Image Credit: Fine Homebuilding
More Musings of an Energy Nerd
Let’s say that your goal is to build a simple net-zero-energy home for your family. You insist that the home be energy-efficient, and you plan to include a photovoltaic (PV) array that is large enough to balance your annual energy needs.
Your main stumbling block is that your budget is very tight. Is your goal attainable?
Perhaps. Many builders have managed to complete a net-zero home that costs only a little bit more than a conventional house. If you want to take a similar approach, consider the following principles.
1. For a zero-energy house, net metering must be available
Aiming for the zero-energy target only makes sense if your local utility offers a net metering contract. If your utility won’t provide you with a one-for-one credit for the kilowatt-hours that your PV system delivers to the grid, it’s going to be very hard to hit net zero.
So clearly, step one is to buy a building lot in a community where net metering is offered by the local utility. (Even if you take this step, there is no assurance that the local utility will honor the provisions of the net metering contract for the life of the house. But it’s still better to start out with a net metering contract than it is to build in a region of the country where utilities are hostile to residential PV.)
2. Use an energy modeling program to optimize your home’s specifications
As you refine your design, you’ll need to consider a variety of tradeoffs. For example, which approach will save more energy on an annual basis: increasing the attic cellulose from R-38 to R-50, or adding $1,000 of PV modules to your solar array?
The best software to help you answer this type of question is BEopt. Fortunately, you…
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18 Comments
Real world results
We attempted to implement many of the ideas in this article for our pretty good house in Maine.
R42 walls
R70 ceiling
R17 under slab
Triple pane R8 windows
1650 square feet of conditioned space.
6.6kw solar array
Electric resistance water heater
Two ductless minisplits.
We just finished our second full year of monitoring energy use.
Oct. 1, 2015-Sept. 30, 2016 we spent $335 for all energy.
Oct.1, 2016-Sept. 30, 2017 we spent $400.
Without the hot tub(which we'll give up when you pry it from our hot wrinkled hands) we'd be net zero, but we'd still pay about $150 just for minimum monthly electric charges.
Response to Stephen Sheehy
Stephen,
Most Americans would be delighted to slash their home energy bills to $33 per month, as you have done. Well done.
GBA readers who want to learn more about Stephen Sheehy's house can read his five-part blog series on his project:
Pretty Good, Not So Big Maine House
Site Work Begins for a Pretty Good House in Maine
At a Pretty Good House in Maine, Siding and Septic
Framed Walls and Air Barrier Membranes for a Pretty Good House
Windows and Floors at a Pretty Good House in Maine
Heating system
I like the idea of moving to a net zero home, or even a pretty good house, but the air source heat pump will only work during shoulder months in my climate. (Zone 7, northern Minnesota). A ground source heat pump is too expensive to install and electric space heating not on some sort of reduced rate program is expensive to operate. Reduced rate programs are available in my area, but require a second source of heat which is not electric and must be thermostatically controlled, eliminating a wood burning fireplace. This brings me back to natural gas if available or propane. Is there another option?
Response to User-6916526
User-6916526,
First, can you please tell us your name?
Ductless minisplits manufactured by Fujitsu or Mitsubishi can provide significant heat output at temperatures as low as -20 degrees F (even though they may only be rated down to -13 degrees F).
Here is a link to an article by Elden Lindamood about his Minnesota house heated with a minisplit: Relative Humidity and Makeup Air at a Tight Minnesota House. Although his blog describes problems with his minisplit, they are issues related to cooling and dehumidification during the summer -- not heating problems during the winter.
Many builders in Minnesota, Quebec, and northern Vermont are successfully heating their homes with ductless minisplits. The key is a good thermal envelope, low levels of air leakage, and high-performance windows.
Usually, when the temperature drops to -30 degrees F, it doesn't stay that cold for more than a few hours. Moreover, very cold weather is usually accompanied by some sunlight during the day, which helps heat the house. Remember, a house with a good thermal envelope won't lose heat quickly -- temperatures indoors may only drop a few degrees in 24 hours, even with no heat.
Finally, many cold-climate builders install one or two cheap electric-resistance space heaters to provide themselves with a sense of security about extreme cold spells. These heaters are rarely if ever used, but they're cheap, so it may be worth buying them for your mental health.
Other possibilities: Self-consumption of rooftop PV output
Where net-metering isn't offered, self-consumption of rooftop PV solar using smart inverters, batteries, and water heaters to prevent export to the grid and using that energy behind the meter is now possible, even cost-effective in high-priced electricity markets. Like the cost of PV, the cost of grid-attached batteries is coming down at a rapid rate.
