Three Superinsulated Houses in Vermont

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Three Superinsulated Houses in Vermont

These three homes designed by Jean Terwilliger are all heated and cooled by ductless minisplits

Posted on Mar 3 2017 by Martin Holladay

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. 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 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.

For more on this wall system, see The Klingenberg Wall. For more information on the historical precursor to the Klingenberg Wall, see All About Larsen Trusses.

While 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’.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. All images (except Image #10 and #19) courtesy of Jean Terwilliger
  2. Image #10: Martin Holladay
  3. Image #19: 475 High Performance Building Supply

Mar 3, 2017 11:00 AM ET

Great examples -- and there are more
by Brian Just

Nice write-up, Martin! Here’s some added info: After a pilot run in 2012, completions in Efficiency Vermont’s High Performance tier had been hovering in the 10-20 per year range. In 2016, however, we saw an astonishing 51 homes certify. All told, over 150 homes in Vermont have either completed (i.e. met those specs) or are currently in design/under construction. Overall, these numbers are a great testament to the building and design community here.

Mar 3, 2017 11:12 AM ET

Response to Brian Just
by Martin Holladay

You're right: Vermont designers and builders are doing a spectacular job.

A shout-out to Vermont designers and builders: If you've completed a new home certified as a High Performance Home by Efficiency Vermont, send us a guest blog describing your project. We'd love to hear about it.

(And all you builders in Maine, remember: We won't know who's doing better -- Vermont builders or Maine builders -- unless you submit guest blogs, too.)

-- Martin Holladay

Mar 4, 2017 10:22 AM ET

Blower Door Numbers
by Kye Ford

The blower door numbers seem really low in the homes as there are wood stoves in #1 and #2 and propane fireplace in #3. I haven't seen a dampener that would seal that tight as to give numbers that low. Did they they seal off the wood stoves with tape/plastic when they did their blower door tests?

Mar 4, 2017 11:24 AM ET

Edited Mar 4, 2017 2:45 PM ET.

Cost Comparison for Walls Way Off
by Kye Ford

I re-ran the numbers for the walls for homes #1 and #2 . The costs I calculated represent a 100 s.f. section of wall as described above. #1 $501, #2 $354. The arctic wall will cost 40% more! For house #2 I included 1/2" zip wall which I assumed the builder used and I also didn't add tape in either cost scenario but if anything you will also need twice as much tape in the arctic wall (#1) as you are taping both your air and WRB layers.
The assumption that labor costs between these two walls would be close is not realistic either. I can build everything in a double stud wall with a nailgun, fast. Having to screw all the TJI's to the side of a building is just going to take longer. Then you have to do twice as much taping, install the solitex, then strap again. I can frame two walls way faster.

Mar 4, 2017 4:44 PM ET

by Andy Kosick

Is the annual energy use in the table actual metered use or a software projection? Also, just to be clear, it does include everything, heating, cooling, lights & appliances?

Mar 4, 2017 5:16 PM ET

Response to Andy Kosick
by Martin Holladay

Actual metered energy use. Includes everything.

-- Martin Holladay

Mar 5, 2017 11:11 AM ET

Dedicated outdoor air for the wood stoves
by Aaron Beckworth

Do you know if dedicated outdoor air ducts were installed for the wood stoves in houses 1 and 2?

We are considering a small Morso wood stove for our project. There are two models we are considering, one of which allows for an outdoor air supply. Is dedicated outdoor air pretty much a requirement for wood stoves in high performance homes?

Mar 6, 2017 10:55 AM ET

Response to Aaron Beckworth- Dedicated outdoor air
by Jean Terwilliger

Yes, dedicated outdoor air ducts were installed in both houses. House 1 has a small Morso stove that is working well. There has been discussion on GBA about whether outdoor air is necessary for HP homes, but I have seen cases of backdrafting of wood stoves from clothes dryers running so definitely prefer to provide outdoor air.

Mar 6, 2017 11:30 AM ET

HRV ducting
by Jean Terwilliger

A clarification on HRV ducting- the week of installation time was for a 6" metal duct system for the "river view" house. Zehnder plastic ducting is more expensive for materials, but installs much faster. Overall ducting cost may be a wash between the two- is also very dependent on the experience of installers in your area.

Mar 6, 2017 2:22 PM ET

Response to Kye Ford
by Jean Terwilliger

Woodstoves/outdoor air intakes were not sealed off for the blower door tests, but the Lunos and other ventilation units were taped over. They are tight houses!

