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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.
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 ACH@50 (106 cfm@50), 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.
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 SRECs, we 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.
“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.
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28 Comments
Well done, thanks for the article!
Cheers
Klark from https://www.core77.com/
Great article, Eric. I agree that the typical home buyer doesn't understand the value of many of the practices that GBA advocates. It's hard to sell them on comfort and efficiency, for example, when they have never experienced those things. The only solution may be a national regulatory framework that imposes a value-add for a PGH or a NetZero house.
Thanks, Steve.
Agreed. It's difficult to convince homeowners that comfort/long-term durability/energy savings are worth the upfront costs, in part because they seem too abstract and the benefit is in the future. When they're looking at a build budget, those line items for air sealing and insulation represent immediate costs, so it's easy to pull back on these items.
It does seem like until significant changes in the code happen, or local municipalities offer strong incentives to pursue air tightness and increased levels of insulation, things aren't likely to progress much, especially here in the Midwest.
Great job, been following along since the beginning. Like you, we also didn't account for the summer dehumidification required. Our summers in Newfoundland (Canada) are quite short but often many weeks with humid weather. Although the house rarely met the criteria for overheating events according to the model, humidity in the summer was a big problem. This being said, running the a minisplit on dehumidification mode is all that is needed to maintain comfort.
Thanks, David.
Congrats on your completed home as well! Really enjoyed reading about your landscaping/gardening, along with the house itself.
I keep reading that the models (PHPP/WUFI) don't produce entirely accurate results for building owners. I'm sure that, in some cases at least, occupant behavior plays a part.
We tried using our individual wall units in 'dry' mode, but without much luck. They easily brought down the temperature of each room, but humidity levels remained elevated. We've had no issues with excess humidity once we started running our dehumidifiers. Thankfully they don't run all summer.
Nice to see y0u and David exchanging complements. Definitely two of the most thoughtful bloggers here on GBA, who ended up with excellent houses. Cheers to you both. I've enjoyed being along for the ride.
Thanks, Malcolm. I appreciate the kind words.
I can't say enough how much I appreciate resources like GBA, Building Science Corp., and Hammer and Hand. Those were the main ones that allowed us to DIY our build.
And many thanks to the regulars on here, like yourself Malcolm, who put in the time to answer questions every day. It was a real comfort to know that when we hit sticking points, in either the design or construction phases, that the Q&A section here on GBA could help us find answers. This site proved to be an invaluable lifeline on many occasions throughout our build.
Thank you for the informative article, Eric. Your blog has a wealth of helpful information. Having gone through a passive build a few years ago, I only wish I found your blog much earlier.
Our experience in climate zone 4A is similar with respect to the need for dehumidification during summer months. We experimented a little bit with using our Carrier HVAC system to perform dehumidification (it has a dedicated mode for that), but found that it would sometimes cause sweaty ducts in the basement (as I understand it, to dehumidify the system has to run at very low temperatures). On the other hand, running the dedicated whole house dehumidifier works well. The benefit of running the house at 45-50% humidity is reduced need for HVAC as we can tolerate higher temperatures when the air is dryer.
Our experience is also similar with yours on needing to change the ERV intake filters frequently.
Thanks for checking out the blog, and for the kind words.
We hope the blog can make someone else's build a little easier to navigate.
Hopefully the many included links stay relevant for at least a few years. Many of them direct readers back here to the wealth of information available on GBA.
The need for some dehumidification appears to be fairly consistent across most climate zones for Passive House. I'm guessing the southwest US is probably the one clear exception.
Although it is shocking just how much 'stuff' the supply side filter catches, it's also impressive --- otherwise all of that material was destined for our living areas.
Thank you, Eric This is how we learn, the monitoring of energy bills after the fact. A number that interests me is Btu's per square foot per heating degree day (Btu/sf/hdd). This is hard to shake out when heat pumps are the heating source due to the COP. Any chance of you putting a number on the Btu/sf/hdd for your house? Substitute the heat pump for straight electric resistance heat. I am interested in the building envelope efficiency to compare it to my superinsulation builds.
