Urban Rustic: A Passive House Energy Scorecard

Passive House, as a building strategy, requires meticulous air-sealing, along with ample amounts of insulation, carefully placed to eliminate or reduce the impact of thermal bridging through the building envelope. Once the air barrier of the building has been established, it requires mechanical ventilation to meet air quality needs, along with high-performance windows and doors to avoid undermining all of the air-sealing and insulation.

All of these elements together, if successfully managed and implemented, should achieve a building that requires significantly less energy to operate and maintain at comfortable temperatures than any conventionally built structure of similar size and shape.

Our 1500-sq.-ft. home was built to the prescriptive model published by Passive House Institute U.S. (PHIUS), although we have not sought official certification. A blower-door test showed air leakage at 0.2 [email protected] (106 [email protected]), and R-values for the structure are as follows:

  • R-16 below the basement slab
  • R-20 exterior of the basement foundation
  • R-40 exterior walls
  • R-80 attic

Energy usage profile

In 2019—our first full year of occupancy—we used a total of just over 11,000 kWh of electricity. This included lighting, the heat-pump water heater, heating and cooling, all other plug-in loads; plus countless hours of power tool usage, as I finished up interior trim, doors, and shelving/storage projects. Record-low temps during a polar vortex event in late January and into early February added to the total as well.

For 2020, after the outbreak of COVID-19, a substantial increase in overall energy use might have been expected. Surprisingly, even after stay-at-home guidelines were in place beginning in March, we ended up at 10,446 kWh, which is slightly less than the previous year.

This was in keeping with our usage during our first nine months (April 2018 through December 2018). If the polar vortex was an anomaly (and everyone hopes that it was), then going forward most years should be around 9500-10,500 kWh for total annual demand. In part, we think that going over 11,000 kWH in our first full year reflects how a colder-than-normal winter can impact overall energy use significantly in a Passive House, not to mention heating demand more generally (whether it’s a Passive House or not).

Moreover, for a family of three and a structure of this size with similar performance specs, it seems to suggest that our 3000−4000 kWh of annual usage per person is mostly baked-in—meaning there’s not much we could do, in terms of occupant behavior, to lower these numbers any further.

It’s fair to question the point of the air-sealing, insulation, and triple-pane windows and doors if they don’t result in a more comfortable day-to-day living experience. If simply chasing energy use were the main objective, reducing it no matter the consequences, then removing all the windows and doors and replacing them with continuous R-40 walls would be a good place to start, but hardly worth considering for obvious reasons. If there’s a payoff for pursuing Passive House, it has to be in the combination of lower energy costs and increased occupant comfort when compared to a similar, conventionally built home or structure.

The unexpected 

The only unanticipated energy use was the need for dehumidification on the hottest and most humid days of the year. After our first summer in 2018, when part of the excess humidity was likely due to moisture present inside the newly constructed structure, we’ve been averaging about 30−40 days a summer, including a few random days in spring and fall, when the dehumidifiers are running intermittently. We set the units to 50% relative humidity, but normally they shut off around 55% based on gauges placed around the house. We try to keep the house under 60% RH. The risk for mold increases above 60%, but it’s mainly at that point when humidity levels make us feel uncomfortable.

Also, we didn’t think about the energy use associated with power tools for woodworking and arts and crafts projects. Without tracking it, we can only guess that it represents a few hundred kWh a year. Even so, along with the potential for an electric vehicle charger, it’s something to think about when designing a new home or retrofitting an older one, especially if renewables are part of the equation, and you’re trying to predict annual demand.

Actual energy use: demand and costs

Energy use has been surprisingly consistent month-to-month and across seasons, regardless of activity level in the house (e.g. guests staying over, vacations away, power tool use, etc.). For instance, in 2018, during our first June when the house was still drying from new construction-related moisture, and we felt compelled to use two dehumidifiers to control excessive humidity (one in the kitchen and one in the basement), total energy use for the month was 616 kWh. The following June, in 2019, we ended up with an even higher number, at 786 kWh of demand. For June of this year, even with the stay-at-home restrictions, we ended up at 605 kWh.

Without a granular study of day-to-day conditions it’s hard to explain this deviation with any level of certainty. Suffice to say, we can expect June usage to normally be in the 600−800 kWh range.

In other words, even in a year where the weather remains milder than normal for a full 12 months, and we’re all exceedingly busy and rarely at home, our total energy use for the year, at best, will likely still end up in the 9000−10,000 kWh range. And even if there was just one person living here, it’s hard to imagine they could keep total energy usage much below 4000-5,000 kWh on an annual basis.

