ASHRAE Journal:
ASHRAE Journal presents.
Drew Champlin:
Welcome to this ASHRAE Journal podcast episode. My name is Drew Champlin. I'm the ASHRAE Journal editor. Today we will be joined by David Traxler. If you read his article in the March 2025 ASHRAE Journal Analysis of 50 Energy Efficient Net Zero Buildings, that article was selected ASHRAE Journal Paper of the Year by the publications committee. David was presented that award at the recent ASHRAE Summer Conference in Austin, Texas. And we're going to take a deep dive into that article, and then another article that David had published in the June 2026 ASHRAE Journal called Decarbonization with Hybrid Geothermal.
David, congratulations on the award, and we're happy to have you on the podcast. Why don't you just tell the listeners about yourself?
David Traxler:
Thank you so much to start with having me on the podcast, and I was just really grateful for the award, and it's quite the honor. I am a consulting engineer, and I'm currently with Burns & McDonell, but I really got into engineering because of an older brother of mine. I came from a place where not a lot of people went to college, and my older brothers dropped out of high school. And so, I was talking to him one day and I said, "If you could go back and finish high school and go to college, what would you do?" And he said, "I would be an engineer." And I had never heard of an engineer before. My working class town in West Virginia didn't have many engineers. I asked, "Well, what's an engineer?" And he said, "It's somebody who designs things, and they're good at math." And so, at that point, I decided I want to be an engineer. And so, after college, I worked in the HVAC industry for a while, and then I went into consulting, and I'm still in the HVAC industry. And that has brought me where I am today.
Drew Champlin:
All right. And how long have you been with ASHRAE? What's your experience there?
David Traxler:
I started with ASHRAE probably in 2004. And so, I've been with ASHRAE quite a while, and I love the organization. The beautiful thing about ASHRAE is that it broadens your horizons, and opens up your perspectives to designs outside of what you may normally see in your own engineering firm, or what you would learn through a mentor. You get to see so many different projects and things that are going on in the world, and learn about those projects. The ASHRAE guidebooks and handbooks are also fantastic, and I owe a lot of where I am today in my career to ASHRAE.
Drew Champlin:
Great. Well, let's get started a little bit just with the basics because you never know, people of all experiences and skill levels listen to this. What is a net zero energy building?
David Traxler:
A net zero energy building is a building that has renewable energy, that produces more electricity than the building consumes over the course of the year. These buildings are typically all electric. The heating, the cooking, the water heating will all be by electricity. And over the course of the year, that building, whatever it uses in electricity, it has generated more with the renewable energies that are associated with that building.
Drew Champlin:
This study was on 50 buildings. What inspired you to study these 50 buildings?
David Traxler:
These 50 buildings, the reason I studied them was because they were published on the new Building Institute website, or many of them were. It was a place I could go and I could find data on emerging net zero energy buildings and verified net zero energy buildings. Instead of having to scout journals, and it was a list that I could start working from. And then the list had all sorts of net zero energy buildings. They had ones that were fairly inefficient buildings with a lot of renewable energy to make up for the inefficiencies. And it had some that were very efficient. And my main goal in entering this venture and this study was to figure out how to build and design energy efficient buildings. That was not directly related to the renewable portions of it, so I honed in on the energy efficient part of it.
And then, I selected the buildings I selected because I wanted a diverse set of building sizes, locations, and types. And the reason for that is because I really started this endeavor as my own personal research project, because I was tasked in my work to focus more on sustainability. I'd been designing sustainable buildings for a while, and I'd worked on net zero buildings and geothermal projects, but they wanted me to really focus on it. And I took that as a pretty significant responsibility.
I wanted to use real data on what is working in the world. I went, and I found the list, and then I searched for different building sizes, different building types, different locations throughout the country to try to get a decent picture of if I'm building a school in the Northeast, what proven heating and cooling types and what proven insulation and other design factors can go into making a very energy efficient building? And so the net zero part of it is first focusing on energy efficiency, and then you have to have much less renewables. And so, that's where it came into capping the EUI on those buildings studied. I really wanted to focus on efficient buildings.