Thanks, Doug.
If my math is right, it looks like I'm around .52 - .7, depending on if you include total interior sf, or subtract out for basement/stairs as you would for PHI yearly heat demand. Assumes around 3,000 kWh for annual heating, and around 6,500 HDD for here in Palatine, Illinois.
Hopefully we get a couple of 'normal' years to get a better understanding of energy use. In our first three years we've had 2 significant Polar Vortex events and then COVID-19 of course. Unless this is the new 'normal'.
Have you had any issues selling your super-insulated homes? Are they staying on the market any longer than more conventional builds?
Eric,
Your Btu/sf/hdd number is fabulous, thank you for fleshing it out. My theory is in a cold climate if you superinsulate or build to PH your Btu/sf/hdd will be right around the ACH50. This seemed to be the case for double wall homes I built many years ago. Not currently involved in new home construction, this is a young persons game. I turned down the opportunity several years ago to consider building a new PH. Planning to maintain my MN builders license and work with my existing building customers for a time. I am watching with interest the next wave of progressive builders, Randy Williams, Paul Abueva, Under The Sun, they are leading the way.
Thanks for sharing Eric. Your words and process have been an inspiration. I am located a little south of you here in Illinois. Finding your blog was like stumbling into goldmine! I really appreciate all the time you have put into sharing your project details. Keep up the good work!
It's nice to hear you're finding the info helpful.
Feel free to contact me directly via my blog if you have questions about anything. I'd be glad to help if I can.
How often are you having to replace the filters for the ERV? Do you think they're getting dirty from soot coming off the charred siding?
Do you think you'd have the same humidity issues with a HRV?
Hi John. In spring-summer-fall I replace the supply side filter just about every month. I don't wait until the filter is 100% gray-black, but it's usually about 80% covered.
In winter, it's more like every other month.
In the summer there just seems to be more 'stuff' floating around in the air outside --- pollen, insects, dust and dirt generally. Of course, that's also when everyone around me is trying to do landscaping projects, construction, and lawncare chores as well, so that's probably the main culprit.
The charred cedar was initially sealed with tung oil, and then later with black pine tar, so there isn't any soot that can come off like it would if it had been left untreated.
We mainly chose the ERV because of the benefit in winter, allowing us to retain some humidity when it's really desirable (we're usually around 40% thru the winter months). Arguably, it lessens total humidity in summer as well:
https://www.zehnderamerica.com/hrv-versus-erv-how-do-i-choose/#:~:text=Your%20climate.&text=But%20an%20ERV%20is%20not,hot%2C%20humid%20summers%20as%20well.&text=However%2C%20an%20ERV%20can%20actually,can%20during%20the%20summer%20months.
Even so, the root of the problem is combining a low air leakage rate (presumably 1.0 ACH@50 or less) with above code levels of insulation. Because you can't really avoid the need for a constant fresh air supply with such homes/buildings, no matter outdoor weather conditions, you inevitably introduce some unwanted humidity in summer. Also, the heat pump AC is so efficient that while it has no problem maintaining comfortable indoor temperatures, it doesn't run hard enough to keep up with the latent load, so indoor humidity can become uncomfortable on the most humid days of the year.
Eric,
Great piece. Thank you.
I found the humidification part interesting. I wonder if it's possible to quantify how much the HPHW reduced your need for dehumidification (clearly it was still needed)?
Also, it seems that adding more solar panels sooner than later is a no brainer at this point given the precipitous drop in array costs, already established system and future EV.
Thanks, Will.
Our Rheem HPHW tank sits in a fairly open 1500 sf basement. It's also next to a roughed-in bathroom, so it's about 8' away from an ERV exhaust port. Any air coming out of the HPHW is fairly quickly headed towards that ERV exhaust port.