Here is the monthly breakdown of energy use for the first full year we were in the home for 2019:

January: 1738 kWh (includes the 2019 polar vortex; the following January was only 1374 kWh)

February: 1483 kWh (the following year was 1237)

March: 837 kWh (the following year was 561—clearly it was a bitterly cold winter)

April: 681 kWh

May: 473 kWh

June: 786 kWh

July: 612 kWh

August: 608 kWh

September: 630 kWh

October: 812 kWh

November: 1166 kWh

December: 1237 kWh

Total energy use for 2019 was 11,063 kWh.

In this same period, our 2.9 kW solar array produced 3863 kWh, so net demand for the year was 7200 kWh (this requires some math using the billing statements from our utility company and the Enphase Enlighten solar app).

Our monthly bills for electricity in 2019 totaled: $1075.89.

Because of our Solar Renewable Energy Certificates (SREC), which for us totaled $848 for the year (paid via quarterly checks), our net energy costs for 2019 were $227.89 (an average of $18.99 per month).

For comparison, the numbers for 2020 were: 10,446 kWh of demand, while solar production for the same period was 3675 kWh, for a net energy demand of 6771 kWh. After SREC payments (again, totaling $848 for the year), our net total cost for 2020 was $189.36 (an average of $15.78 per month).

The SREC payments, which are based on a five-year contract, reduced our annual cost by $848 each year, with a net average cost for our first two years of just $208.63 per year for all of our energy needs (a roughly $17.39 per month average). Without any solar panels or SRECs, our electric bill would be roughly just under $1500 per year based on current rates.

Our approach has been to live normally, enjoying the benefits of the air-sealing, insulation, and our HVAC setup. We set, then mostly forget about, our heat pump at 70ºF in winter, 75ºF in summer.

Numbers for heating and cooling

In spring and fall, when there’s less demand for heating or cooling, our baseline monthly energy usage is below 500 kWh (this has been fairly consistent over the course of the last 2-1/2 years.)

In our case, summer months typically run about 600−800 kWh of actual usage, depending on the number of days above 82ºF, when we typically turn on the AC. Even on these days, we will turn it off if there’s a sufficient drop in outdoor temperature overnight, which allows us to open the windows (dependent on outdoor humidity or rain).

Even though we thought we’d regularly open our windows whenever the weather was remotely nice, this hasn’t turned out to be the case. Between having to monitor indoor humidity levels, and the ability of our ERV to deliver continuous, filtered fresh air (it’s shocking how quickly our fresh-air-supply filter turns black), windows stay mostly shut.

Photograph of a painting and a minisplit head on the wall.
This single minisplit head does most of the heating and cooling for the house. Photo courtesy Eric Whetzel.

On the plus side, it’s not uncommon for us to wait until there are two or three successive days when temperatures rise above 82ºF before we feel the need to turn on the AC. In other words, there is some truth to the idea that Passive House buildings take some time to heat up or cool down based on outdoor conditions.

During the heart of winter, our total energy demand is in the range of 1000−1500 kWh per month. Even in January of 2019, with a polar vortex event, we used less than 2000 kWh for the month. During this same week, however, we saw minimal benefit from our solar panels since they were covered by several inches of snow.

These elevated kWh numbers during winter reflect just how much harder our Mitsubishi heat-pump system has to work. And we can hear the difference: while in summer the system is virtually silent, in winter, especially as temperatures head towards zero, we can hear the compressor outdoors working to keep up.

Cooling is similar to what it would be in a conventionally built house. In summer, the Passive House thermos-like structure is mostly a hindrance rather than a benefit to keeping the interior comfortable. All the free sources of heat in winter (such as our body heat or heat given off by computers, TVs, and appliances) actively contribute to the overall cooling load, however small their impact might be.

In addition, because cooling loads are relatively low, and the efficiency of the minisplit heat pump is so high, it leaves us with a latent load that we need to address with two standalone dehumidifiers, indirectly adding to the overall cooling load.

So, of our roughly 10,000−11,000 kWh per year of total demand, without an actual energy use monitor on our main panel, it looks like just over 3000 kWh is used for heating, with another 800−1000 kWh used for cooling. That, of course, could change with weather spikes, hot or cold.

Additional solar panels to achieve net zero

Based on what we’ve been paying for energy so far, we don’t feel compelled to add more solar panels, even though our system is relatively small. Should the SRECs dramatically fall in value with a new contract, or disappear altogether, it might encourage us to purchase more panels. But even so, at less than $90 per month, even without the SRECs, it makes our energy bills a relatively painless expenditure.

Because of the upfront effort and money for air-sealing and insulation, we’ve managed to whittle our energy costs down to something highly affordable and resistant to significant cost increases. This should remain true, regardless of what’s happening in the market in terms of prices for natural gas, coal, or nuclear power. Worst-case scenario, we add additional solar panels to get to net zero or even net positive.

We average between 3500−4000 kWh of solar production per year, nearly 40% of our annual demand. Combined with SRECswe nearly end up at net zero, at least in terms of total cash spent for energy. As a result, there’s not much financial incentive to purchase additional solar panels to achieve absolute-zero energy consumption (site energy).

