Net Zero ICF House Cheaper to Build Than Stick-Frame

Zero Net Now’s ICF-based project keeps its cool whenever it needs to.

Builder Anthony Aebi has been building homes from insulated concrete forms (ICFs) for more than a decade because of their energy efficiency and disaster resistance. Until this year, affordability was not considered one of their strong suits. But, with pandemic-related shortages driving up the cost of lumber, ICF construction has become more cost-effective. 

Owners of “The Cooper Street Residence,” a stylish 3,718-square-foot home in New Paltz, N.Y., will get a house with highly thermally resistant ICF walls at a cost of $7,500 less than a similar-sized conventional stick-framed enclosure built to the minimum energy code, according to the builder.

Aebi’s company Zero Net Now, formerly known as Greenhill Contracting of Esopus, N.Y., constructed its first zero energy home in 2007 and has built 39 zero energy custom homes to date. 

The company has been a partner in the U.S. Department of Energy (DOE)’s Zero Energy Ready Home (ZERH) program since 2013 and has committed to certifying all of its homes to the DOE Zero-Energy Home label. ZeroNetNow routinely achieves among the lowest Home Energy Rating System (HERS) scores in the country. 

On the HERS Index, a new home built to code would typically achieve a score of 80 to 100. A net zero energy house would score under 10, which is made possible by building a very efficient model and then adding solar panels that produce as much energy as the home uses over the course of a year. 

Zero Net Now’s homes often score below 40 before including the photovoltaic (PV) panels. Zero Net Now’s home this year achieved a HERS score of 26 without PV and minus 13 when the 13.74 kilowatts (kW) of solar panels are added.

ZeroNetNow’s Cooper Street Residence achieved a Home Energy Rating System (HERS) Index score of 26 without photovoltaics (PV) and minus 13 when the home’s 13.74 kilowatts of solar panels are added.

Blocking Basics

The homes start with the highly insulating properties of the ICFs, which are hollow foam blocks that stack like Legos to form a wall reinforced with steel rebar, then filled with concrete. This home was constructed on 11.25-inch-wide ICF blocks consisting of a 6-inch concrete core with about 2.6 inches of rigid EPS on each side, for a total wall insulation value of R-22.

The solid walls are airtight and the continuous foam layers on either side limit thermal bridging or heat transfer through the walls. The ICF blocks were used for the below-grade basement walls and the above-grade walls, providing a continuous thermal barrier from the footing to the roof line. They also provide R-22 of slab-edge insulation around the basement floor slab. 

Before pouring the floor slab, the builder applied 4.5 inches (R-29) of closed-cell spray foam directly onto the gravel base, where the foam serves as a vapor barrier and under-slab insulation. ICF construction eliminates framing and sheathing at exterior walls. The ICF blocks have integrated plastic splines, which allow for direct fastening of the exterior cladding without the need for furring strips or nailers. 

Aebi uses the ICF blocks for the entire wall up to the eaves and to the roof line on the gable ends. Along the eaves, he installs pieces of 1.5-inch insulated coated oriented strand board (OSB) sheathing between the rafters at the roof-to-wall interface to serve as a backer board for the attic spray foam insulation while providing a complete R-6 thermal break to the roof truss framing.

The ICF blocks are sealed at the seams to provide a continuous air barrier. They also serve as the drainage plane on the exterior side of the walls, so no house wrap is needed. To protect the framing where windows or doors will be installed, an elastomeric waterproofing compound is applied with a caulk gun and putty knife to provide a seamless, jointless flashing layer around the openings. Composite wood clapboard siding is used for the exterior cladding.

Aebi constructed a sealed, unvented attic that is insulated on the underside of the roof deck with two types of spray foam. He sprays 10 inches (R-44) of open-cell spray foam followed by 2.5 inches (R-18) of closed-cell foam insulation which completely covers the open-cell spray foam, providing a total attic insulation value of R-62. 

The closed-cell spray foam also serves as a Class II vapor retarder. Above the roof deck, a self-adhered bitumen membrane is installed at the roof edges and valleys to provide additional moisture protection.

Additional energy savings come from the home’s 100 percent LED lighting and ENERGY STAR-rated appliances.

Leak-Free Environment

The Cooper Street Residence was designed with large windows on all sides to maximize views and daylighting of the interior. To allow for an abundance of natural light without sacrificing too much in thermal performance, the builder opted for high-performance triple-pane windows. 

The windows are argon and krypton-filled, vinyl-framed, fixed, casement, and sliding style windows with an insulation value of U-0.17 and a solar heat gain coefficient (SHGC) of 0.21. Even the sliding doors are triple paned. They have an insulation value of U-0.20 and an SHGC of 0.25.

The home is so airtight that a blower door test of whole-house air leakage showed a leakage of only 0.23 air changes per hour at 50 Pascals pressure difference (ACH 50). That level of airtightness (which is typical of Aebi’s homes) is far below the 3 ACH 50 required by the 2015 International Energy Conservation Code and below the 0.60 ACH 50 required in the Passive House U.S. standard.