In winter, there's no discernable cooling effect from the HPHW tank. In the summer, I don't think it has much chance to dehumidify very much.
I think Matt Risinger had a video showing a HPHW tank in a mechanical closet off a kitchen. He was ducting the exhaust air from the unit into the kitchen for some added cooling/dehumidification benefit. Makes sense, especially in hotter regions like Texas. Important to remember, though, that unless it's a very busy household (a lot of cooking, laundry, and showers), this effect may only be marginally beneficial, rather than something you can always rely on.
I recently brought up the idea of additional solar panels, even just to get fairly close to our annual 10,000 kWh demand, but my wife wasn't thrilled about the idea. We had an awful experience with a PHIUS-certified builder early on, before it became a DIY build, so the process of building our house was pretty stressful. Things were getting back to normal, but then COVID-19 hit.
It looks like we'll be spending some money this spring to mostly finish up our yard, but any additional solar panels may have to wait a year or two. I'm also curious to see what happens with net metering, or even SRECs, in the next year or two:
https://energynews.us/2020/04/14/as-illinois-begins-shift-from-net-metering-advocates-say-slow-down/
It will also be interesting to see how battery technology continues to develop:
https://www.energysage.com/solar/solar-energy-storage/what-are-the-best-batteries-for-solar-panels/
https://www.bbc.com/future/article/20201217-renewable-power-the-worlds-largest-battery
Thanks for sharing, here's data from Western MT 2020, noting I lived next to O'Hare for about 20 years. Seems like we have the same Mitsubishi Hyper-Heat ASHP, though I have three ducted zones 15, 12 and 9. Water is on-demand gas, dryer is heat pump. A small twist on the kitchen sink, it has a thermostatic on-demand 2.4kW electric where flow can be low to not trigger the on-demand gas and/or use less on-demand gas/water (hot water line inside envelope so not too much delta T). We do have a Napoleon direct vent fireplace (Vector 38) in a great room but it only gets run in the evening on some cold nights. I have an Efergy meter on the non-ASHP circuit so I know we're at 340 kWh per month for two people (working from home with computers/multiple displays). Oven is electric, cooktop gas. Looking to get thin film solar.
House is 2,100 sq ft (1,400 down, 700 up), conditioned volume 22,000 cu ft, attached garage, 12'x12'x3' concrete crawl, rest on piers. Walls R-26, ceiling R-49, floors R-38, windows (incl. 6 sliders) less than 0.3-U. Think ACH50 is around 2 or so (have one hole still to fill). And, here's the usage data, kWh, ASHP kWh and Ave Temp (noting that billing is shifted mid-month):
2020 ASHP Ave Temp
1533 1193 29
1257 917 32
1230 890 33
937 597 40
613 273 49
551 211 56
573 233 62
800 460 70
572 232 65
484 144 54
1121 781 33
1506 1166 29
11177 7097
Total for 2020: 11,177 kWh
Total ASHP 2020: 7,097 kWh
For Jan 15, 2021 to Feb 16, 2021:
1295 HDD (highest 63 HD, with ave temp of 1.5 F and sustained winds over 20 mph gusts to 30 mph) Total ASHP 1395 kWh (for 32 days, again sometimes DV gas fireplace on for a couple hours in the evening).
It seems like your ASHP may be right-sized to slightly oversized and that you see a bigger difference (improved) versus my house as temperature drops (thermostat winter 70F, summer 72-73F). PH/PGH pulls ahead as temperature drops (heat transfer driving force increases and better insulation shows its strength).
Thanks for posting your numbers. It's always interesting to see someone else's energy demand spelled out like this.
It also emphasizes that the main energy savings to be had with PH/PGH is heating demand, since even an older conventionally built home could be converted to all LED lighting, a heat pump water heater, and more efficient appliances.
PH/PGH advocates also hang their hat on increased durability of the structure with better detailing for air/water sealing and flashing, and the somewhat more elusive notion of increased occupant comfort with more consistent temperatures from room to room.
