Walls under construction on Oberprillers' ICF home.

Finished house.

Net zero energy refers to a building that is capable of capturing the free energy on a site in balance with the amount the residents use. It is distinct from a home that is off the grid in that it uses the local utility grid essentially as a battery. Off the grid homes must generate and store all their energy requirements, and the storage capacity available in batteries has been one of the physical and cost limits of this approach. A net zero energy home can send power into the grid for others to use when their on-site system generates excess electricity, and draw from it when on-site power is unavailable. Solar photovoltaic (or PV) is the most common on site electricity generation, though depending on the site, small scale wind and micro-hydro may have potential as well. This is the ‘blue’ for Oberpriller; who plans to install photovoltaic capacity, micro-hydro, and wind turbines. “I plan to get a plug-in electric car. My goal is to be energy independent in 2 years.” To make that work he needed to reduce the power needed for heating, cooling, ventilation and hot water.

The specific design and material choices that allow a family to operate their home at or near net zero energy are varied. But several strategies and elements are central and have to do with the building envelope and its shape and location on the site. Thermal mass, window choices, properly sized overhangs, air sealing, and insulation, along with an orientation and exposure towards the south all contribute substantially to achieving the lowest energy demand possible for thermal comfort and daytime lighting. Heating and cooling are the two largest home energy uses and therefore the greatest opportunity for building in savings. Having built wood-framed homes for decades, Oberpriller had a good understanding of how challenging it would be to use that system to build a very tight, highly insulated shell with enough mass to retain heat from the sun during the day and release it slowly at night. So he began to look for alternatives that would be easier to assemble and achieve better thermal performance.

His investigation took him to the annual trade show “World of Concrete” held each year in Las Vegas. Insulated concrete forms (ICFs) are relatively uncommon in the Northeast (New England), used primarily for foundations. But the national show provided ample examples of the technology and how it worked. Rigid foam insulation pieces create a form for pouring concrete that stays in place as part of the building system. Quad-Lock, one of several major ICF manufacturers, was exhibiting their 4 inch panel system that can deliver up to R-40 walls in a seamless envelope and Oberpriller had found his alternative. The common element of ICFs are EPS (expanded polystyrene) blocks which are placed and connected to provide an interior and exterior side to the wall. Concrete is poured into the cavity between them and dry-wall and sheathing or stucco are applied to finish the walls. Manufacturers vary in the thickness of the EPS panels (and the insulation that they provide), as well as the connecting systems to hold them in place while the concrete sets.

Designing and building for an energy demand that is at or below the renewable potential of a site requires looking more broadly at both energy and building systems. It is useful to step back and look at the energy system of a building and site as an integrated whole. The interplay between an array of factors determine how much energy use can be avoided or provided by the natural, ‘free’ resources of the site and building 

- the building materials, shape, location on a site,

- the window materials, size and location,

- the mechanical and plumbing systems,

- the occupant’s use patterns, and

- the available solar and wind resources.

The ‘grid’ and renewable generation address electrical power; households often use natural gas, propane, or oil for heating, cooling, and hot water. By remembering that energy use is not the end, but a means to have light, thermal comfort, hot water, and electricity for plug loads, a designer can use a site’s natural assets to meet these needs. Careful design uses passive solar energy and natural air flow to provide light, thermal comfort and ventilation without first converting it to electricity. Solar water heaters, either evacuated tubes, or flat plate collectors, can use active solar energy to provide thermal comfort and hot water.