The underlying battery technology for home storage and electric vehicles (EVs) has dropped 60% in less than five years, and as production ramps up to satisfy the EV market it is anticipated to continue falling even further. Even conservative analysts are projecting at least another 35% drop by 2025, but that may look embarrassingly conservative by 2020, if recent history is any guide.
Grid-aware water heaters are being remunerated by utilities and grid operators in many local markets, some are even being bid into ancillary services markets by aggregators, who both install the grid-smarts on dumb water heaters, and pay the homeowner for the use of their thermal mass as a switchable load/power dump to use it for maintaining voltage and frequency on the regional grid. But even if those options aren't available in your area, using similar smarts to be able to select when the water is heated based on when it's needed, the state of charge on the home battery, and the real-time output of the PV array are possible, and have even been marketed in Hawaii as a complete self-consumption package by solar companies in (high electicity priced) Hawaii.
The "price learning curve" of manufactured technology is a well documented phenomenon. What's cost-effective in Hawaii today will become cost effective nearly everywhere within a decade, can be part of the planning process for any new Net Zero Energy project.
Response to Dana Dorsett
Dana,
Thanks for your comments. GBA readers interested in more information on one of the topics you raised may want to read this article: PV Systems That Divert Surplus Power to a Water Heater.
Collapse
Just to be a rotten tomato here... there was a time not long ago when 'zero energy' would imply that when the systems around us were collapsing, the home would remain inhabitable and resilient. I still worry about how a house will perform in a power down situation. It seems the pendulum has swung far towards 'house as delicately balanced battery bank.' What about a good old wood stove with fresh air intake to cut those electric bills by 50%?
A couple things
I'd like to add that integrated design can save a lot of money also. Pre-planning where ducts will run in the home and bringing a structural engineer onboard early on can save a lot on unnecessary beams/posts/footings (and thermal bridges). With a simple house like Martin describes, a panelized wall system- if it is available in your neck of the woods- and trusses craned in can save a lot of time (and money). Using readily available local materials and designing one's assemblies to fit the local trade vernacular can help a lot too. Teaching old dogs new tricks can be expensive.
I understand this is an article about the most cost-effective way to create a net zero home, but some things are worth the extra cost. Some of the HRV/ERV's Martin mentions tend to also be noisier and harder to balance than others. It may be great to have a net zero home, but if you have a constant noise issue- especially noticeable in thick-walled homes- one may kick themselves for not considering this. And if one wants to save energy, being able to really balance the ventilation system should not be overlooked. Same goes for the windows in regards to comfort- a triple pane window in a mixed climate may be worth the extra cost. It all depends on what is most important to any one individual, whatever floats one's Mayflower.
Great article. I think bringing net zero construction to cost parity with 'code level' construction is the nut that needs to be cracked. Thanks.
Response to Ethan T (Comment #7)
Ethan T,
The topic of this article is "how to build a zero-energy house." The usual definition of a zero-energy house assumes that the house will be all-electric, so that you don't need to argue about the math involved with producing more electricity than you use, in order to balance the purchase of natural gas or firewood. (Of course, that type of math is always possible, although you end up having to decide whether you are talking about site energy or source energy. Remember, too, that if you produce more electricity on an annual basis than you use, most utilities won't give you a credit or pay you for the excess electricity production. The excess electricity production is a donation to the utility.)
It sounds like you are interested in a different topic -- not "how to build a net-zero house" but "how to build a resilient house." An interesting topic, for sure -- one covered in many GBA articles -- but not the subject under discussion here. GBA readers who live in rural areas, and are interested in installing a wood stove to help them ride out the next hurricane or ice storm, may want to read this article: All About Wood Stoves.
One final point: You implied that installing a wood stove could "cut those electric bills by 50%." But that would only be true in a house where 50% of the annual electricity bill was devoted to space heating. That would not be possible a very well insulated house -- only in a leaky, poorly insulated house. Marc Rosenbaum explained that point in his GBA article, "It’s Not About Space Heating." Rosenbaum monitored a group of "pretty good houses" in Massachusetts, and learned that on average, they used only 19.7% of their electricity for space heating and cooling.
So a wood stove won't save you 50% on your electricity bill unless you build a bad house.
