Building a Low-Cost Zero-Energy Home

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Building a Low-Cost Zero-Energy Home

Follow these guidelines to keep costs as low as possible

Posted on Oct 6 2017 by Martin Holladay
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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) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) 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 zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations..

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 can download BEopt for free. For more information on BEopt, see BEopt Software Has Been Released to the Public.

In recent years, the cost of PV has dropped. If you compare the energy savings of your proposed energy-efficiency upgrades with the energy yield of extra PV modules — using either BEopt or a pencil-and-paper calculations — you may be surprised to learn that extra PV modules often yield a better return.

Of course, just because a $1,000 investment in PV yields greater energy savings than a $1,000 investment in thicker insulation or better windows doesn't mean that you have to choose the PV option. But whatever option you choose, it's good to understand the value of the tradeoff before you make the decision.

3. Design a small house

A small house is cheaper to build than a large house. Moreover, since a small house requires less energy to heat and cool than a large house, it will require a smaller PV system to cover the home’s annual energy budget.

4. The house should have a simple rectangular shape

If your goal is to achieve high performance on a budget, your house should have a simple rectangular footprint.

If you are itching to design a house with bump-outs, ells, and dormers, resist the urge. For a low-cost home, stick to a rectangle.

Should it be a one-story house or a two-story house? Regional preferences seem to guide this decision. Homeowners in New England and the upper Midwest seem to prefer two-story homes. Homeowners in the South and in California seem to prefer one-story homes. Either approach can work.

5. Orient the long axis of the house in an east-west direction

If you are building a net-zero house, orienting the long axis of the house in an east-west direction has nothing to do with passive solar design principles (even though passive solar advocates have been urging this approach for years). The main reason this is done to ensure that the south side of the gable roof has the optimal orientation for a roof-mounted PV array.

If you are planning to install PV modules on your roof, you don’t want any plumbing vents or other penetrations on the south side of the roof. All penetrations go on the north side.

If your lot has setback limitations that prevent the long axis of your house from being oriented in an east-west direction, you may prefer to install a ground-mounted PV array (assuming, of course, that your lot has a suitable unshaded area of lawn).

Net-zero-energy house in Wheat Ridge, Colorado. [Photo credit: NREL]

6. Specify a slab-on-grade foundation

The least expensive foundation will be a slab. This slab should have vertical insulation (either rigid foam or semi-rigid mineral wool) at the slab perimeter, unless fears of termite infestation preclude that step.

In a cold climate, the slab should have a continuous layer of horizontal insulation (again, either rigid foam or semi-rigid mineral wool) under the entire slab.

In a very cold climate, it probably makes sense to specify a highly insulated raft slab. (For more on raft slabs, see Foam Under Footings.)

7. Plan for an all-electric house

If you are concerned about global climate change, you understand the need to eliminate the burning of fossil fuels. That’s reason enough to avoid appliances that burn natural gas, propane, or oil.

Here’s another reason: Natural gas pipelines leak, and leaking methane has a high global warming potential. For more information on this issue, see Natural Gas Pipelines Are Leaking.

Most green builders agree that in the near future, most homes will be all-electric, and a high percentage of our electricity will come from renewable sources. It makes sense to plan for that future now by designing an all-electric house equipped with a PV system.

Remember, if someone in your family wants a single appliance that burns natural gas — for example, a gas range for the kitchen — you’ll have to pay a minimum monthly charge for the privilege of being connected to a gas pipeline, even during the months when your home doesn’t use any gas. If you care about low operating costs, it’s best to avoid hooking up to the gas pipleline in the first place.

8. Locate the bathrooms close to the kitchen

You’ll get the best performance from your water heater if the home’s bathrooms are close to the kitchen, and if the water heater is located close to the rooms where hot water is required. You want your hot water pipes to be as short as possible.

If you design a two-story house with upstairs bedrooms and bathrooms, you’ll probably want to include a drain-water recovery device. For more information on this issue, see Drainwater Heat Recovery Can Lower Your HERS Score.

In a net-zero-energy house, either an ordinary electric-resistance water heater or a heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. can work. In many cases, the decision depends on whether the house has a mechanical room that is large enough for a heat-pump water heater.

9. Include a vented attic

The least expensive way to include high-R ceiling insulation is to specify raised-heel roof trusses. Create a vented attic with a deep layer of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. on the attic floor.

10. Use fixed windows where possible

To keep costs low, size windows wisely. Large areas of glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. waste energy, and may lead to complaints over comfort and glare.

While some windows certainly need to be operable — especially windows designed for emergency egress from bedrooms — many can be fixed. Fixed windows cost considerably less than operable windows.

