VISION House® TucsonWhite Paper

VISION HOUSE t u c s o n

®

Prepared by: Charles Wardell

This super-efficient home is testing innovative approaches to HVAC, ductwork, and the building envelope.

The purpose of a demonstration home, as the name implies, is to demonstrate new products and technologies. But many of these projects also have research goals, allowing builders, designers and manufacturers to field-test new products and building systems. That was certainly the case with Green Builder® Media’s VISION House® Tucson, built by John Wesley Miller in the city’s Armory Park Del Sol neighborhood. Although the home show-cased a variety of green and energy-saving products and building methods, this paper focuses on the creative approaches it took to energy efficiency.

The home’s energy saving technologies include super-efficient windows, thick attic insulation, thermal mass walls, and high-efficiency HVAC and Water Heating equipment. Most of the electric needs are satisfied by grid-connected photovoltaic (PV) panels. While these elements aren’t unique by themselves, this project optimized them in unusual ways.

That optimization earned the home an impressive score from the Home Energy Rating System, or HERS. The HERS scale compares new homes to a reference home built to the 2006 International Energy Code. The scale defines a Net Zero home as one that gets a score of zero, which means its PV and thermal solar panels generate enough energy to offset most of the energy used by the home over the course of a year, as compared to the ref-erence home. (The HERS rating can’t predict actual energy use, because it makes assumptions about occupancy and energy use that may not match those of the as-built home.) VISION House Tucson went much further, earn-ing an impressive score of -17 – that is, the panels will generate 17% more energy annually than the reference home will use.

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Miller calls it the most efficient structure he has ever built. “I’m not aware of a home in the Southwest with a better rating,” he says.

The key to achieving this level of efficiency was in the details used in the building envelope. Those details re-duced energy demand to the point where it was practical to experiment with a new approach to space heating.

The Envelope PleaseThe home has an average electric demand of 5.5 KW, which is more than satisfied by a 7.4 KW grid-tied solar ar-ray from Hanwha Solar and its solar leasing partner One Roof Energy. Conext™ TX inverters from Schneider Elec-tric convert the panels’ DC (direct current) output to AC (alternating current) while also connecting the home to the electric grid. The grid connection provides supplemental power when demand exceeds the panels’ output, and allows the home to sell power back to the grid when the panels are generating more power than the home needs.

The ability to operate mostly on solar is thanks in large part to a super efficient building envelope that lowers heating and cooling loads to a minimum.

The Conext™ TX inverter is easy to install, easy to service, and integrated with innova-tive features. The Conext TX can be installed as a single inverter for a single PV array or in a multiple-inverter configuration for large PV systems or three-phase applications. The sealed inverter enclosure is detachable which allows the wiring box and its AC/DC connections to remain intact for easy rein-stallation. The Conext TX is integrated with Fast Sweep MPPT technology to reduce the impacts of shading on harvested energy. It is a great value in a compact, high-frequency design.

OneRoof Energy offers solar leasing and loan options available with zero-out-of-pocket expense.

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Some of the products in the envelope might be found on any project. These include Boral Madera 900 concrete roof tiles, thick Johns Manville attic insulation (more on that below) and MI EnergyCore™ Windows from MI Windows and Doors (made from material that blocks thermal conductivity 6x better than fiberglass, 4x better than rigid PVC, and 3x better than wood).

One big difference with this project was how Miller configured the wall system.

While the typical energy efficient home has wood framed walls with thick insulation, VISION House Tucson relies on the thermal mass effect, where concrete walls and floors reduce the load on the heating and cooling equipment by absorbing excess heat on hot summer days, then releasing it during the evening when ambient temperatures have dropped.

To create the needed mass, Miller built exterior walls with 8-inch thick concrete masonry units, or CMUs. Af-ter placing steel rebar and electrical conduit, he filled the CMUs’ voids with concrete. Of course mass walls on single-family homes are nothing new (although they’re rare even in the hot Southwest); the difference in this project was how they were insulated.

Most builders who use mass walls either place the insulation on the inside or don’t insulate them at all. Miller says that the latter approach effectively creates a Trombe Wall, a mass wall that collects solar energy during the day and releases it to the interior at night.

Madera 900 is a Cool Rated ProductReflectivity: .13Weathered Ref (3 yr Aged): 0.15*Emmisivity: .87Weathered Em. (3 yr Aged):SRI: 11

Concrete masonry units were used in the walls to create the thermal mass effect. MI EnergyCore Windows have patented AirCell™ PVC frame technology, TMAX™ insulating glass, and a design that incorporates numerous physical barriers against air and water infiltration.

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VISION House Tucson took a third approach, with two inches of rigid polyisocyanurate insulation on the outside of the wall. The strategy, says Miller, was to totally isolate the inside from the outside. During the heating sea-son, heat generated by the heating system during the day is stored in the walls and then released at night; during the hot months, heat from the sun is blocked by the exterior insulation. The loss of solar gain in winter is more than offset by the savings gained by keeping heat out of the wall in summer.

“We do take advantage of solar energy in this house,” he says, “but it’s with the photovoltaic and solar water heating panels, not with passive solar.”

Although Miller has used this wall system before, he doesn’t yet have a good way of modeling how much energy it will save in a particular home. “We know that it works,” he says. “But we can’t quantify its effectiveness.” One of the projects’ research goals is to assign hard numbers to the wall’s performance.

That research is being done by the Concrete Sustainability Hub at the Massachusetts Institute of Technology (MIT) in partnership with the Portland Cement Association. MIT researchers placed sensors around the home to record temperature and humidity inside and outside, along with the heat pump’s power consumption and the inlet temperature at the hydronic air handler unit.