Mar 6, 2017 3:54 PM ET

Cost Comparison Kye Ford
by Alex Carver


It would be great to see the breakdown of your numbers. I couldn't believe how close the were in materials only. Let's see how the two sets of numbers differ. I'm sure there are plenty of folks who want to know. Certainly the Architects because they are counting on us to give accurate estimates of framing costs.Every job is different, and I continue to maintain the labor of providing the insulation cavity on the outside of the building is faster and less interrupted so will therefore be less to install.

Mar 6, 2017 4:09 PM ET

Kye In the comparison I have
by Alex Carver


In the comparison I have done I am assuming the double stud walls have the following: , Air sealing by taping the exterior, a vapor smart membrane inside, either WRB or zip wall, taping required for both systems of the fabric used, and strapping for both walls for an air chase. This will bring the cost of the walls closer. Are you considering having the interior vapor smart membrane in your system?

Mar 7, 2017 6:10 PM ET

Edited Mar 7, 2017 6:16 PM ET.

Wall Costs
by Kye Ford

Here is my breakdown of the two walls 100 sq ft
Arctic Wall-
9-2x4x10- $42
1-2x4x10PT $6
3-7/16 osb $32
6- 9 1/4" TJI $93
12-10' Furring $17
100 sq/ft solitex mento $25
100 sq/ft x 9 1/4" cellulose $121
100 sq/ft 3 1/2" Roxul $132
Total $463 per 100 sq/ft

Double Wall w 2x6
9-2x6x10 $65
1-2x6x10 PT $8
3- 1/2 zip wall $60
10- 2x4x10 $42
100 sq/ft x 12" cellulose $155
100 sq/ft dense pack fabric $10
Total $340 per 100 sq/ft
If you want to add a smart vapor barrier in side I would delete the dense pack fabric and add solitex so $ Total would be $355

I looked more closely at my numbers that is why there is a difference from my previous post. But it still looks like a difference of 30%.

Don't get me wrong......Arctic wall is a good wall, and it could be argued by many that the air tight layer is in the correct middle position of the wall assembly but I think it is a more expensive alternative to a double stud wall.

Mar 7, 2017 6:31 PM ET

Great Work
by Ken Levenson

Wonderful to see these professionals and projects featured. Great designs and craftsmanship are not just inspiring but can become day! Love it.

Mar 11, 2017 2:13 PM ET

Dedicated make-up air for wood stoves
by Dylan Kinsey

I'll second what Jean said about providing dedicated outdoor make-up air for a wood-stove in an HP home. My company completed an 1,110 SF High Performance home in Wolcott, VT last year, with a 2 zone, Mitsubishi Ductless Mini Split as primary, and a small Pacific Energy wood stove for back-up and outages. Even with directly-connected ducted make-up air to the wood stove, the kitchen range hood can cause back drafting under the right (or wrong) conditions. I wouldn't recommend using a wood stove in a building with exhaust-only ventilation either. They don't generally play well together in tight homes. Happy to elaborate......Hope this helps!

Mar 11, 2017 2:53 PM ET

Arctic vs Double Wall
by Dylan Kinsey

Alex and Kye,

Thanks for sharing your thoughts and cost break-downs of the two walls. Great for comparison. My experience with the double 2x4 wall approach was that it made for fast and light work compared to some wall systems. Adding an 1-1/2" service cavity to the inboard side of a double 2x4 wall, seems like it makes the two approaches that you're comparing very close in complete cost.

Mar 26, 2017 7:28 PM ET

Kye Wall comparison
by Alex Carver

Hey Kye,

I've been working up comparisons for Habitat and other jobs of the double stud and arctic wall again. It is still more but interesting to note the totals are closer when the whole house is used as a basis rather than 100 sf wall. Either way as you say both good walls. I prefer the air barrier to be nice and toasty on the inside. I like to look at it as $1.08 sf more in the 100 sf wall scenario.

Apr 21, 2017 8:28 AM ET

Alex, are you OK with just having a wrap outside?
by Ethan T ; Climate Zone 5A ; ~6000HDD

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.” In hindsight... do you sleep OK knowing that the outside of your wall is just a glorified housewrap? I am trying to get comfortable with this idea, but keep having sneaking reservations. Maybe I've just had too many rat and raccoon infestations over the years... Raccoons clawed through 1x boards, I'm sure they would make quick work of a fancy European housewrap.

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