Triple glazing only makes economic sense in the coldest parts of the United States (for example, Minnesota and northern New England). Most homeowners are satisfied with the performance of double-glazed windows with low-e coatings and argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. gas fill.

11. Pay attention to airtightness

Sealing leaks in your home’s thermal envelope will add to your construction cost — but it won’t add much. Yes, you’ll need to purchase tape, caulk, gaskets, and canned spray foam, and someone will have to devote a few hours to installing these products — but the investment probably has the best economic return of any energy-saving measure you undertake.

12. Heat the house with ductless minisplits or ducted minisplits

You’ve designed a compact house. You’ve built a thermal envelope that is close to airtight — an envelope equipped with insulation that exceeds minimum code requirements. That means that you can easily heat and cool your home with one or two ductless minisplits. As an alternative, you can consider installing a ducted minisplit unit.

This type of heating and cooling system is inexpensive to install and energy-efficient to operate.

13. Install an affordable HRV or ERV

An increasing number of building scientists warn builders of the deficiencies of exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. systems, and are instead recommending the installation of a balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). system that includes an HRV or ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV..

While a European HRV (for example, a Zehnder) has lots of bells and whistles, it’s also going to be expensive. You’re building a low-cost house, so you should purchase an affordable HRV or ERV from Aprilaire, Panasonic, RenewAire, UltimateAir, or Venmar.

14. Install a rooftop PV array

For the best deal, you want to own, not lease, your PV system.

Bargain hard when seeking bids for your PV system. Prices are dropping, so it pays to shop around.

How much extra did it cost?

Your PV array probably cost you $12,000 to $15,000 after taking tax breaks into account, and some of the other measures described here may have added another $10,000 or $15,000 to your construction costs.

But by keeping your house small, and designing the house as a simple rectangle, you also saved money. Moreover, your energy bills are likely to be so low that the investment in energy-efficiency features will probably yield a positive cash flow from day one.

GBA articles that will give you design ideas

Check out the following GBA articles for examples of simple net-zero homes that were built on a tight budget:

Martin Holladay’s previous blog: “Henry Gifford Publishes a Book.”

Click here to follow Martin Holladay on Twitter.


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  1. Fine Homebuilding

1.
Oct 6, 2017 7:47 AM ET

Real world results
by stephen sheehy

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.


2.
Oct 6, 2017 7:50 AM ET

Edited Oct 6, 2017 9:03 AM ET.

Response to Stephen Sheehy
by Martin Holladay

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


3.
Oct 6, 2017 8:02 AM ET

Heating system
by Randy Williams

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?


4.
Oct 6, 2017 9:15 AM ET

Edited Oct 6, 2017 9:17 AM ET.

Response to User-6916526
by Martin Holladay

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.


5.
Oct 6, 2017 12:15 PM ET

Other possibilities: Self-consumption of rooftop PV output
by Dana Dorsett

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.


6.
Oct 6, 2017 12:41 PM ET

Response to Dana Dorsett
by Martin Holladay

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.


7.
Oct 7, 2017 11:08 PM ET

Edited Oct 7, 2017 11:11 PM ET.

Collapse
by Ethan T ; Climate Zone 5A ; ~6000HDD

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%?


8.
Oct 8, 2017 12:09 AM ET

A couple things
by joshua salinger

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.


9.
Oct 8, 2017 4:46 AM ET

Edited Oct 8, 2017 4:59 AM ET.

Response to Ethan T (Comment #7)
by Martin Holladay

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.


10.
Oct 8, 2017 4:51 AM ET

Response to Joshua Salinger
by Martin Holladay

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.


11.
Oct 8, 2017 3:52 PM ET

Wood Heat
by Malcolm Taylor

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.


12.
Oct 9, 2017 8:49 AM ET

Please make available to the public
by Daniel Beideck

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.


13.
Oct 9, 2017 2:44 PM ET

Edited Oct 9, 2017 2:44 PM ET.

Response to Daniel Beideck
by Martin Holladay

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.


14.
Oct 12, 2017 1:09 PM ET

Radiant heat
by user-6750330

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?


15.
Oct 12, 2017 1:16 PM ET

Edited Oct 12, 2017 1:19 PM ET.

Response to Richard Stein
by Martin Holladay

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.


16.
Oct 12, 2017 2:28 PM ET

Martin is right
by stephen sheehy

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.


17.
Oct 13, 2017 3:28 PM ET

Thanks Martin and Stephen
by user-6750330

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?


18.
Oct 13, 2017 4:05 PM ET

Response to Richard Stein
by Martin Holladay

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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