Drew Champlin:
Okay. David, I remember when you emailed me about this study, and you were getting ready to publish it, or maybe you had just submitted. How long did this take? It seems like this would have taken quite a while.
David Traxler:
It took me about three years to gather all this information. Like I said, originally it wasn't meant to be an article. It was just my own personal research project. And I'm a dad like many people out there who are parents, a mother or father, and I have children. And so, it was nights and weekends. When I had an hour here, two hours there, I would sit down at my computer, and I would do research, and try to find another building. My goal was to sit down and find another net zero building, and to get enough information from the building to be able to fill out a spreadsheet I developed with infiltration, insulation, what kind of windows did they use? Did they do anything special with the electrical system? Do they have enough information on their heating and cooling?
Every single building was this like mini research project where I had to go out and try to find publicly available information. And then, I would read through the information and there were many buildings where maybe I had some but not enough to be like a decent example. It was a lot of trial and error. And through that and being that this wasn't my full-time job, it took quite a while to be able to assemble all this information into something that could be considered an article.
Drew Champlin:
Yeah, absolutely. And we're glad you spent the time doing that. I know the ASHRAE readers appreciate that. Why were only buildings with a certain energy use intensity studied? It seemed like you went up to a certain measurement.
David Traxler:
Yeah. The reason is the main focus of my endeavor was the efficiency portion of it. Because you can build a terribly inefficient building, and if you want to buy enough renewable energy, or install enough on your site, or buy it from someone, you can make most buildings net zero energy, but that's not the goal of the project. The goal of the project was really to figure out if there are any patterns or basic things that most really energy efficient buildings use. And then, what kind of renewables do these energy efficient buildings use? A lot of times, clients, they—most clients don't want their buildings to be energy hogs. They want energy efficient buildings. And then some clients want to go so far as go net zero. They want to involve the renewable portion of it. And so, I wanted to be able to focus on what I think most clients are interested in, which is a healthy, safe, energy efficient building that does what it's supposed to do so that the building can serve the people that it's meant to serve.
Drew Champlin:
Well, David, what patterns can be learned from existing buildings to apply to new and future buildings?
David Traxler:
The main patterns are focused on efficiency first. Infiltration is a big one. Many of these buildings talk about very low infiltration rates. And as I heard it put one time to somebody who was installing radiant heating under driveways in a very cold place, he said it's very hard to heat the great outdoors. When you have a lot of infiltration in your building, you are basically heating the great outdoors, because you're just losing your heat or your cooling that you're putting into the building. A lot of these buildings started with a very, very tight building, irrespective of the insulation values. And then, on top of that, depending on where it was, the insulation value had a more or less impact. For farther northern climates, those insulation values made a very significant impact. And a lot of those buildings were insulated above code minimums, some to significant portions, and others just a little bit above.
And so there was a little bit of a variety. And then the further south you go, insulation still has an impact, but it's a different impact. And where you insulate may be different with more roof insulation, because you have more heat came through the roof, and a lot more focus on windows. A lot of it was efficiency focused in these buildings. That was the number one goal to build a very efficient building. The next step in a lot of them was to only use what you need to use. And what I mean by that is a lot of them set up good lighting from exterior. You had less need for electric lighting in your buildings, or they would provide natural ventilation at times, in times where it was comfortable outside. Almost all of them turn things off when they're not being used, which seems very common sense. But, a lot of our buildings today, depending on the equipment that's used, is designed to run continuously. In these buildings, the net zero buildings, it was very, very rare to come across a system that wasn't designed to be on demand, that you walk into a space, and then that space is served, you leave the space, and shortly thereafter, the heating and cooling system, and the lighting, and the electrical for that space, as much as possible, are turned off. And so, just these real fundamental efficiency measures were so much of what was used in so many of these buildings.
Drew Champlin:
What system types did the buildings in this study use to reduce energy consumption?
