ALL HOMES WILL EVENTUALLY BE ZERO NET-ENERGY

The Rocky Mountain Institute states in their 2018 paper “Economics of Zero-Energy Homes” that; “… ZE (Zero-Energy) and ZER (Zero Energy Ready) homes comprise less than 0.1% of the current US housing stock”.  Deduct from this figure the ZER component (which likely is the largest component), and further deduct any house that uses fossil fuels, and you are left with a very tiny (unknown) number of net-zero energy, zero carbon homes.  One can also visit the Net-Zero Energy Coalition (www.netzeroenergcoalition.com) and the New Building Institute (www.newbuildings.org), both of which attempt to track these types of numbers, but their figures are more useful in gauging trends, not absolute numbers.  Although the present number of zero net-energy (“ZNE”) homes is low, eventually nearly every new single-family house will be ZNE.  Here is why…

Houses built to circa 2019 building stretch codes in Massachusetts are nearly ZNE if a renewable energy source (such as a PV system) is added.  In other words, the 2019 stretch code is almost to the point where a house built to that code would be considered net zero ready.  For example, THE HAYFIELD HOUSE reached ZNE in a manner that (except for the solar PV system and thicker insulated walls) was not that much different than if the house had only met current stretch code requirements.  In addition, THE HAYFIELD HOUSE didn’t ‘cheat’ its way to ZNE by merely pairing an energy inefficient house with an overly large PV system.  Most ZNE houses are designed to reach an energy usage index (“EUI”) of below 20 (preferably 10-15).  THE HAYFIELD HOUSE met this target in 2021 (EUI 15.6) and again in 2022 (EUI 14.04).

If we look at the differences of getting to ZNE between THE HAYFIELD HOUSE and a ‘standard’ house, much of the differences are due to the design elements and the PV system, and not due to extraordinary measures.

First, let’s look at the elements of THE HAYFIELD HOUSE (Building Metrics) that are the same as a circa 2019 stretch code compliant house, and/or reasonably consistent with materials and appliances already used in most new houses.  THE HAYFIELD HOUSE:

• Employed all Energy Star appliances and LED lights to reduce electrical demand.  I’d argue that for new construction, this is virtually the current norm.  For example, the cost of LED lights have fallen so much that it makes little sense to use other lighting choices.  In fact, it is now very difficult to buy lights that aren’t LEDs.  Also, for new construction, the house will likely come with all new appliances which (even if not Energy Star rated) are much more efficient than older models. 

• Used double pane windows (not triple pane) windows.  Double pane windows are what are used in virtually all code compliant homes.

• Used a standard electric hot water heater; which is actually less efficient than most houses in New England which use natural gas to heat water.

• Insulated the roof to an R value that barely meets the current stretch code for roof insulation.

• Insulated the concrete slab’s edge to R10 value which is equivalent to code.  (Although the underside of the slab and the exterior of the slab are insulated to R10 and R5 respectively which is above code.)

Next, let’s look at the differences between THE HAYFIELD HOUSE and a circa 2019 stretch code compliant house. THE HAYFIELD HOUSE:

• Has an air infiltration rate of 0.98 ACH@50, whereas stretch code requires less than 3 ACH@50.  I’d argue that this lower infiltration level of about 1.0 ACH@50 is probably already within reach of most code compliant houses being built in New England.  My personal observations are that many houses are being built using the Zip panel system, which should produce a tighter shell than the Tyvek system that was used to wrap THE HAYFIELD HOUSE.  In addition, I’m noticing a large shift to the use of spray foam insulation and away from fiberglass insulation. Spray foam produces a much tighter home. Consequently, most of the incremental ‘cost’ of going from ACH 3 to 1, is probably largely just a matter of diligently plugging leaks as already mandated by code.  I’d also argue that a lot of air leakage is merely a result of sloppy work.

• Used an air source heat pump, mini-split heating (and cooling) system.  Most houses in New England use natural gas or oil fired heating systems.  But, there is a growing trend to use air-source heat pumps in New England, especially when they can be coupled with a solar PV system.  My guess is that this trend will continue, especially due to tax incentives and the downward trend in the cost of solar panels.

