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Posted by Grant in Blacksburg, VA on 9/6/2008

Are Zero Energy Homes Possible? Practical? And do they provide a payback?

A LinkedIn Discussion group [Energy Priorities "Thought Leaders"] posted the above question and I thought I would share my answer here, particularly since I referenced this website.

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ZEH is possible. The biggest key is actually not the on-site energy production part of the equation, but energy conservation. However, even with the highest energy efficiency possible, ZEH does not have a direct monetary payback in most parts of the country. Distributed energy costs are still lower than on-site production.

It does pay to build a home capable of becoming a ZEH home, however. The energy savings of a high efficiency, "near ZEH" will have a monthly payback, as well as being a naturally more comfortable place to live. Additionally, the majority of the cost of living will be "fixed" into the mortgage payment allowing the homeowner to secure their standard of living into retirement, without having to fear future changes in the cost of energy. Such a "near ZEH" can be built today with a lower monthly cost (mortgage, insurance, taxes, and utilities) than for a less comfortable traditional home.

I am designing a high-efficiency "near ZEH" with a tight envelope and high solar mass. I am incorporating solar tubes for daylighting. I will be using CFL fixtures and the MOST efficient "energy star" appliances. I will be installing solar water heaters (a solar technology that provides payback today). I will be using a "home run" PEX manifold system to further conserve hot water (despite the essentially "free" solar hot water) and water, in general. I hope to also install a grey water system and a cistern for irrigation water. I also intend to rely heavily upon xeriscaping principles to control life-cycle costs of the landscaping. Additionally, I am considering using pervious paving to reduce the stormwater impact of impervious surfaces.

I am going back to antebellum architectural principles with a wrap-around porch and a belvedere and cupola. Antebellum architects here in the south knew how to make a house as comfortable as possible with minimal energy consumption. Combined with modern advances in tight construction and humidity control, antebellum design can lead to exceptionally energy-efficient homes today.

The belvedere (above the living areas) will have windows on all four sides to intentionally heat the air and cause it to rise out of the cupola. With all other windows in the house closed, this "solar chimney" will pull air in from the cooler north basement wall windows and circulate the air throughout the house for passive cooling. I will also install an attic fan at the base of the cupola to actively flush heat from the house during cool summer evenings. (One word of caution about such passive cooling and ventilation with exterior air... air quality concerns can frequently override the reduced cooling load benefits. Exterior humidity levels and allergen concentrations must be considered prior to "open" ventilating with exterior air.) Such passive-cooling measures will significantly reduce the total number of active-cooling days for my climate.

The south side porch of the home will shade the interior during the summer as a screened porch (windows opened), and will be closed in the winter to function as a solarium (passive-solar thermal collector). I've designed the air flow of the house to permit natural convective air circulation when all of the windows are closed. This will cause cool air to be pulled from the basement as the solarium heats air on the south side causing it to rise. Replacement air will be pulled back into the basement from the relatively cooler air falling on the north side of the home. In this manner the thermal energy collected on the south side will be able to naturally, convectively circulate to the whole house. The solar mass in the home will help moderate the temperature (prevent uncomfortable temperature swings) and reduce maximum required cooling and heating loads when active conditioning systems need to be used.

While I am incorporating passive heating and cooling design, here in the southeast, humidity control will be my biggest energy consumption obstacle. 50% of the cooling loads of a home in the southeast can be attributed to dehumidification of the air. To avoid mold and mildew issues from high interior humidity, systems must be sized to run long enough to control humidity. Also, once air is dehumidifed, you don't want to let in high humidity air. Even if the evening is cool, it may actually cost more conditioning dollars to open the windows to let in cool night air, that also happens to be high in humidity. You really have to put some thought into it before you use the whole house attic fan if you are trying to save money. Often, it is actually cheaper to leave the conditioning system running and keep all of the windows closed and the house sealed even if it is a "comfortable temperature" outside. Only when the conditions outside are favorable for extended periods (weeks or months) may it make sense (cents) to turn off the conditioning system and open up the house.

Unfortunately, even with efficient passive cooling design, humidity control (which is required for a healthy living environment) in the south still demands active energy-consuming cooling systems. This is our biggest obstacle to achieving a cost-effective ZEH in the southeast. Dry climates such as the southwest can have significantly lower costs for climate control, and can more readily take advantage of favorable ambient air temperatures which makes such climates more practical today for achieving a ZEH. Many areas of the southwest and the coastal regions also have abundant (and relatively cheap) wind energy and/or geothermal energy available. The southeast only has solar energy, and that requires careful siting; taking into consideration our obstructive rolling hills and forests.

With the rising hot air, which exits out of the cupola and which holds higher moisture content than the cooler entering basement air, I will receive some passive humidity control, particularly in the summers. (Antebellum homes were not airtight and many were designed to pull cool (lower humidity) air from the crawlspace which would warm up and exit out of the attic cupola taking additional humidity with it. The high air-infiltration and air-exchange rates of these older homes thereby helped to control humidity and provided healthier interior air. Many an antebellum home has been ruined by insulating and sealing it in the name of energy efficiency. Many "modernized" antebellum homes have had to contend with rot, mold, and mildew caused by resulting higher interior moisture content.)