David Traxler:
It used a wide variety actually. And it varied a lot depending on where in the United States it was located. One of the things I studied is I broke it down by a geographical region. And so, in the Northeast, it was mostly—well, there was a VRF, ground source heat pumps. Those styles of systems were used predominantly in the Northeast. In the South, it was similar. Out West, there was a widest variety of things, especially in the California or that coastal region. I think it's because the climate there's more moderate than some of the other portions of the United States. An air source heat pump, and a ground source heat pump, there's going to be a difference in efficiency, but an air source heat pump could still be used on a net zero building, because you don't have the extreme winters that require a significant amount of energy, and you don't have as many hot summer days as you could have in a place like Arizona or Florida.
Now, the one place I found to be pretty interesting is all of the net zero buildings studied in the Midwest. And the Midwest is known for having very cold winters, and very hot summers, and being humid. They were all geothermal. And so, I'm not saying that that's the only way to do net zero, but the buildings studied in the Midwest had that one common thing. Where out in California, you had four or five different system types, and in other places in the United States, there was more of a variety.
Drew Champlin:
Yeah, so you studied 50 of these buildings. I guess, they were all over the country, different regions, maybe balanced as best as you could for that?
David Traxler:
Yeah, I tried. Yeah, I would look, and I would try to get a fairly decent balance on climate zones and geographic regions. And I tried to balance out building types also. That was a little more difficult, but the regions, it wasn't so bad. I could find net zero buildings all over the country by the time this has been published, and this data was being made available. And that was a big part of this endeavor was at the time I worked for a company, and I still do, do work all over the country. The firms I've worked with have been unique where they don't do just one work in Florida or the mid-Atlantic.
I've worked for firms that are nationwide, and especially with specialties like sustainability and geothermal all over the country, and even all over the world, projects will pop up, and they'll reach out to me, and they'll say, "Oh, our client has asked us to give them a really efficient building at a reasonable cost." And wherever that project is, they look to me to be able to help guide them through that process. I was really looking for the data to help me in those situations to be able to give good sound advice based on empirical data versus theoretical data.
Drew Champlin:
What were the differences in the buildings as it pertained to different climates and geographies?
David Traxler:
A huge part of it was insulation and windows. That was probably the biggest difference when it came to climate and geography. One other big difference though was the way the outside air was handled. In more extreme climates, the outside air—well, on a lot of these buildings, the outside air was dedicated. I think that was one thing also that sets net zero buildings apart is a traditional building will combine the outside air with the rest of the HVAC system. But, many of these net zero buildings did not do that, especially in harsh climates. And the reason being is because in harsh climates, you want to try to really turn off your heating and cooling system as much as possible.
And anytime you can treat your HVAC system air, your outside air independently, that allows that to happen. And especially when you have really cold air coming into your building to be able to pretreat that through a DOAS unit and to be able to recover energy on a really cold winter day, or to cool it on a really hot summer day through dedicated outdoor air with energy recovery, that seem to make a, it makes a big difference, and it was a common theme in a lot of these net zero buildings.
Drew Champlin:
You may have answered this a little bit ago when you mentioned the geothermal in the Midwest, but what was the most surprising thing that you learned during this study?
David Traxler:
The building's efficiency isn't directly tied to the cost of the building. A building's efficiency is more closely tied to the system types used than what was paid for the building. I didn't have the granularity in my data to say like, "Oh, they paid 20% for their HVAC system of the total building cost." It was all just building costs of the data that I could find, and it was publicly available data. But, the trend was that the simpler HVAC systems, and when I say simple, I mean their ground source heat pumps, through water to air heat pumps, or those types of systems that are distributed throughout a building with dedicated outdoor air units, those typically were on the lower end of the cost scale, even though it's somewhat unintuitive. Climates where they could use air source heat pumps, or rooftop units, so there were some in Arizona and California that they just used traditional rooftop units, and they did energy efficiency measures throughout the building in other ways, through lighting and reducing plug loads. Those were great also.