• Used a double wall construction that allowed for much higher insulation levels in the walls than is required by code.  I doubt the ‘standard’ house will move to a double wall construction, however some ‘standard’ houses are already using insulated sheathing, such as insulated Huber Zip panels to increase the wall insulation and reduce thermal bridging.  In short, higher wall R values is an incremental cost problem, but not a big design problem.

• Used a whole house energy recovery ventilation system (“ERV”) to improve air quality while reducing heat loss.  This system is expensive and was not required in MA for a 2019 stretch code compliant house.  Instead, the then applicable code required a tightly air-sealed house (such as the HAYFIELD HOUSE) to have at least two exhaust fans running constantly.  This code requirement is good for improving indoor air quality but runs counter to the goal of minimizing air leakage.  Anyhow, this prior regulatory silliness is being fixed in the MA 2023 code whereby instead of requiring simple exhaust fans, the code requires single unit ERV or HRV systems.  Granted, the end result would not be as good as using a whole house ERV system as employed in the HAYFIELD HOUSE, but using single unit ERV or HRV would address most of the air circulation problems at a reasonable cost.

That then leaves us with the biggest difference between THE HAYFIELD HOUSE and a standard house; the PV system.  Most builders (unless forced to by future regulation) are unlikely to invest $60K that it cost to install the 16.64kW system on THE HAYFIELD HOUSE.  Granted, the cost of the PV system expressed after tax, after incentives, after net-metering, etc., was quite reasonable when analyzed on a net present value basis.  But, most PV systems don’t make much sense if analyzed on a pre-tax, pre-incentives basis, with many potential buyers experiencing sticker shock when seeing the upfront cost to install a PV system.  Another problem is that many houses don’t have enough unshaded roof space to fit the 52 solar panels that were used on THE HAYFIELD HOUSE.

So what needs to happen in order to meet my thesis that virtually every house eventually will be ZNE?

• The building codes need to tighten to require more wall insulation, lower air infiltration rates, and the inclusion of ERV or HRV single unit fans (not necessarily whole house systems).  As the regulators seem to be on a perpetual three-year cycle to update building codes, I’d expect we’ll see further tightening of the codes which will result in a house that could reach ZNE if it only had a PV system.  Part of this tightening will be a requirement for better design to capture passive solar energy and allow for enough south facing roof area to install solar panels.

• Better insulated windows would be a huge benefit to reaching zero net-energy.  Currently, windows are the weak link in creating a well-insulated house.  Most current double pane windows are only R3.  Fortunately, technology will save us on this front.  There are super windows in development^[1] and some manufacturers are already selling windows with R values in the range of 8-10.  The barrier to installing super-efficient windows is mostly due to their high cost.

• That then leaves us with the problem of the high cost and roof space requirement of the PV system; but technology will again save us.  The current efficiency of production solar panels tops out at about 23%.  (THE HAYFIELD HOUSE’s panels are 19% efficient).   Unfortunately, using current silicon-based technology, the theoretical maximum efficiency is capped around 30%. It would be great if we could roughly double the current solar efficiently to reach at least 40%.  That would mean THE HAYFIELD HOUSE would only need half as many panels (i.e. only 26 panels).  Most new houses (with a proper roof design) can easily accommodate 26 panels, which solves the space limitation on most roofs.  But how do we get to 40% efficiency?  We eventually need to move away from silicon-based PV solar panels to other materials that have higher efficiencies.  Such materials are already in development.  For example, solar panels based on Perovskites have efficiencies that are expected to eventually reach 40%.  Other futuristic materials (such as Nanotube based panels) are touted as theoretically reaching 90%.

• Over the last several years, solar panel costs have plummeted, largely due to economies of scale as more solar panels are produced.  I expect this trend to continue, making an investment in a solar PV system less of a sticker shock for most homeowners.

In summary, the short answer of why all houses will eventually be net zero is that over time building material technology will improve (and costs will fall) to the point where it would be hard not to build a ZNE house.  Furthermore, regulation will continue to push us in this direction. — Technology is the key.

So the moral of this story is:

If you wait long enough, anything can happen.

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[^1] Berkeley Lab, “Super Window Could Save $10 Billion Annually in Energy Costs”, June 2018