I'm hoping the new solar thermal recharged-desiccant waterfall technology (University of Maryland research project) will advance sufficiently before I build to provide a commercial, near zero-energy solution to the humidity problems we face here in the southeast. With such a system, a ZEH would become almost as practical in the southeast as it can manage to be in the southwest. (Although we still don't have as many on-site energy production options as the southwest.)

I am considering pre-plumbing a PEX radiant heating system into my basement slab and into other floors as practical. Pouring a stamped concrete floor instead of using stone tiles may make the poured concrete flooring cost-beneficial to enable the radiant floor. My house design has plenty of south facing roof to enable solar thermal collectors to heat my radiant floor system. While heating loads aren't as critical as cooling loads in my climate, every bit of energy savings is a plus in a grid-tied home. Unfortunately, on-site energy production will still need to be sized for the controlling cooling loads in my climate, so additional heating-load savings won't reduce the expense of on-site energy production needs.

My house will be rear southern facing with an 8:12 roof slope (ideal for solar collectors in my region). I will install a metal SIP roofing system with a 16" standing seam metal roof. I will pre-wire for photovoltaic panels, even though I won't install them at the time I build. Power here in the southeast is still among the cheapest in the nation and the payback isn't there for installing PV today. (In locations where wind power is available, wind energy can be surprisingly competitive. Unfortunately, we don't have adequate wind resources here.) However, with the roof at the proper slope and the house properly oriented to an open southern exposure, I can adhere a sufficient number of PV sheets to the standing seam metal roof in the future to handle most of my on-site electricity generation needs to someday make this a zero energy home. While the standing seam metal roof will cost more initially, it will have a lower life-cycle cost than most other roofing systems, and the relatively inexpensive future mounting of PV sheets versus expensive racks with heavy PV panels, makes it a wise upfront investment in my opinion.

Because my home will be so energy efficient in its design, the payback for super-high efficiency systems such as a geothermal heat pump and an energy recovery ventilator will be even longer than for a normal home design. I have yet to decide if the cost/benefit will be there over the life of the home. If I intend to someday make it a ZEH, then the resulting smaller on-site energy production costs will more than pay for the higher costs of such energy saving features.

One thing I am discovering is that general contractors charge a premium for the "learning curve" for doing such non-traditional construction. Building such a home, being an Owner-Builder pays even bigger equity dividends than with typical construction. I am looking at using highly engineered, pre-manufactured systems that provide greater quality control and reduce on-site labor costs. I am considering Owner-Builder friendly technologies that simplify residential construction project management. So far, I like either TF Systems' vertical ICF or All Wall systems for my exterior walls. I like metal SIPs for the roof. These systems can be pre-assembled (and better quality controlled) in a warehouse and rapidly constructed on site.

In certain states with significant government incentives for investing in renewable energy the homeowner's return on investment for a ZEH can at least come close to happening today (but this is via a government subsidy and not reflective of "true cost," but then again the "true cost" of our distributed energy is also government subsidized!). Unfortunately, my state has no incentives beyond the national incentives that may be expiring soon.

I know of several owner-builders on the OwnerBuilderBook.com website who have already built near ZEHs that could easily be converted to ZEHs once the payback is there for investing in on-site energy production. The OBB website also has a handful of owner-builders who built ZEH homes in off-grid sites where the "payback" and "practicality" of ZEH wasn't the biggest consideration. In their case, gaining access to distributed energy would have cost more than investing in on-site energy production.

So to summarize a very long comment... Yes, ZEHs are possible today. They are currently only practical in a few select environments. But it is immediately practical to build a house so that it can be converted to ZEH when the payback begins to make sense as energy prices continue to rise. Energy efficiency has payback today and makes future conversion to ZEH practical. (For every dollar spent in improving energy efficiency you save multiple dollars in the cost of on-site energy production needs.) Besides, energy efficiency is already environmentally preferable and has additional immediate indirect benefits for all mankind. But building such a home takes a personal commitment and a somewhat impractical learning curve and significant owner-builder involvement and oversight, unless you can afford to reduce your payback by paying a premium to have an expert do all of that for you. Unfortunately, the average architect and general contractor are not prepared to design and build such a home. I've had to do most of the research and design work myself. I couldn't afford to pay someone else to do it. It is understandable that not everyone would have the time or commitment to invest in learning such things the way I have. It has become my most time-consuming hobby.

I hope to break ground within 18 months. I will be meeting with my architect in October. I will get a return on my initial investment immediately [a much higher appraisal than cost to build, a lower composite monthly expense (for mortgage, insurance, taxes, and utilities), a rising resale value as the energy efficiency is proved out with annual energy bills, and additionally the payback of an extremely healthy and comfortable home], and do not plan on adding PV to make it a ZEH until I am able to get a comparable return on that investment as well. So yes, I will see a return on investment from my house from the day I build it, and I expect my ROI to continue to grow into the future.

Society needs to understand that more can be done to conserve energy via energy efficiency in our homes and buildings, than by energy efficiency in our cars! Energy-efficient homes are a much smarter investment for consumers and for the environment than the much-hyped hybrid and alternative-fuel cars. Most greenhouse-gas concerns are driven by increasing coal consumption, which is largely for architectural energy use. The principle means available to solve the energy and climate crises is architectural energy efficiency. It's time we all stop building energy-gluttonous McMansions that will be unlivable within 50 years, and build for sustainability and energy efficiency.

Regards,
 
Grant


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