According to the Hub’s executive director Jeremy Gregory, this research is just one part of a larger effort that in-cludes several projects. The ultimate goal is to quantify the environmental impacts of concrete over its entire life cycle. However he says that the results from VISION House Tucson will help his team create a simplified energy-use model that architects considering this wall system can consult during the design phase.

The research is ongoing but will eventually be published at the Hub’s website: https://cshub.mit.edu/.

Bury The DuctsTwo big energy drains in the average home are duct leakage and heat gain or loss through unconditioned attics. This is certainly the case during Tucson’s hot summers, where most heat gain is through the roof. Here again, VISION House Tucson addressed the problem in an unconventional manner.

The concrete walls are wrapped with a 2-inch layer of rigid Johns Mans-ville AP™ Foil-faced Polyiso Foam Sheathing - a polyiso-cyanurate insulation.

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Ducts routed through unconditioned attics need to be wrapped in insulation and their joints carefully sealed. Even then, the fact that most ducts are only insulated to R-8 means that energy losses are still a problem on very hot days. That’s why most building scientists recommend keeping ducts inside the conditioned space instead of running them through the attic.

In this home, however, Miller didn’t want duct chases breaking up the architectural lines on the interior. So at the suggestion of insulation manufacturer Johns Manville he decided to try something different. He ran insulated ductwork through the attic then covered it with 32 inches of blown fiberglass insulation. Before installing the ducts, a layer of foam insulation was sprayed on top of the drywall to eliminate air leakage in the ceiling plane. The end result was a total attic insulation value of around 55.

Miller says that while he had not used this approach before he is sold on it and will look for opportunities to do it again. “From a performance standpoint, it’s the same as putting the ducts in the conditioned space.”

Mechanical EngineeringIn addition to thermal mass and good insulation and air sealing, VISION House Tucson also minimizes energy use with super-efficient mechanical equipment. The Ventilation system consists of WhisperGreen Energy Star-rated ventilation fans from Panasonic, which incorporate highly energy efficient DC motors. Heating, cooling and do-mestic hot water are provided by Rheem’s new Net Zero Air and Water System.

Panasonic WhisperGreen ventilation fans incorporate highly energy efficient DC motors. These models come with variable 2-speed and delay timer controls built-in or as a single speed fan. A motion sensor model is also available. The WhisperGreen, WhisperGreen LED and WhisperGreen Lite models were all used in VISION House Tucson.

JM Corbond III® Spray Foam Insulation was sprayed onto the attic floor and JM Climate Pro® Formaldehyde-free™ Insulation was then blown-in to achieve an R-value of 51.

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The Air and Water system isn’t a product per se. Instead, it’s custom-designed solution. In this project, the system’s main elements are thermal solar water panels, a hot water storage tank, an air source heat pump, a hydronic air handler, a demand water heater and a control system that coordinates all these elements. “None of the equipment is new,” says Rheem engineer Shaun Thomas. “What’s new is how this is all integrated together.”

Space conditioning accounts for 46% of a typical home’s energy use, and water heating another 18%. In VISION House Tucson the efficient building envelope and the customized Rheem system slash those percentages by lowering equipment run times.

For instance the home’s high-efficiency 16-SEER heat pump would use about 3000 watts of electricity per hour during the cooling season if it ran continuously. However run times have been reduced to just 10 or 12 minutes, with 30 to 45 minutes of idle time between runs.

During the heating season the heat pump shouldn’t be needed. That’s because the primary heat source is the rooftop thermal solar water heating panels. During the day the panels heat water in the 120-gallon storage tank to as much as 160 degF. When the thermostat calls for heat, water from the tank is tempered down to 120 degF then pumped to the hydronic air handler, where the water’s heat is transferred to the supply air stream. If the home needs more hot water than the storage tank can provide, the demand water heater makes up the shortfall. The demand heater draws 27kw of power but only comes on infrequently.

“We wanted to validate using solar heated water to actually heat the home,” says Thomas. “This project con-firmed that it works. We can gain enough Btu capacity for thermal storage during the day to provide adequate hot water during evening as well as overnight heat for the home.”

Rheem uses a software program called Design Star to calculate heat loss and size the system. It starts by pulling the home’s square footage, location and year of construction from public records. This returns a basic calcula-tion based on that data. The results can then be fine-tuned for the particular home, as well as the needs of the builder and homeowner. This last point is important: Thomas emphasizes that the same home would have a different system configuration if it had been built in Denver rather than Tucson. Indeed, the configuration used here wasn’t the only one possible.

Rheem Solaraide™ Storage Tank

Rheem Solpak™ Solar Collectors

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Of course the practicality of using solar hot water for space heating will vary by climate. “The farther north you go the more you have to increase the solar array and the hot water storage capacity depending on the heat loss of the home,” says Thomas. “VISION House Tucson has two panels, a total area of 16 square feet. Further north you would have to increase that 3 to 6 times depending on how far north you go.”

ConclusionMiller hopes that VISION House Tucson’s unique combination of mass walls, thick attic insulation, and cutting edge mechanical equipment will provide valuable lessons for builders and designers. A typical home of this size would have an electric bill of $300 per month and more,” he says. “This home actually pays the homeowner.”

He believes that the builders who thrive in the future will be those who can offer such benefits to their cus-tomers, and that more builders will come on board once they understand that there are different approaches to building a green home. “The big challenge in this business is to be able to get your arms around the whole picture,” he says. “How does everything come together to make a better, more livable space?”

VISION House Tucson (photo: lathamarchitectural.com)

Builder: John Wesley Miller Companies | Architect: Hank KrzysikPhotography: lathamarchitectural.com; Rubin Layne Ruiz; Hannah Bergeron

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