all of the design decisions should be fully discussed and worked out before the start of construction, and followed through during construction by someone with high performance building experience.”
Martin Holladay’s previous blog: “Zero-Energy Construction is ‘Set to Explode’.”
- All images (except Image #10 and #19) courtesy of Jean Terwilliger
- Image #10: Martin Holladay
- Image #19: 475 High Performance Building Supply
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