Daniel McKinney is reaching four decades into the past for two important features of a new house he plans to build. Both notions were mostly discarded after early attempts at energy efficiency led builders in new directions, but McKinney thinks they may still have some merit.
“OK, here’s the basic idea,” he writes at Green Building Advisor’s Q&A forum. “I would like to use an earth tube system to bring fresh air into a very tightly sealed home. I’m also designing the home with a ‘solar stairwell,’ a stairwell that’s exposed to the sun with big windows, with the back wall of the stairwell being made of dark-colored cast concrete.
“During cold months,” McKinney continues, “the concrete wall acts as thermal mass, gathering heat during the day and shedding it during the night. Exterior louvered shades above the big windows would keep this wall from getting direct sunlight during the summer. So, the question is this: Would it make sense to duct the earth tube air vertically through the concrete wall?”
An earth tube, which is simply a pipe buried in the ground, alters the temperature of incoming air because soil temperatures well below grade don’t change much seasonally. Incoming air is warmed in winter and cooled during the summer. The other part of McKinney’s plan, a heat absorbing, high-mass Trombe wall, was a common feature of many early passive solar homes. The appeal of both of these ideas lies in their simplicity, but many builders and designers now think their flaws outweigh any potential benefit.
Who’s right? That’s the topic for this Q&A Spotlight.
No, this is the wrong approach
Earth tubes and Trombe walls are dated concepts, says GBA senior editor Martin Holladay. Don’t bother.
“You are entertaining two ideas from the 1970s,” he says. “Both ideas have been substantially discredited, but, like the walking dead, it seems that it takes more than a stake through the heart and a bunch of garlic to keep these ideas where they belong.
“Briefly, the added cost to build a Trombe wall or install an earth tube are so high that there is no way that any conceivable future energy savings will ever be enough to justify the investment,” Holladay continues. “In the case of a Trombe wall, the details you describe may actually increase rather than decrease the annual energy load of the house. Earth tubes are expensive to do right — you need a very large diameter duct, buried very deeply, in a very long trench — and are subject to problems with condensation and mold. They don’t save much energy.”
Stephen Sheehy wonders why McKinney would consider installing earth tubes when he already is considering mechanical ventilation. Further, there is the prospect of a decline in air quality over time.
“I guess what scares me most is that the tube is pretty much impossible to clean, so over time the ‘fresh’ air is likely to get less fresh,” Sheehy writes.
Early inspiration from Canada
McKinney isn’t alone in his interest in earth tubes. Brad Hardie also was intrigued by the technology as he designed a net-zero energy house in New Hampshire. Hardie points to a 2014 guest blog at GBA by Malcolm Isaacs, a Passivhaus pioneer in Canada, who installed earth tubes at his own house.
In researching the topic, Hardie wrote to Isaacs to ask whether he’d changed his mind about earth tubes since he installed them, and whether Hardie should incorporate earth tubes in his own design. His plan was to use the tempered incoming air to lower energy demands by piping the earth tubes directly into a Zehnder heat-recovery ventilator instead of installing a Zehnder ComfoFond to moderate outdoor air temperatures.
“I have been considering the Zehnder ComfoFond as an add-on to the HRV, but have to admit the idea of using earth tubes has me so much more excited,” he told Isaacs. “Is there anything you would change, add, or omit if you could do it all over again? Some other info: I’ve got a fairly level site, with well draining sandy soil, and I will be doing the excavation myself.”
The short answer was, no, Isaacs said. Earth tubes still look like a good idea.
“A couple of colleagues have built much more high-end earth tubes than mine recently, with ‘proper’ components,” Isaacs replied, “but I feel that if you’re vigilant and careful then you don’t need custom products to do this — the [earth tube] approach is remarkably resilient. I did everything ‘wrong’ but it still works very well, so I might use a better quality [polyethylene] pipe, but otherwise not much would change next time.”
That view is shared by Jason Morosko, a certified Passive House consultant who has been living with an earth tube system for four years. From his experience, McKinney might be able to gain between 500 and 3,500 BTU/h of “free energy” from earth tubes. He invested less than $1,200 in his system of two 8-inch pipes 100 feet long.
“Are they worth it?” he asks. “Well, in my climate my [energy recovery ventilator] would require preheat to ventilate at cold outside conditions (times below 12°F). Hence, I would have need of some type of duct heater to keep incoming air warm during cold snaps. Hence, the duct heater is generally an electric resistive device, meaning large instant amp draw for short pulses. Not a lot of energy — but if you are looking at an off-grid house with solar electricity, you do not want spikes of energy use, which are bad for batteries and storage capacity. The load happens at the worse condition (climate timing speaking).
“So that is something to consider,” he continues. “Given my cost and experience of four years thus far — yes, it was worth it. Financially, maybe break even at year 4?”
Taking necessary precautions
The idea has obvious appeal, McKinney adds. Fresh air at a temperature that has been moderated by the earth looks a lot better than ambient air when it’s 10°F below zero.