Response to Joshua Salinger
Joshua,
Thanks for your useful hints. Finding the balance between "low cost" (the topic of this article) and "best quality" (always worth striving for, but not always affordable) is the task of each family planning to build a dream home. Families who are able to afford a Zehnder HRV and triple-glazed windows are usually delighted with their choices. That said, not every family can afford those options.
Wood Heat
I live in an area where one of the primary economic activities is logging. Almost all firewood is taken from the slash on cut-blocks after timber harvesting, yet even here the price of a cord of wood is around $240C. Many people (known locally as "Wood Ticks") cut their own, but like other DIY contributions to costs needs to be included in the analysis at the market rate. I agree that heating with wood builds in resiliency, but it isn't a free source of energy.
Please make available to the public
Green building advisor will occasionally unlock an article to make free to the public. I think this would be a great candidate as it is a basic overview. This is the type of info that needs to get widespread attention to those that don't yet know that this is something they need to know more about. Opening it up will help in that regard. Hopefully, it will generate interest in the many more detailed articles that are referenced that then require a prime account.
Response to Daniel Beideck
Daniel,
I'm glad you liked the article. Unfortunately, the article has to stay behind our paywall. Subscriptions to GBA are the main way that Taunton pays my salary (and pays for all the other expenses related to keeping GBA going).
You can tell your friends that GBA offers a 10-day free trial offer. (Here is the link.)
Once the 10-day trial period is up, it's possible to subscribe for one month for $14.95.
Every now and then, one of my blogs is available to non-subscribers through our "sneak peek" promotion. But we just did that two weeks ago, so it's not going to happen again for a while.
In case you missed it, here is a link to our last "sneak peek": GBA Prime Sneak Peek: Bathroom Design.
Radiant heat
by Richard Stein
We're building a small single-story house with a slab-on-grade foundation in New York's lower Hudson Valley. We have been urged to install radiant heat in the floor. It is considerablly more expensive than mini-splits and has no summer air-conditioning feature, but its proponents say that the floor — even with insulation around and beneath it — will feel cold without it. Are they right? Also, Joshua Salinger's comment mentioned that some ERVs are much noisier than others. Any hints about how to avoid those?
Response to Richard Stein
Richard,
Almost all complaints about cold concrete floors are related to the fact that these uncomfortable floors aren't insulated. A continuous horizontal layer of rigid foam under your slab will go a long ways toward achieving the comfort you desire.
Needless to say, most slab-on-grade homes don't use the concrete as the finish flooring (although you can certainly do that if you want). You can install a wide variety of flooring types on top of your concrete.
Homeowners in search of the elusive warm floor are often disappointed by a concrete slab with hydronic tubing. The main reason is that if your house is well insulated and has a low rate of air leakage -- features worth striving for -- your space heating needs will be so low that your floor will almost never get warm enough to feel warm. If the floor ever got that warm, your house would overheat.
Radiant floors perform well in leaky homes that are poorly insulated. But you don't want to build that kind of home.
For more information on this topic, see All About Radiant Floors.
Considering the facts I've just outlined, the key information you provided in your question -- namely, "A radiant floor is considerablly more expensive than minisplits and has no summer air-conditioning feature" -- should make your choice of heating equipment easy.
Martin is right
Richard Stein: Our new, pretty good house is slab on grade, with the slab as our finish floor. No complaints about comfort. The heat hardly ever goes on. We're in Maine. Nights have been in the 30s and 40s, days in the 50s and 60s. We have yet to turn on the minisplits this Fall.
Our previous house had radiant heat in the floor and, even though it wasn't particularly well insulated or tight, the floor wasn't nice and warm very often because the area got lots of sun, which heated the space, and thus the boiler would be off during the day. Since heated slabs are slow to react to calls for heat, the floor would only get warm late at night, making the cat and dog happy, but not doing much for the people.
Thanks Martin and Stephen
By Richard Stein
I appreciate the advice about radiant heat in a slab on grade. If I go with mini-splits, do you think 2" of rigid foam would be appropriate beneath the slab?
Response to Richard Stein
Richard,
It sounds like you are in Climate Zone 5. In your zone, I would say that 2 inches of horizontal rigid foam would be the minimum. The Building Science Corp. recommends a minimum of R-7.5 of horizontal rigid foam for slabs in your zone, plus a minimum of R-10 of vertical rigid foam at the slab perimeter (assuming there are is no hydronic tubing in the slab).
If you have hydronic tubing in the slab, I would double these R-value minimums.
For more information, see R-Value Advice from Building Science Corporation.
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