The Risks of Building Net Zero Energy Homes
High-performance enclosures can pose higher risks to builders. Here’s how to assess and mitigate them.
This is Sam Rashkin’s latest in a series of articles based on his second book, “Housing 2.0: A Disruption Survival Guide.” It is intended as a roadmap for high-performance builders to become the most successful in the industry.
While recruiting engaging and supporting builders with the U.S. Department of Energy (DOE) Zero Energy Ready Home (ZERH) program, I would always lay out the greater risks when constructing high-performance enclosures that are critical to ZERHs.
When senior management became aware of this message, they got extremely upset. Viscerally upset. How could the chief architect of the Building Technologies Office be telling housing professionals that zero is greater risk, and impugn DOE’s efforts at promoting more rigorous codes requiring high-performance enclosures?
The answer is that early failures are devastating with any market transformation effort, and the truth had to be shared. That’s what I’m doing with this article.
Net Zero Building Is Here to Stay
Before I get to the higher risk, it’s critical for all housing professionals to understand that “zero has left the station.” Consider the following observations:
According to RESNET, more than 300,000 new homes were HERS rated in 2021, with an average HERS Index Score of 58.1 This is significantly better than the average HERS Index score of 70 for the 2015 IECC code.
Speaking of the IECC codes, they have gotten 40 percent more rigorous since 2009. The latest codes (2015 and greater) meet the national program requirements for ZERH.
Seven states and territories have announced plans to use zero energy codes: California, Oregon, New Jersey, New Mexico, New York, Puerto Rico, and Washington.3 Of course, California’s zero energy code is already underway.
Eighteen states have 100 percent clean energy or renewable energy policies substantially aligned with zero energy buildings.
As of 2020, nearly 28,000 housing units had been certified to zero energy and zero energy ready performance programs.
The Inflation Reduction Act locks in the 45L Tax Credit from 2023 to 2032, with $5,000 per single-family home and $1,000 per unit for multi-family homes certified to ZERH.
Environmental, social, and governance (ESG) factors are increasingly driving investment decisions (follow the money). It is already attracting 10 publicly owned builders to prepare reports. Zero energy performance will be a significant opportunity for these builders to attract investors.
Maybe even more compelling than any of the above indicators is the superior user experience (UX) associated with ZERHs compared to code homes, including:
- no/ultra-low energy bills.
- comfort taken to a new level.
- enhanced convenience.
- low maintenance.
- greater safety.
- healthier living.
- enhanced resilience.
When these factors are integrated with best practices for UX-optimized communities and design from Housing 2.0, you get new zero-energy homes that are the equivalent of the Tesla car. Everyone will want one.
Why is a Zero Energy Home a Greater Risk?
When performance is significantly ramped up with any product, higher risk typically comes along for the ride. Consider airplanes: Going from open-cockpit single-engine prop planes to modern jets was a huge jump in performance, including faster, better, and more comfortable travel.
But instead of flying at 10,000 feet elevation at 140 miles per hour, modern jets fly at 35,000 feet elevation and 500 miles per hour. This higher performance results in far greater risks, with less oxygen at higher elevations, more structural stress with dramatically higher wind loads, incredibly low temperatures (e.g., minus 70 degrees Fahrenheit), and more exposure to collisions with thousands of planes flying at high speeds.
Of course, we didn’t reject modern jets because of these greater risks; we managed the risks. Airplanes now have pressurized cabins and oxygen masks, incredibly strong and lightweight structural framing, airtight and well-insulated cabins, and advanced radar warning systems to avoid collisions.
Today, airplane travel is statistically the safest mode of transport, with just 0.07 deaths per 1 billion passenger miles. The most popular form of transport, cars, has 100 times greater risk.6
The same increased risk scenario plays out when moving from low-performance enclosures to high-performance enclosures in ZERHs. The superior UX with ZERHs includes ultra-low utility bills, plus more comfort, health, durability, and safety. However, just like with modern airplane travel, high-performance homes and there substantially reduced thermal and air flow, resulting in outcomes that bring far greater risks, including (see figure below):
- Colder surface on the cavity cold side increases moisture risk
- Ultra-low dilution increases indoor air quality risk
- Ultra-durable home increases future readiness risk
The following paragraphs explain each of the resulting risks and how they are easily managed in zero energy and zero energy ready homes.
Moisture Risk In Zero Energy Ready Homes
Because the inside surface on the cold side of a high-performance construction assembly experiences much less thermal and airflow getting to it, it is much colder than with a low-performance enclosure. This means it has much greater wetting potential with greater propensity for temperatures below the condensation point, and much less drying potential, with less thermal flow to dry it out.
In cold climates, this risk occurs on the inside surface of exterior wall sheathing. In warm climates, it occurs on the inside surface of interior drywall.
As a result, any bulk moisture leaks create greater risks in cold climates for moisture damage, and any air flow leaks that allow warm air to reach the cold surface create greater risks of condensation in all climates.
Managing these risks requires diligence with all four building science control layers: airflow, thermal flow, bulk moisture, and moisture vapor. It is precisely this moisture risk that explains the title of Joe Lsitburek and John Carmody’s building science book, The Moisture Control Handbook. It is an outstanding reference for this moisture risk management.
Moisture risk is also managed with HVAC best practices that locate heating and cooling equipment and ducts inside the conditioned space, and tightly air seal and properly size the ducts. Lastly, whole-house ventilation is important for diluting internally generated moisture (e.g., breathing, cleaning, cooking, and washing), but the impacts of bringing in outdoor moisture during humid weather also have to be fully considered.
Seven states and territories have plans to move forward with zero energy codes, while 18 states have 100 percent clean or renewable energy policies aligned with zero energy buildings.