ICF technology provides mass that can absorb radiant energy from the sun, as well as energy radiated from occupants and equipment. Concrete’s slow collection and release of heat effectively flattens out temperature changes within the building and distributes heat more evenly throughout the day and night. The combination of mass, high insulation value and a seamless, solid wall structure provides the kind of envelope that a building needs to get the best thermal performance. While not impossible, it is very difficult to get similar performance from an assembly of separate framing, insulation, air sealing and vapor barrier components. The ability to get all of these features in a single installation system, versus hoping that the efforts of several different subcontractors all end up working well together is what further impressed Oberpriller. “There were obstacles in introducing a new technology, but the installation was easy. ICFs are like building with LEGOS^®. The challenges were finding a local designer/engineer with ICF experience, and educating the local officials. I ended up using an out of state engineer, and battles with local officials and inspectors caused delays this first time through. The next project will be easier in these aspects now that we’ve all learned from the first one. For anyone doing this for the first time there are more and more resources available to help you educate the regulators and inspectors about what you are doing.”

With an envelope that can buffer against temperature changes by storing and releasing heat slowly, the next element of a net zero energy strategy is to work with the site to capture the suns heat when you want it and avoid it when you don’t. Radiant heat transfers by direct waves – if they are blocked, that heat transfer stops. So to capture passive solar heat in the winter, and avoid it in the summer, the location of most of the windows on the south side of a structure, and to a lesser degree on the east and west – and proper shading to allow winter rays in and block summer rays out is critical. The right type and size of trees can provide shading or allow winter sun, and overhangs on the windows need to be sized correctly by someone who is familiar with local, seasonal sun angles. While direct radiation can provide some of the needed heat gain, in most climates additional sources are important. One system that works particularly well with concrete is hydronic radiant floor heat. Solar thermal collectors capture the sun’s energy to heat water, which then runs through tubing in the floors to distribute the heat throughout the living space.

Making use of on-site renewable energy strategies requires some upfront investment in site specific design and the selection of building materials, and equipment in the case of solar water heaters, photovoltaics, or other electrical generators. However this represents more of a shift from on-going costs associated with utilities to ‘first’ cost in design, materials, and equipment. On-going utility costs are subject to inflation, global politics and war, environmental degradation and pollution, and will in all cases get higher over time. First costs are either over with and paid back by on-going savings, or financed through a mortgage or loan at a predictable, constant rate for a finite period of time. Often the on-going utility savings can offset an increase in mortgage or loan payments used to invest in on-site renewable energy and efficiency measures.

The Oberprillers started with a hybrid hot water system – flat plat collectors pre-heat the water before it enters an array of 60 evacuated tubes and finally stands by in a 165 gallon tank. Water can get as hot as 275° - even after a couple of cloudy days, it can maintain the temperature at 65°and gas is available for back up. “This first winter, we didn’t have to turn it on until December.” In the summer, the tubes are covered, and the flat plates provide plenty of domestic hot water. “With heating, cooling and hot water covered with minimal electricity, we can add photovoltaics and micro-wind to try to meet or exceed our electric demand.”

“Do some research on ICF experience in your area to find the right people to work with, and educate yourself to be able to answer common questions and concerns. One concern I often hear is that ants eat it. Certain insects will chew on or through anything left out for them, but installed properly the EPS isn’t accessible and they’ll just run into concrete anyway. It is much less vulnerable to insect, mold, and water damage than other materials. It doesn’t cost more when you factor in the speed of installing, reduced labor costs, and operating savings. It is important to plan plumbing and wiring penetrations carefully since they are harder to fix after concrete is poured. But I know this house will still be state of the art for future generations, while most of what we are building now will be a burden to operate.”

The Oberprillers discovered several additional benefits of a concrete wall system as they moved ahead with their project. One was the ease of creating curved walls and other design styles that are difficult to implement with a framing system. The house has several rounded rooms and archways. Quad-lock was willing to work on perfecting the bendability of their system. “This is a high craft house. Now we are starting a simpler new house, a 1000 square feet rental unit. I’m inviting the local vocational-tech classes to learn how we are doing it so that more people can do this in our area.” Another benefit was the quiet that comes from a solid structure and concrete’s noise attenuation. A final benefit was the simplicity of installing an integrated wall structure.

Project Team

Builder/Owner: Barry Oberpriller, Sawmill River Builders

ICF Manufacturer: Quad-Lock