I'm guessing you don't miss Chicago winters 😁
You guessed correctly, Chicago has some of the worst weather in the world, I remember leaving downtown at 6 PM with gauges reading 105F, while having -20F for a few days in a row. My father was a custom home builder, his father was a builder too, but from the old world. The shift from Euro to US standards, ugh. 2x4 construction was commonplace in Chicago burbs. Weatherwise, Minneapolis was better, sunnier winters less humid summers, but -25F for a few days in a row. Western MT has proven to be quite nice. House was built 2017/2018 has closed cell spray foam in 2x6 walls, ceiling and floor (flash-n-batt) and also batts in all the interior walls, the ducted SEZ are pretty quiet, during the build many commented on how quiet it was inside the house. Comfort has been great. It would be interesting to see power data on your ASHP, mine is probably a the lower side of rightly sized. At night with temp about -10F and wind at/over 20 mph, the morning temp was at 65F or so (less than 70F). Heat load calculation was about 27 kBTU/h and ASHP is MXZ-3C30NAHZ (28.6 kBTU/h at 5F), which I believe you may have? With PH/PGH and 1,500 sq ft, I imagine your heat load was around 20 kBTU/h (or less?). The MXZ-2C20NAHZ does 22 kBTU/h at 5F - but maybe can't handle as many zones (limited to 2). The MXZ-3C24NAHZ2 can do three zones with 25 kBTU/h at 5F. Just wondering if you could reduce usage even more with a smaller ASHP?
We did go slightly oversized. This gave us some room for error.
If I remember right, we were technically 9k Btu for the first floor and 6k Btu for the basement in terms of heating load. We decided to opt for three individual heads on the main floor. This allows us to dial in comfort for the kitchen/family room, along with each separate bedroom.
The 15k Btu unit in the kitchen/family room has had no issues so far. Whether winter or summer, once it gets to the set temperature, it barely runs unless we've just brought home groceries or had the front door open for some other reason, or we've just turned it on and it's trying to ramp up to meet temperature. We probably could've gotten away with a 12k Btu unit, while a 9k Btu unit, I'm guessing, would've really been pushing our luck.
Although we do enjoy some 'free' passive heat through our south-facing windows on sunny winter days, there are plenty of gray days over the course of the winter where this benefit is negligible. Even with the 15k Btu unit, the bedrooms are roughly 5 degrees cooler than the kitchen/family room (when the other 2 heads are turned off), especially overnight. Presumably, this effect would've been more pronounced with a smaller head. Also, there's our entryway and a utility room (laundry/pantry), both just off the kitchen. With the 15k Btu head these rooms are within a couple of degrees of the kitchen/family room. Maybe not with a smaller unit in the kitchen/family room. Overall, we've been very happy with our set-up.
We didn't put a head in the basement. We've had no issues in summer keeping the basement cool. It's always within 3-5 degrees of the main level. In winter it's usually in the low to mid-60's, which we don't mind.
We could've gone with a 1:1 set-up (1 compressor outdoors for each interior head), which is said to be more efficient, being able to modulate down more. I haven't seen anyone quantify the kWh usage (1:1 vs. multi-zone on a single compressor), but presumably there would've been some energy savings if we had.
Thank you Eric, that helps explain the strategy - I keep thinking of the crazy low numbers from Marc Rosenbaum. I know the ducted heads reduce efficiency so wall heads are a plus for you, noting I'm also on the side of a single ASHP with multiple zones (noise/ice/visual limited to one spot - only one thing to fix). I see from your photos that there may be a few inches of each copper line exposed/bare at the ASHP. Mine were exposed/bare too so I wrapped them all the way to the fittings (hope that's not a sin). The three supply lines are so small and at the highest temperature, hate to waste it at the source. If your solar system/utility meter has minute-by-minute readings, an Efergy on your non-ASHP breaker box would theoretically allow you to get minute-by-minute on the ASHP. I'm still thinking about getting another Efergy for my ASHP (it's on a separate breaker box).