And those are low cost systems that still yielded a really good energy efficiency, and still yielded a great EUI. I think there's a common misperception that to have an energy efficiency building, or an energy efficient building, you have to pay a lot more money, but that is not what the data yielded. It was actually more tied to buildings that were net zero and had a central plant that distributed hot and chilled water out throughout the building. Those buildings tended to be more expensive, and buildings that had distributed systems tended to be less expensive.
There's one thing I didn't publish in the article, and for any of the listeners out there, the article is long. I know if you've read it, sometimes you read half of it, and take a break, and read the other half and I get it. Well, it was actually even longer. And so, Drew reached out and said, "You can either make it two articles, or you can make it one article." And so, I decided to trim out a bit, and to really focus on the information that I had, and to make it into one article.
But, one thing that didn't make it into the article was analysis on how the cost, if there was any cost EUI relationship like what I just talked about. And the more granular data that listeners may be interested in is so I broke the buildings down into $100 per square foot increments. For buildings that were less than a hundred square feet, or dollars per square foot, those buildings were mostly retrofits, and they had really good EUIs actually. On average about 15 was the EUI, or the energy use intensity of those buildings.
And then, buildings that were 100 to $200 a square feet were about 20 EUI. The buildings that were 200 to 300 square feet were about 18, and 300 to 400 square feet were about 15. You can see as the price went up from $100 a square feet, to 200, to 300, to 400, it did get a little bit lower, 10 to 20% lower as they jump. This is the thing that I found pretty interesting. After you break $400 a square foot, so there was a large subset of buildings that were over $400 per square foot, probably like eight to 10. And the UI started going back up, and then it went up to 18. The data shows there's some benefit to spending more on your HVAC system if we correlate the cost of the HVAC with the building costs. But, there comes a point where it doesn't benefit you anymore. And so, I thought that was somewhat interesting, and it didn't make it into the article, but I thought listeners maybe find that useful.
Drew Champlin:
Yeah. I wish we could have used a lot more. I had a lot more space to fill in all the images, and the diagrams, and the text and all that stuff. And I appreciate you whittling it down to make it work. I guess, to wrap up this study a bit, what should people know about designing energy efficient buildings?
David Traxler:
Focus on the fundamentals. One thing I haven't talked about yet is domestic hot water systems. Domestic hot water systems are kind of, they're a hidden thing that is using energy in many buildings that people aren't really thinking about. When I say domestic hot water, that's hot water that's used for hand washing, cleaning, showering, domestic hot water systems. A lot of these energy efficient buildings used decentralized domestic hot water systems. Those are hot water systems that just have a tank near the point of use, or they have instantaneous water heaters. Some did use solar thermal systems and those are helpful also, because you're using the sun's energy to heat the water, a portion of the water at least instead of all of the electric energy to do that. What it found is most or all of these buildings used decentralized hot water, and a lot of them used point of use hot water.
And the thing that's really important about that is a lot of buildings, office buildings, schools, and other building types, what they'll do is they'll set up a recirculating hot water system. And what that is reheat hot water, and then you recirculate or circulate hot water through your building at all times. Well, what happens is, in an ideal world, the controls are set up to try to turn it off as much as possible when there's no demand, and to not circulate water on the evenings or the weekends when no one's using water. But, oftentimes, those systems don't work as well. And even when they do work well, there's still the energy loss of circulating water through a whole entire building and creating this large storage system that's constantly using heat. That is one thing to really watch out for with energy efficient buildings is decentralized water heating systems.
Beyond that, it's really a focus on the fundamentals. Building tightness is so very important. The building window type is really important too. One building that I studied, they figured out that if they went to triple pane windows from a double pane, they were able to eliminate the entire perimeter heating system. And by doing that, they ended up saving a significant amount of money even though the windows cost more because the mechanical system reduced so much in price. Other areas are just the insulation. If you go to the net zero design guides that are published by ASHRAE in partnership with the Department of Energy and Better Buildings, a lot of the fundamentals that they go through there are going to be a great starting point. The insulation values, the window values. And so, any of those fundamentals and really focusing well on the fundamentals and the infiltration is a big part of it.
