Before solving the above scenario, the key assumptions are as follows:
House A Requires 48 solar panels for zero Energy and House B requires 14 solar panels for zero energy.
This is a difference of 34 panels or an estimated cost of $35,000.
An air change is the movement of all the volume of air in a given period of time; if a house has one air change per hour, it means that the air in the house will be replaced in a one-hour period. You can also think about it as how many times the air enters and exits a room from the HVAC system in one hour. Or, how many times a room would fill up with the air from the supply registers in sixty minutes.
You can then compare the number of room air changes to the Required Air Changes. If it’s in or below the required range, you can proceed to design or balance the airflow and have an additional assurance that you’re doing the right thing. When a home is designed for less leakage, it results in lower ACH and lower energy demand.
But, how is ACH or envelope leakage reduced without adding costs, delaying schedule, or increasing labor? Anthony Maschmedt, Principal of Dwell Development in Seattle, believes he has found the solution. (See Table 1.) Dwell builds homes based on the Passive House model, targeting the lowest possible ACH50 (Passive House requires 0.6 ACH or better). Seattle code requires a 5.0 ACH50 or better. This means Dwell’s standard ACH is dramatically better than code. Maschmedt explains, the solar panels and investment needed to reach zero energy depends on energy demand. He says, “What we know is, by reducing demand, less energy is needed to achieve zero energy. A symbol of reduced demand, or energy efficiency, is the HERS Index - but, low ACH is also a symbol of energy efficiency.”
The typical Dwell home achieves a 45 HERS Index before solar, which is 30% better than code. The Dwell energy efficiency strategy is simple, focus on high impact – lowest cost, energy improvements. The typical Dwell energy strategies include:
(Note: Seattle’s energy source is mostly hydropower.)
Included in the Dwell energy efficiency strategy, constantly striving to implement innovative strategies. Design and product innovations are needed for Dwell to push the energy envelope to their high expectations. For example, Dwell’s innovative wall system is R-40. Meanwhile, code requires only R-21. Maschmedt says Dwell is always considering ways to lower the carbon footprint of the homes they build, including the carbon footprint of the occupant; a recent Dwell multi-family project has no parking, targeting those who will rely on public transportation or bicycle.
Maschmedt believes he has uncovered the key to the next generation of building performance for Dwell homes; his most recent net-zero energy home achieved a mind blowing, 0.22 ACH50, or nearly zero leakage; dramatically better than Seattle code, or 5.0 ACH. And, nearly three times better than the Passive House requirement of 0.6 ACH.
As of February, Dwell has made advanced envelope sealing standard practice. This is accomplished by adopting AeroBarrier, an aerosol spray developed by the University of California, Davis. The AeroBarrier mist fills the house and seals any remaining leaks that would normally escape visual inspection. By making the envelope extremely tight, mechanical load and energy demand is dramatically reduced. Minimizing demand means the homes do not need much solar power to meet zero energy. (See table 2.)
Tadashi Shiga, CEO of Evergreen Certified, is the energy rater for all Dwell projects. Shiga says prior to AeroBarrier, and when targeting Passive House leakage requirements, it meant weeks of air sealing — literally. Shiga said the old strategy for meeting 0.6 ACH or better, added countless man-hours to the project and a construction delay of two weeks or more. Why the delay? The Passive House leakage requirement of 0.6 ACH50 meant a strenuous search for the tiniest of leaks, including those not visible to the eye. The search included filling the home with smoke, pressurizing the area, and then observing the exterior of the project for points of smoke escaping. Shiga says the ‘smoke routine’ was sometimes conducted two or three times over three weeks before realizing the 0.6 ACH. Before insulation and drywall, the average Dwell project achieves a .8 – 2.0 ACH. Shiga says before AeroBarrier there was anxiety surrounding every blower door test, not knowing if the project would be tight enough. He says, "it was always a nail-biter and point of anxiety for the entire team. The sealing process has reduced from weeks to just hours.”
The former real estate broker turned energy rater, Shiga says the market has responded to Dwell’s extreme energy efficient homes; realizing a 5-10% premium compared to the market. The average price per square foot for a Dwell home is $525. Dwell homes sell between $620k – 1.4M.
One might assume the Dwell strategy for meeting the most stringent air leakage is justified by a premium sales price. But, the formula for a tightly sealed envelope as a means to achieving zero energy is being rolled out on 500 new homes in Arizona, priced as low as $240k, or less than $120 per square foot.
Mandalay Homes broke ground on Mountain Gate in Clarkdale, Arizona. Every home will feature near-zero energy as a standard. How is it possible to offer near zero energy homes at a relatively low sales price without relying on incentives or a buyer premium? Mandalay’s Founder is Dave Everson. Everson responds to the critics of zero energy by saying, “with innovative air sealing technologies, affordability of solar, and incorporating storage, we are able to scale zero energy on every home we build, no matter how low the sales price, and without passing a premium onto the homebuyer.” (See Table 3, provided only as an example, and applies to the Mandalay model only.
Actual numbers require modeling and vary by project.)
Could advanced envelope sealing become the new standard, especially for builders targeting zero energy? Dwell and Mandalay are early adopters, but interest is growing. Especially, in states that pay a premium for electricity.
There is no better way to make zero energy and solar more affordable than to begin with less energy demand. Now, reduced energy demand is made affordable and practical with new technologies like AeroBarrier.
There is some controversy surrounding overly tight homes.
Mandalay and Dwell install balanced ventilation with heat recovery. At a minimum, when designing airtight homes, balanced ventilation is required, as is properly sizing the mechanical system.
Balanced ventilation with heat recovery introduces fresh air and exhausts indoor air with an air-to-air heat exchanger. With this, the outgoing house air will precondition the incoming outdoor air. This air-to-air heat exchanger—more commonly referred to as a heat-recovery ventilator or HRV is great benefit in colder climates.
A slightly different version, known as an energy-recovery ventilator (ERV), is similar but transfers moisture as well as heat from one airstream to the other, keeping a more desirable humidity in the house in the winter, and reducing the amount of humidity introduced from outdoors in the summer. Mandalay uses ERV and Dwell uses HRV.
For renewable energy to be financially scalable and not dependent on incentives, it begins by building a home that requires less energy to operate. The key is a tight envelope.
Mandalay Homes is using the formula to meet the demands of the consumer. Everson says, “tell me a homeowner who does not want a near-zero electric bill, and without costing them extra money or a premium sales price?” An added bonus to the super efficient and tightly sealed homes includes particulate and noise control. Such control becomes ever important in unique applications, such as hospitals, hotels, and multifamily compartmentalization.
Builders no longer need incentives or fancy financial schemes to make zero energy homes affordable to buyers. As market innovators such as Dwell and Mandalay are demonstrating, the quickest and most practical discount on solar panels is reducing envelope leakage.
Paul Springer, MBA CPM, is a business-builder for Aeroseal, LLC.
The Journal of Ventilation published the article, "Recent Applications of Aerosol Sealing in Buildings" by Curtis Harrington and Mark Modera in March 2014. (Vol 12, No. 4).