“I don’t think that the requirement that a system be designed well should disqualify the idea out of hand,” he says. “A poorly designed and constructed roof can spell disaster for a new home, but does that mean that the idea of a roof is a bad one?”
Excavation costs, an argument against earth tubes, will be lower because he plans on doing the work himself, and he can make the earth tube trenches do double duty by using them for utility and water lines as well as earth tubes.
Further, he could use a material that’s more thermally conductive than plastic, and design the system so that condensed moisture can’t collect or cause mold growth.
“Obviously, the pipe must be pitched at an angle that will allow condensation to flow and not puddle, and that condensate must be able to drain at one end or the other,” McKinney adds. “I’ve seen designs where the tube pitches down toward the building and the condensate drain is in the basement. This seems like a good idea, allowing for that line to be cleaned, and allowing access directly to the earth tube, as long as the desired depth can be achieved and still have the end of the tube accessible inside the basement.”
Installing ultraviolet lights at one end of the tube would be another hedge against the contamination of income air by mold, bacteria and fungus, he says.
“I am quite fond of more than a few ideas that were born in the ’70s,” he says. “That decade did generate some useful stuff.”
Making the right choice for the type of pipe
McKinney has been considering a type of pipe called PCD, or polyvinyl coated ductwork, which is a G-90 galvanized steel pipe primed with epoxy and finished with a 4-mil polyvinyl coating. If buried above the water table, McKinney says, it would provide much more thermal conductivity than PVC pipe.
It’s not the thermal conductivity of the duct itself isn’t the limiting factor in heat transfer, writes Charlie Sullivan, but the thermal conductivity of the dirt and the thermal transfer between the air the earth tube.
“And in other resects,” he says, “the PVC/galvanized steel sounds like the worst possible combination to me: the steel will eventually rust out, and in the meantime, the PVC will outgas into your ‘fresh’ air. I would want a smooth-walled seamless polyethylene tube. (Polyethylene is hard to glue, and so regular HDPE drain pipe would not be ideal — the joints could leak soil gas into the system.)”
For these reasons, Sullivan suggests a smaller-diameter buried line in which a glycol mixture is circulated, eliminating the potential for mold that comes with an air system.
“Then you spend less money on pipe and take advantage of the better heat transfer with the liquid, which you then connect to a liquid-to-air heat exchanger,” Sullivan says. “It’s pretty clear that that’s not cost-effective, but if you are wanting to do a DIY system and you aren’t too concerned about cost effectiveness, you could probably DIY one of those for less than the Zehnder system.”
It’s true, Holladay adds, that a glycol loop brings with it a lower risk of low indoor air quality. But the bottom line is that neither a glycol loop nor an earth tube approach is cost-effective. “They both cost more to install than can ever be justified by future energy savings.”
Try it, and then let us know
McKinney seems confident that earth tubes are still relevant, and seems unwilling to let go of the idea. With that in mind, Holladay suggests he just give it a try.
“Here’s my advice: go ahead and install an earth tube,” he says. “Keep track of how much it costs to install the earth tube (including an estimate of excavation costs, backfilling costs, and landscaping costs). Install some monitoring equipment for temperature and relative humidity [RH]. You want to measure the temperature and RH of the air right before it enters the HRV. Ideally, you also want to log outdoor temperature and outdoor RH.
“After you live in your house for two years, write a blog for GBA about the experience,” he continues. “And remember — if you have mold problems, you can always cut off the earth tube in your basement with a hacksaw and cap it. Then you can install an air intake for your HRV somewhere else.”
Our expert’s opinion
GBA technical director Peter Yost added this:
I must admit that my building science background creates an almost knee-jerk reaction against earth tubes: To what gain would you intentionally run your fresh air system through underground ducts? And certainly early work with earth tubes was rife with indoor air quality problems, either from the get-go or over time.
But there are fully-engineered earth tube systems, such as those being designed by George Sullivan of Net Zero Analysis & Design Corp and by Jason Morosko of UltimateAir that I think should not be dismissed out of hand.
Sullivan has done more than a half-dozen earth tube designs, tempering the air exchange in his ERVs with tubes running through 55°F soil temperatures. “In Climate Zone 5 and colder zones, it’s worth it because of the combined heat and cooling effect,” says Sullivan. “In really cold climates, the defrost cycle avoidance is a big plus.” Sullivan has developed his own modeling program to design and predict his earth tube systems.
Another consideration is the defrost cycle associated with ERVs and HRVs, which vary by device. For example, the graph shown in Image #2, below, shows the energy associated with defrost cycles from BuildingGreen founder Alex Wilson’s Zehnder’s ERV system.
Finally, a word about PVC and CPVC outgassing: While it’s true that both PVC and CPVC pipe outgas, these “rigid” plastic pipe products should not be confused with PVC shower curtains, PVC vinyl siding, etc. These more flexible PVC products contain plasticizers, such as phthalates, which are much more volatile and of much greater concern.
[Editor’s note: In an earlier version of this article, GBA published an inaccurate report of energy savings data from an earth tube installation. The error was mine. GBA regrets the error. — Martin Holladay.]