Then, efficient lighting is great, and that's become ubiquitous in most modern design because that LED lighting, decentralized hot water is really an important one to be able to reduce your losses, and designing systems that can turn off. There may have been one or two exceptions that had central chillers. But, for the most part, every building was using some type of heat pump and every building was trying to reuse or reclaim a lot of heat. And what I say is traditional HVAC systems, they'll just take heat out of a building and then put it outside when they don't want the building. And then, they'll do the opposite. When it's cold in the building, they'll try to extract heat from somewhere and put it in the building. These buildings, a lot of the effort was done to be able to recover that energy.
You need less in the first place, because you're more efficient. You're covering what you're using so you're using less. And then, after you do those things, and commissioning the building really well, that was another theme through a lot of these buildings is at first, the buildings, they worked well, but often, it would take a period of six to 12 months of commissioning and going through the controls to make sure the building was performing optimally. And so, that was a really big part of these buildings.
And then, once you do all of those things, you are typically going to have a really energy efficient building. Definitely want to focus on decentralized HVAC systems, ones that are zoned so that they can turn off when people aren't using it, and they easily turn off in the evening and the weekend. And separating your heating and cooling system from your outside air, especially in harsh climates, because you only want to bring in what you need for outside air when you're having to heat zero, or below zero air or cool really humid, 90 degree air. You don't want to bring in excess air, or run your HVAC all the time when you only need to bring in outside air.
Those would be my fundamental tips for anybody who's interested in designing energy efficient buildings, or any owners who want to build energy efficient buildings. A lot of it's the fundamentals. And then, after that, you figure out your EUI. And then if you want to, as a bonus, you can choose to put renewables on it. And then, the renewables, almost all the renewables were solar for all these buildings, whether it was rooftop or parking lot. There were a few wind turbines, but they were relatively small, and they were more experimental, so the vast majority of net zero buildings that I studied used solar as the energy source.
Drew Champlin:
I do want to ask you a couple of quick questions about the article in this June 2026 ASHRAE Journal, Decarbonization with Hybrid Geothermal. Did this previous article inspire you to explore hybrid geothermal systems in more detail or something else?
David Traxler:
It was not the previous article. It was a little bit of something else. When you work in sustainable buildings, often the owners want you to do something called a lifecycle cost analysis. And what that is, is you figure out how much extra or how much each system costs from highest to lowest. And then, you figure out the efficiency gains, and the cost reduction of using those types of systems over a period of time. That was a very common thing. And often, with those life cycle cost analysis, you have to justify the cost of whatever energy efficiency measure you use. You can't just do it just because you want to do it. You have to do it because there's a business case behind it.
And so, cost is a big driver in all of our lives, and with building owners also. And so, there's a huge perception in the industry, I think, especially with people who don't do a lot of geothermal, and maybe some who do, who think that geothermal is always going to be too expensive, and to not even consider it in a life cycle cost analysis, or not even consider it for an owner in the south, because geothermal only makes sense in the north.
And so, what I wanted to do is I wanted to dig into it and figure out, okay, when does geothermal make sense, and how can it be more cost-effective? A hybrid system for geothermal, what that means is instead of getting all of the heating and cooling for a building from the ground, which is a traditional geothermal system, you have some other source of energy input or place where you can get rid of energy. And so, for a hybrid geothermal system, we'll use a ground source heat pump system, and then I'll use some other system as a support to that ground source heat pump system. And so, I wanted to figure out, okay, how much of a difference does it make in cost in different locations in the United States from really hot ones like Texas. I say Houston, Texas, very hot, very humid, would not normally think of doing geothermal there.
What does it look like the comparison between a traditional geothermal system and a hybrid? And then, I steadily moved into more north. I moved into more four season climate, and then I moved into like a northern, very cold climate. And I modeled each building, so I just did a traditional office building, and I ran the model on it. And then, I would run the analysis for each building, and the different climate zones as either a traditional geothermal building, or a hybrid geothermal building to look at things like how much did it affect the size of the bore field, and how much does that affect the cost of the project, and cost of doing geothermal? That was a real motivator is to be like, okay, how much impact for good can it have in certain locations to do hybrid geothermal, and does this make it make sense to a building owner or to an engineer to consider when they're doing lifecycle cost analysis?
