The concept of a net-zero energy home is simple enough: start with a building that’s as airtight and efficient as possible, then provide enough renewable energy to power it. The challenge, however, is that conventional building technologies just aren’t enough to get us there. Together with Klopf Architecture, Matarozzi/Pelsinger Builders is constructing a new home in Cupertino that deploys an innovative set of technologies and construction techniques to get to net zero
Conventional stud walls, even when brand new, are still quite “loose” in terms of air infiltration. The countless field-assembled joints and connections facilitate many unintentional “air changes” per day, putting an unnecessary strain on the building’s heating and/or cooling system. Even when packed to the gills with the best insulation available, the studs in conventionally framed walls are still relatively conductive and function as “bridges” for heat energy to move through the wall.
Instead of traditional framing, the house uses Structural Insulated Panels (SIPs) for the exterior walls. SIPs are an innovative way to make a better wall from the same readily available raw materials. By sandwiching a slab of rigid insulation (EPS, XPS or PU foam) between two sheets of plywood (typically OSB) a structural wall panel is created that is stronger, better insulated and more air-tight than traditional stud walls.
Because they’re pre-fabricated off-site, SIPs are also faster to erect than traditional framing. While on-site installation times are relatively fast, the panels do require an additional degree of coordination prior to fabrication to ensure proper panel dimensions and correct location of wiring chases. If planned and managed well, a SIP house can easily be cost-neutral compared to traditional framing. The final result, however, is a much more efficient building envelope that will save energy (and money) for years to come.
Air Source Heat Pump
Even in mild climates like the Bay Area, conventional boilers and A/C units require more power to operate than can typically be produced with on-site renewable energy. The HVAC system for the Cupertino house employs an electric Air Source Heat Pump for both heating and cooling. Traditional furnaces and air conditioners burn relatively large amounts of energy in order to create or remove heat, respectively. Instead of trying to directly create or remove heat, a heat pump uses a relatively small amount of energy to simply move (or pump) heat from one place to another. Conventional boilers and furnaces are only able to convert about 75%-95% of the energy they use into usable heat. Because heat pumps move heat rather than generate heat, they are able to provide up to 4 times the energy that they consume (click here for more info on how heat pumps work).
Although SIP panel construction successfully creates a far tighter and better insulated building envelope compared to conventional stud walls, it simultaneously creates a new challenge for building ventilation. The looseness of conventional construction allows for many unintentional air-changes per day. While this is clearly a big problem for energy conservation (air leaking in needs to be heated or cooled to maintain the desired indoor temperature), it nevertheless contributes to keeping the house well-ventilated with fresh air.
Simply opening the windows of the SIP panel house is certainly an effective way to ventilate when indoor and outdoor temps are comparable, but on cold or hot days when the Heat Pump is working keep the indoor environment comfortable, the introduction of unconditioned air through windows would only increase the heating/cooling load.
Enter the HRV, or Heat Recovery Ventilation, system. These systems not only pull fresh air into the house and push stale air out, they also recover heat (or "coolness" in the summer) from the stale air in the process. Odors and pollutants can quickly be removed, but energy used to condition the air is recycled in the heat exchanger within the HRV unit. HRV systems can recover up to 80% of heating and cooling energy used to condition the indoor air.
Heat from the heat pump is distributed throughout the Cupertino house via in-floor radiant tubing. Because the heat originates at the floor level and rises, in-floor radiant is far more efficient than ducted forced-air systems that often distribute over-heated air from the ceiling (where it tends to stay). Moderate climates like the Bay Area are ideal for radiant systems powered by Air Source Heat Pumps. Climates that experience seasonal temperature extremes, however, are better suited for Ground Source Heat Pumps that utilize the relatively constant temperature beneath the ground as an exchange medium (click here for more info on Ground Source Heat Pumps).
Insulated Concrete Forms, or ICFs, were used to form the basement-level walls of the house. ICFs are modular EPS foam blocks that lock together like Legos. They take the place of disposable wood formwork and provide a high insulation value for the wall when complete. Using ICFs saves both time and resources, making them a practical and sustainable choice for forming concrete.
Finally, the roof of the Cupertino house will have an 11 kW solar array sufficient to offset all the energy used in the home. The innovative construction methods and technologies mentioned above combine to create a structure efficient enough to achieve a true Net Zero energy use.