Thinking About Net Zero Energy
Do investments in photovoltaic arrays make sense?
UPDATED March 28, 2013
The average new home is so poorly built, it’s enough to make an environmentalist weep.
Windows are routinely installed without any consideration of orientation. As a result, south windows fail to take full advantage of free solar heat during the winter, while west windows worsen summer overheating. Windows are often installed in unshaded walls, even in hot climates. In the absence of legal requirements for high-performance windows, builders regularly choose windows with appalling U-factors and solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. coefficients (SHGCs).
In the southern U.S., air-conditioning ductwork is still routinely located in attics. Most builders insulate walls by hiring a low-bid subcontractor to stuff fiberglass batts between the studs. In the U.S., unlike in Sweden, most new homes receive occupancy permits without ever undergoing a 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..
Builders don’t pay energy bills
Yet the low standard achieved by U.S. builders is understandable — even logical. Many builders ask themselves, “Why should I pay for foam 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. ?” After all, builders don’t pay heating bills — homeowners do. In other words, the interests of builders and homeowners are not aligned.
This divergence of interest between builders and homeowners is best addressed by ratcheting up our energy codes — codes which currently allow new homes to be shockingly leaky and poorly insulated. Last summer, a coalition of partners — including governors, mayors, and the U.S. Department of Energy — gathered in Minneapolis to propose energy code changes that would reduce energy use in new homes by 30%. The proposal was defeated after vigorous lobbying by the National Association of Home Builders.
Now that you’re depressed …
Fortunately, some far-sighted builders are already implementing cost-effective measures that reduce homeowners’ energy costs, including:
- Insulation (like cellulose) that performs better than fiberglass batts;
- Exterior foam sheathing that interrupts thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. ;
- Very thick attic insulation;
- In cold climates, windows with a very low U-factor;
- In hot climates, windows with a very low SHGC;
- In hot climates, features like porches to shade west-facing windows;
- Ductwork located within a home’s conditioned envelope; and
- A meticulously detailed 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..
Assuming these measures are well chosen and properly implemented, they should all be cost-effective. In other words, the incremental cost of these measures will be more than offset by future energy savings.
The problem with solar electricity
Once a house has been designed to minimize energy bills, some builders are eager to go further. They dream of building a home with a rooftop photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) array that produces enough electricity to meet the home’s annual energy needs.
There’s at least one problem, however, with the pursuit of net-zero-energy homes: PV systems are not cost-effective. In other words, PV is a very expensive source of electricity.
Successful net-zero-energy homes have PV arrays in the 6 kW to 9 kW range; such systems generally cost $48,000 to $72,000. These costs are so high that they raise the question: Why do homeowners decide to saddle themselves with very high electricity costs?
[Author's note (March 2013): Since this article was written in early 2009, the price of PV modules has dropped sharply; in fact, prices have been cut in half. The cost of a 6-kW PV system has dropped from $48,000 to $24,000. Needless to say, falling prices for PV modules are a game changer. For more information on this topic, see PV Systems Have Gotten Dirt Cheap.]
There are at least two reasons that grid-connected homeowners choose to install a PV system:
- Some homeowners are willing to pay more for energy that comes from an environmentally benign source; and
- Many homeowners receive PV incentive payments, so they don’t have to pay for the true cost of their solar equipment.
Tax-credit and rebate programs established by utilities, state governments, and the federal government shift a portion of these PV costs to utility ratepayers and taxpayers.
While these subsidy programs are a financial boon to PV manufacturers and installers, it’s worth considering a few points:
- PV incentives disproportionately benefit middle-class and upper-class homeowners.
- By artificially lowering the cost of expensive technology, PV incentives warp investment decisions and draw money away from more logical investments like efficient appliances and improved ductwork.
- As long as the government neglects investments in low-hanging fruit — for example, by failing to enact improvements in residential energy codes — it makes little sense to invest in energy-efficiency measures that are demonstrably not cost-effective.
Going forward with our eyes open
Of course, there’s no shame in choosing to spend money on measures that aren’t cost-effective. It’s useful, however, to do so with our eyes open. My advice: if you’re going to install a residential PV array, be sure that the house incorporates every efficiency measure with a shorter payback period than PV.
Such a house will need to be designed with a much sharper pencil than typical new homes. Anyone contemplating a $25,000 investment in PV should first investigate what could be achieved by investing an equivalent sum in improvements to the home’s thermal envelope.
Last week’s blog: “Can Swimming Pools Be Green?”
- Riverdale NetZero Project
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