Drew Champlin:
When deciding between ground source heat pumps, air source heat pumps or hybrid geothermal, what key factors should engineers prioritize early in design?
David Traxler:
Well, one of them is just the land, where it's located. Some places are harder to do geo than the others. Urban environments, you can do it and it's done often. They even put them under buildings. And so, an existing building in an urban environment, it's a bit harder. Then there's also the climate. Air source heat pumps, they are really progressing significantly. In the last 10 years, and even in the last five years, the ability for air source heat pumps to heat in ultra cold climates has significantly improved, in part because of the Department of Energy's challenge for cold climate heat pumps versus residential and now rooftops, so they're really making a lot of progress. The manufacturers are really focused on this also with creating chiller size heat pumps that are able to work in cold climates. There's a huge amount of progress in that area, but it's still developing technology, and there's still people are learning lessons from these things.
It's still a bit harder to apply those types of systems in extremely cold climates and depending on the building type. But, a ground source heat pump is very good at utilizing the steady earth temperature in those climates. Then you can reverse course and go to the real far southern climates where air source heat pumps, they really work great. The only issue you have the further south you go is that air source heat pumps, they don't like it when it's too hot. If it's really, really hot outside, the refrigerant can end up getting too hot, and it can affect the longevity of the compressors and the air source heat pumps. And so, I've worked on projects where they're really, really hot climates, and they're struggling with their air cooled chillers, which is just a form of air source heat pump that doesn't heat, having compressor failures.
The ground source heat pump also mitigates that. You are using the ground's temperature as a stable source. You have less strain on your equipment. Your equipment lasts longer than your air source. Now one area, and this goes back to the net zero article I wrote, where both the air source and ground source heat pump performed fantastic was in mild climates in coastal California, Oregon and Washington. In those areas, the air source heat pumps, they were great. They were very efficient. Even air source heat pumps in Phoenix, Arizona performed really well. Even though it is a really hot climate, and the building was efficient, and it had a good EUI. I don't know about the equipment longevity on that project though. Air source heat pump is more affected by the climate zone than ground source heat pump, because it interacts with the air temperature continuously.
Drew Champlin:
To wrap this up, David, how do you see hybrid geothermal influencing the next generation of net zero energy buildings?
David Traxler:
I see it making them more affordable. The more it can be applied, it can decrease the cost of geothermal, and geothermal is traditionally more efficient than other types of heating and cooling systems. And so, by using hybrid geothermal, you could have a more efficient building with a much less expensive cost than if it were a traditional geothermal system, or a climate that was so hot, or building type that was so hot where you couldn't use geothermal in the past, because you would just overheat the ground and the ground heat exchanger would be so big that it wouldn't make sense financially to do it.
Drew Champlin:
Well, that will wrap up this ASHRAE Journal podcast episode. You can find all of our podcast episodes on ashrae.org. Go to the technical resources tab. Look under ASHRAE Journal, you'll see ASHRAE Journal podcast, and our other podcast, Hot Air, which David was featured on in 2025 about this article in a more digestible, shorter version. You can also search for ASHRAE Journal podcast on any of your favorite podcast platforms such as Apple, Spotify or Google. Want to thank everybody for listening, and want to thank David Traxler for your time and for your expertise.
David Traxler:
Drew, thanks so much for having me. It's been a pleasure.
ASHRAE Journal:
The ASHRAE Journal podcast team is editor, Drew Champlin; managing editor, Linda Rathke; producer and associate editor, Allison Hambrick; assistant editor, Mary Sims; associate editor, Tani Palefski; technical editor, Rebecca Norris; and creative designer, Teresa Carboni.
Copyright ASHRAE. The views expressed in this podcast are those of individuals only and not of ASHRAE, its sponsors or advertisers. Please refer to ashrae.org/podcast for the full disclaimer.