Is There a Comfort Risk in Zero-Energy Homes?
High-performance enclosures that are air-tight and well-insulted result in dramatically reduced heating and cooling loads. This significantly increases comfort risks in a housing industry where quality HVAC installation has been a huge problem.
Research on hundreds of homes by the National Renewable Energy Laboratory revealed that 70 to 100 percent of field measure HVAC systems evidenced at least one performance-compromising fault, including 31 to 93 percent with oversized equipment, 29 to 78 percent with low refrigerant charge, 67 to 100 percent with duct leakage, and 50 to 93 percent with inadequate airflow.
Ultra-low HVAC loads could be expected to significantly increase risk with an HVAC installation infrastructure that is quality challenged. This is because there will be substantially less air flow in ducts that have a high propensity for sizing and air leakage issues, shorter cycles resulting in coils that are less likely to be cold enough to meet latent loads where there is a high propensity for over-sizing, and longer swing seasons that may reduce sensible cooling on the front and back end of the summer cooling season when significant latent cooling is still needed.
Again, these risks can readily be managed with HVAC quality installation best practices for ensuring proper airflow, refrigerant charge, and duct airtightness. Further, builders add insurance by using variable-speed equipment that can throttle up and down and effectively mitigate issues with oversizing. This may also protect production builders who seek to specify one size per HVAC system per model, where variation in orientation can result in more than a one-ton variation in sizing requirements.
And lastly, dehumidification, which has been ignored or addressed as an afterthought, needs to be an integral part of comfort systems in all climates with humid summer conditions, including northern states that are getting increasingly hot and humid summers with climate change.
Indoor Air Quality Risk In Net Zero Homes
Substantial progress has been made in improving airtightness in all homes constructed today. ZERHs are becoming ultra-airtight. As a result, contaminants introduced in homes will accumulate without adequate whole-house ventilation. These contaminants include those embedded in construction materials (e.g., paints, carpets, sheathing, cabinets, adhesives) and those introduced by occupants (e.g., furnishings, shoes, cooking, moisture, and cleaning products).
This risk is managed first with good air sealing best practices that keep outdoor contaminants from entering the home (e.g., dust, pollen, pests, and moisture), HVAC quality installation best practices that ensure airtight ducts inside the conditioned space and proper air balancing, and a comprehensive indoor air quality strategy that ensures source control, dilution, and filtration.
The best recommendations for ensuring good indoor air quality are to certify all homes to the EPA’s Indoor airPLUS program, along with educating homebuyers about effective behaviors for healthy living (e.g., using kitchen exhaust fan when cooking, using bathroom exhaust fan when bathing, taking shoes off at the entrance, vacuuming carpets regularly, checking furnishings for dangerous fire retardants, and cleaning gutters).
Surprising Risk for Future-Ready Homes
Of all the risks discussed here, this one may be the most surprising. If a home is “ultra-durable,” why is “future ready” a risk? The answer is that a ZERH with a high-performance enclosure is now ready to last longer, with greater durability and being able to meet future home buyer expectations.
With these capabilities comes additional responsibility to stand the test of time—this is a home that should last hundreds of years. However, there are now far greater risks to this persistence associated with rapidly increasing disaster risks, drought-induced water risks in a growing part of the country, and a major movement to all-electric homes. All ZERHs should be prepared to meet these future challenges.
These risks can be readily addressed by integrating comprehensive resilience strategies (e.g., Institute for Business and Home Safety’s FORTIFIED Home Program), water conservation strategies (e.g., EPA’s WaterSense certification), and electric-ready capability including high amp outlets for fossil-fuel appliances (e.g., space heating, cooktop, oven, and clothes dryer), solar ready measures in homes with significant access to solar insolation (e.g., ZERH solar ready checklist), electric vehicle ready (e.g., wiring for Level 2 chargers), and battery storage ready (e.g., 225 amp busbar, back-up power circuits, transfer switch, and subpanel).
We know for certain this is the future of housing. Why bake in obsolescence in the ultimate consumer product that should last more than a hundred years?
The benefits of Zero Energy Ready Homes (ZERHs), such as this Department of Energy Housing Innovation Award finalist from 2021, include ultra-low utility bills plus more comfort, health, durability and safety. Courtesy S.D. Jessup Construction
The Risk Managed Zero Energy Home Building Blocks
Now it is time to put all of this risk management together and start building ZERHs that will stand the test of time. Why? Because people, communities, and the planet will all live better.
What we must follow is a four-step process outlined in the figure below.
- Optimize efficiency. This includes an energy-efficient enclosure, equipment, and components throughout the home.
- Minimize risk. Now that we have a high-performance enclosure, it is critical in the next step to manage the risks discussed earlier. This includes comprehensive water protection to minimize moisture risk, ensured comfort system to minimize comfort risk, and a comprehensive indoor air quality system to minimize health risk.
- Be future-ready. Integrate regionally appropriate resilience strategies to address prevailing disaster risks, comprehensive water efficiency in locations likely to experience water shortages, and electric-ready features that allow a smooth transition to all-electric homes at minimal cost and disruption.
- Get on the path to zero carbon. Start with the basic high-performance home programs: ENERGY STAR, Indoor airPLUS, and WaterSense. When you’re ready, move up to zero energy-ready performance with ZERH or Passive House. Next is zero energy homes with on-site renewables or micro-grids (e.g., special recognition labels available from PHIUS and soon ZERH). Complete the journey with zero-carbon homes that are all-electric and minimize embodied energy. There should be no judgment about where you are for any step of the process. The important part is to begin the journey and to manage your risks.
The next Housing 2.0 workshop is November 11, live at The University of Denver. Click here to attend.