To provide fresh air for the home, an energy recovery ventilator (ERV) runs 24/7 at low speed to exhaust air from the bathrooms, kitchen, laundry, and attic. The ERV is equipped with sensors that will trigger higher levels of ventilation when CO2 levels exceed 1,000 ppm. 

The main controller for the HVAC system allows occupants to increase (or decrease) ventilation as desired. Boost-speed controllers are also located in each bathroom and the kitchen for higher-speed exhaust when desired.

Fresh air brought into the home through the supply duct is filtered by a MERV 6 filter as it enters the ERV heat exchanger. Then, it is ducted into the return trunk of the air handling unit, where it is filtered again via a set of electro-static and MERV 11 media air filters. The redundant air filters, zero-VOC paints, and non-combustion HVAC all contribute to indoor air quality.

The home is equipped with a highly efficient ground-source heat pump with one 3-ton central air-handler unit, ground-loop configuration with a rated efficiency of 5.1 coefficient of performance. The air handling unit is fitted with an electrical heating coil for emergency backup only. 

The duct system is made from rigid metal and is all located in conditioned space. The properly sized HVAC equipment includes an air handler with a variable-speed compressor and high efficiency ratings. 

Variable speed compressors modulate the fan speed and produce the output needed based more precisely on the demands of the thermostat, which allows the equipment to run longer and more efficiently and to increase the lifespan on the system as it doesn’t need to turn off and on as much.

Hot water for the all-electric home is provided by the water-to-water ground-source heat pump with an 80-gallon storage water tank to provide 100 percent of the domestic hot water for the home. The storage tank is fitted with an electrical element for emergency backup only, should the heat pump fail (e.g., due to an electrical surge).

The heat pump is in a mechanical room within the conditioned basement. The heat pump has a coefficient of performance of 3.10 and then is de-rated for the thermal standby loss of 0.63 person in the storage tank, resulting in a system energy factor rating of 2.66, per the EF calculator of the supplemental energy modeling toolkit.

Low-flow EPA WaterSense-labeled plumbing fixtures reduce water and water heating demand. This home is a part of a development that was designed to handle the storm water from multiple lots. Therefore, 100 percent of storm water runoff is managed onsite through design.

Other Ways to Save

Additional energy savings come from the 100 percent LED lighting and ENERGY STAR-rated appliances. The builder specified a ventless heat pump clothes dryer, in part for efficiency but primarily because the home is so airtight that a standard vented clothes dryer would have depressurized the house too much.

Another feature of this home is the innovative solar shingles that were installed on the roof. This solar roof system was the second installation of its kind in New York state. Engineers from the product manufacturer were required to come to the site to train the crew on how to properly install the solar shingles. 

Despite the initial cost to invest in this solar array, the engineers claim that the roof lasts up to 200 years and that the durability of the solar PV shingles is three times stronger than asphalt shingles, which need to be replaced every 15 to 20 years.

“The best kilowatt of energy is the kilowatt not used,” says Aebi. “It’s critical that builders focus on constructing more-robust thermal enclosures and optimally efficient mechanical systems.” 

With ZeroNetNow’s highly efficient ICF building envelope and high-performance HVAC, he is practicing what he preaches. But Aebi goes beyond construction with continued monitoring, because “our job doesn’t end when the homeowners receive their keys.” 

The builder installs energy and climate monitoring systems on each home to track energy use and production, operation of the mechanicals, and indoor air quality measures. 

“We can monitor, identify, and troubleshoot equipment in a quick and efficient manner and assess whether or not homes are performing as expected,” Aebi says. “Based on those assessments, corrective action can be taken to prevent challenges that a homeowner may face. The data collected is proof of what works and what doesn’t.” 

Key Home Features

Air sealing: ACH50 = 1.48 CFM50. Spray foam under slab and on attic walls and ceiling; ICF walls with liquid-applied flashings at all rough openings and penetrations.

Appliances: ENERGY STAR refrigerator, dishwasher, clothes washer and dryer.

Attic: Unvented attic, vaulted ceilings: 10-inch R-44 medium-density spray foam topped by
2.5-inch R-18 high-density spray foam.

Energy management system: CO₂ sensors in master bedroom, return duct, and kitchen trigger boost setting on the ERV. Wi-Fi monitoring of energy production and end uses.

Foundation: Insulated basement: ICF blocks R-22, 11.25 inches wide.

Hot water: Ground-source heat pump water heater, 80-gal, 2.66 EF.

HVAC: Heat pump, ground-loop configuration, 5.1 COP, variable-speed compressor, 34 EER.

Lighting: 100 percent LED, motion sensors, large windows.

Roof: Gable truss roof, coated OSB sheathing, 1-by-3-inch furring strips, self-adhered membrane, solar PV shingles. Solar: 13,74-kW roof shingles.

Ventilation: ERV with MERV 11 and 6 filters, demand-controlled ventilation, CO₂ sensors.

Walls: ICF walls, R-22 total: 11.25-inch ICF blocks, engineered wood siding.

Water conservation: EPA WaterSense fixtures; central manifold, PEX piping.

Windows: Triple-pane, argon and kypton filled, low-e, vinyl; U=0.17-0.20, SHGC=0.21-0.25.

Other: Low-VOC products. ICF thermal mass holds temp through power outages. EV ready.