Passive House and net zero

In addition to designing for Passive House, there is the question of net-zero or even net-positive buildings. Passive House strategies eliminate a significant portion of overall demand by requiring a significant outlay of upfront funds for air-sealing and insulation. Once this pill has been swallowed, it’s normally cost-effective to incorporate renewable energy of some kind to cancel out the remaining energy bill.

A quick side note: An excellent resource—one that I found only as our build was coming to an end—is William Maclay’s book The New Net Zero. It contains a wealth of information, but, in particular, many specific construction details vividly illustrated. Also worth noting, if this approach (Passive House and net zero) were adopted on a national level, including renovations, it would eliminate a large portion of aggregate energy demand, thereby having a meaningful impact on greenhouse gas emissions and global climate change (up to 40% for construction and existing buildings).

Based on what we know at the moment, a combination of approaches—including Passive House building principles, zero carbon goals, and the use of renewables—could be the way out of the climate crisis over the long haul. In addition, if adopted as part of building codes, it could mean properly training the next generation of tradespeople (like European-style apprenticeship models, which would also improve the build quality), while being a tremendously effective jobs program.

Passive House cost premium

Even though Passive House construction offers a significant reduction in energy costs, the numbers may not be compelling when faced with higher costs for things like air-sealing and extra insulation.

In our case, the annual energy savings compared to something code-built would likely be in the $2000−$3000 range. Fairly significant, but if the purchase price of the home is $500,000−$1 million-plus (fairly typical here in the Chicago suburbs for new construction), then even a $100,000 savings over the course of a 30-year mortgage may not convince someone to move beyond conventional construction practices. The upfront costs associated with meticulous air-sealing and added levels of insulation—if not viewed as an investment in build quality—will likely appear frivolous to the average consumer.

Writing in a GBA Q&A post, Travis Brungardt put it this way:

“One of the issues we face here [in Kansas City] is the fact that energy is cheap, like most things in the Midwest. We don’t have the financial burden placed on us that the coasts do—real estate-wise and energy-wise. So there is not much enthusiasm around green building on a financial level; it’s almost always an ethical issue. The people who are interested want to do a good thing for the environment, as opposed to saving money on their utility bills.

“Another thing is that people are accustomed to discomfort—we have drastic and frequent temperature swings. It’s really humid in the summer and freezing in the winters, when drafty windows are just accepted. They are used [to] it, so it is hard to sell them on high-performance windows to be more comfortable; or taking measures to keep a basement from being wet—they just aren’t concerned about it. There’s a complacency that we fight against; there’s not enough financial gain to incentivize making upgrades.”

Looking solely at upfront costs can discourage prospective homebuyers from pursuing Passive House (or even Pretty Good Houses), whereas looking at the cost of ownership, including the cost of monthly utilities, produces a more accurate comparison (note, however, this assumes the homeowner can stay put for at least the next 20 to 30 years).

A cost-of-ownership calculation should also acknowledge lower maintenance costs year-to-year. If the structure is detailed well, it should experience far fewer issues (none ideally), especially damage caused by bulk water intrusion, mold, or even air leakage. Granted, it may take a decade or more before this kind of damage is found in a conventional home, but when it is, it’s rarely (if ever) inexpensive to correct.

Hard choices

The American consumer has been taught by the market, realtors, and builders to believe cost per square foot is the gold standard of value. As a consequence, little emphasis is placed on building science basics such as air tightness, proper moisture management, thermal performance, and indoor air quality. In layman’s terms, this means the average American home is leaky, parts of it have likely been damaged by bulk water or mold, and it’s uncomfortable in terms of indoor temperatures and humidity, all while delivering subpar air to its occupants.

In terms of quality construction and green building (Passive House or not), there really is no free lunch. Quality, of any kind, has its price. Only those who recognize its value will be willing to pay for it.

Regardless, as homeowners, we either pay upfront for the air-sealing and insulation, along with high-performance HVAC systems for better indoor air quality, or we pay monthly (and perpetually) in the form of higher energy bills. This normally comes with less occupant comfort and far inferior air quality. Either way, the money is going to be spent, it’s just a question of when (upfront vs. long-term month-to-month) and for what (air-sealing and insulation vs. mediocre systems and underwhelming outcomes that require costly maintenance over time).

This post is one of a series by Eric Whetzel about the design and construction of his house in Palatine, Illinois, a suburb of Chicago. For more details and more photos, see Eric’s complete blog, Kimchi & Kraut.

Read More

Leave a Reply

Your email address will not be published. Required fields are marked *

Previous Post

Postdoc in Mechanics of Soft Materials and Biological Tissues

Next Post

Soft robot swims in the Mariana Trench

Related Posts
Close Bitnami banner