Link to some info from the guru (Marc): https://nesea.org/user/6685/presenter
One of my favorites: "Minisplit Heat Pumps: Lessons from the Field" https://nesea.org/session/minisplit-heat-pumps-lessons-field The constant set point versus on-off data are really telling . . .
And, another good one: https://nesea.org/session/air-source-heat-pumps-measured-performance-and-best-practices
Seems like capacity less than heat load is OK, noting Marc has a 7% oversize on his 14kBTU/h 1,334 sq ft deep energy retrofit (DER) project in Martha's Vineyard.
@PBP1, I read about some of Marc's reports when I was planning our HVAC layout.
In addition to the reports you linked to, I also found this one interesting:
http://www.energysmiths.com/resources/documents/EliakimsEnergyMediaReportFullwCopyright.pdf
A project for Marc and South Mountain Company that goes back about ten years.
The retrofit work that he did on his own house was also impressive. I think he may have had an article here on GBA about it at some point.
Hi Eric,
Were these results consistent with your energy modeling? I starting the process of a build similar to yours in Kentucky and I am grateful to have your blog as a resource.
Hi John. We started with a PHIUS-certified builder, but parted ways when things went poorly (pre-construction):
https://kimchiandkraut.net/2016/05/05/review-of-evolutionary-home-builders/
They told us they had done the energy modeling (presumably PHPP and/or WUFI), but we never saw any documentation of this.
The more we researched Passive House details, the more we were unhappy with their construction drawings. In the end, we pursued a 'prescriptive path' before one really existed at the time (2016-17):
https://www.phius.org/phius-certification-for-buildings-products/project-certification/phius-2021-emissions-down-scale-up
We based our R-values and roof/wall/foundation assemblies off of their previous Passive House projects, along with many other projects, most of them on the East and West coasts (particularly Hammer and Hand's Madrona House project).
We also found The Passivhaus Handbook to be helpful, along with the Passivhaus Trust website (UK based):
https://www.amazon.com/Passivhaus-Handbook-Constructing-Retrofitting-Performance/dp/0857840193
https://www.passivhaustrust.org.uk/
I would argue a prescriptive path is fairly straightforward with a residential build that closely follows basic Passive House principles, although having someone model the construction-ready drawings can't hurt.
Energy modeling becomes more of a necessity once the project grows in scale (e.g., multi-family, commercial, or institutional size), or even a residential project that's trying to break some rules (e.g., extensive glazing to the north, curtain walls, no overhangs, etc.)
If I was designing/building today, I would consult resources like https://naphnetwork.org/ and https://passivehouseaccelerator.com/
Feel free to contact me directly through my blog if you have questions during your design or construction phases. I'd be glad to help if I can.
Are you building in the Louisville area? I have some family there. Or are you building in a more rural part of Kentucky?
Eric,
Glad to hear the great results of your Almost Passive House. I followed your articles with interest as we were in the process of building a Pretty Good House just a bit behind your schedule. Our PGH is R10 under slab, R23 basement walls, R40 in our double stud walls, R60 attic. We also used triple glazed krypton filled windows with U-value around 0.16 -- our bldg inspector said he had never seen such efficient windows before. He also had never seen an air exchanger. I guess we're kind of on the cutting edge, haha.
We've been moved in for about 6 months now although there is still some work to do. But I am excited to read about your kWh results because we are getting similar numbers as well. Of course there are some differences -- we have about 2100 sqft of conditioned space but our climate here in Lancaster Co, PA isn't as brutal as yours. We are Zone 5 but adjacent counties are 4, whereas it looks like you are Zone 5 but your adjacent counties are 6. Anyway congratulations on your successful build. God bless your home !!!
Thanks for checking out the blog, Dennis.
Congratulations on your new house! Hope you're enjoying your new home!
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