Can We Live Happily Underground?
An owner-builder looks for advice on plans for an earth-sheltered house
Earth-bermed houses built with the Passive Annual Heat Storage (PAHS) approach are a little off the beaten track for most builders and prospective homeowners. These houses go back a bit: John N. Hait described the construction of an early "umbrella house" in the 1980s.
As unusual as they may be, PAHS houses have their advocates. One of them is Laurel Davison, who is planning to build one in Missouri on a gently sloped lot with an unimpeded southern exposure.
In a Q&A post at Green Building Advisor, Davison outlines the basics of her proposed new house: a shell made by Terra-Dome Corporation, heated mostly with passive solar with a minisplit air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. for backup heating and cooling, and a variety of energy saving electrical appliances.
Davison sees several advantages to this approach, including little to no required exterior maintenance — not something she'd get with an above-ground house — and protection from tornados, always a threat in this region.
"Please review my plans... and pick holes in them," Davison writes. "I don't guarantee to change my mind, but I do promise to research varying opinions thoroughly and supporting data would be appreciated ... I have an interested builder, land, money, and a quiet winter to get all the details addressed."
The merits and pitfalls of this type of construction are the topic for this Q&A Spotlight.
No data to back up claims
"I hate to be the bearer of bad tidings," writes GBAGreenBuildingAdvisor.com senior editor Martin Holladay, "but I am going to give it to you straight: You have decided to adopt a discredited approach to residential construction. The design approach you are discussing (the passive annual heat storage approach) has passionate advocates but little data to back up its claims. In those respects, it resembles the earthship approach."
Elements of energy-efficient construction are already well known, Holladay adds: a compact house shape, not too many windows, a tight building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., and lots of insulation in the ceiling, walls, and floors.
"That's basically it," he says. "You don't need many cubic meters of soil around your house, nor do you need to install buried insulation to isolate this soil from other soil. Sorry, but it's true. The PAHS approach requires large investments in measures that are irrelevant to your home's energy performance — expensive measures with no energy yield."
While it's true that underground houses eliminate most exterior maintenance, the cost of waterproofing details are likely to be higher than maintaining siding in conventional construction. Insulation should be located in the shell of the building, not located some distance away from the house as is the case with a PAHS house, Holladay adds. And if tornado protection is a concern, an option worth considering is building above ground with insulated concrete forms (ICFs).
"There are so many aspects of the PAHS approach that make no sense that it's hard to list them," he says. "However, it's probably worth warning you that many homes with earth tubes [a feature of the underground house shown in the illustration above] have abandoned and sealed the earth tubes because of problems with air quality and mold."
Another PAHS advocate responds
Bruce Lepper says he is building his own half-buried PAHS house with rammed-earth walls in Toulouse, France, a process he describes in his own construction blog.
Lepper was convinced to try the building approach by someone who's lived comfortably in one for years. "He says it's the most comfortable house he's ever lived in, and his neighbor, who ordered a copy from him, says the same," Lepper writes.
He mentions other PAHS success stories, including that of Tom Beaudette, founder of Beaudette Consulting Engineers in Montana, who built one for himself. Lepper also cites the experience of Joe LaCour, who he says converted a 1970s era house in the Pacific Northwest into a PAHS structure (see comment #13 in this GBA post).
"There are many other isolated examples, but as Martin would point out it tends to be anecdotal," Lepper says. "I'm hoping to add a little data with the buried temperature and humidity probes around our house, but that will be in the future."
Higher costs and more concrete
A big problem with PAHS construction, Holladay replies, is higher construction costs. Because it's underground, lots of concrete will be required, he says, and that will require waterproofing in order to keep the house dry. Because a large amount of soil around the structure must be insulated, rather than just the building envelope, insulation costs also are higher. Finally, all that extra excavation will mean more spending.
"If you have a fat enough budget to cover these expenses, and you trust the veracity of the anecdotes you've heard about PAHS comfort, go right ahead," Holladay says. "Readers of GBA who are thinking about different approaches to energy-efficient construction, however, deserve an analysis that considers whether these added expenses yield any energy savings. In my opinion, they don't."
You're generalizing, Lepper replies.
"You are wrong with your sweeping statement about PAHS houses being 'much more expensive than other forms of construction,'" Lepper says. "That's like saying that rammed earth houses are expensive. They are if you have them built by a professional in California.
"You are wrong about PAHS houses needing lots of concrete because they are underground, for similar reasons. PAHS houses can be bermed, they don't have to be underground. Mine is on a south-facing slope, and digging out the hole for it was certainly no more expensive than pouring lots of concrete to make the foundations for an above-ground house on a slope."
Earth-bermed houses need no more waterproofing than the typical basement or underground garage, Lepper says, and the earth surrounding a PAHS house is protected by a waterproof, insulated umbrella.
"Your next assertion, that 'you'll need more rigid foam because you're trying to heat the whole neighborhood' makes me wonder if you are really interested in a serious discussion about PAHS (which is what Laurel was asking for)," Lepper addis. "This remark, and your vague reference to 'physics,' seem to indicate that, despite the evidence from homeowners, you think heat cannot be stored in the vicinity of the house and is just going to dissipate around town.
"If that was the case those homeowners would be shivering for six months of the year. Why do they say their house is the most comfortable one they've ever owned?
"Is Hait a crackpot, or could he be the genius of the insulated umbrella?" Lepper asks.
Davison is looking for specific recommendations in key areas: the specs of the insulating and waterproofing membrane over the soil surrounding the house, and the ventilation the house will require. But GBA reader Brendan Albano suggests she is looking for answers in the wrong place.
For advice on the membrane, Albano says, go to the company supplying the components for the shell. For ventilation advice, consult someone who's familiar with PAHS construction rather than the third-year civil engineering student Davison has been speaking with. Read Malcolm Wells' book on earth-sheltered construction, Albano advises.
"And just to be clear, I'm not trying to discourage you from posting here, just pointing out to be cautious about the information you receive if you are asking for experts in one subject (standard above-ground energy efficient building, and off-grid living in these sorts of buildings) for advice about a related but different subject (below-ground PAHS energy efficient building and off-grid living in those sorts of buildlings)," he says.
"And just to be clear, my biases would be to build a standard insulated home for efficiency purposes. And if you want to have it be underground for aesthetic purposes, just put it underground! Even if it's just a big pile of dirt, PAHS is still a complex system. I would argue that if you're after simplicity, a well insulated wall and a wood stove is much simpler than PAHS. But if you want to build a PAHS home, definitely seek out advice from people with the expertise!"
Stephen Sheehy has one additional thought: recouping costs through the sale of a PAHS house might not be very easy.
"One thing about PAHS is that I suspect such houses, once complete, are probably not salable at a price that would recoup much of the significant cost of construction," Sheehy says. "So long as one is either rich enough not to care or old enough to be sure this is the last house, resale isn't important."
Malcolm Taylor adds that PAHS construction is likely to remain a niche building technique, if only because it requires at least a large suburban lot and is unsuited to urban or multifamily construction, which makes up the bulk of the housing market.
"They also use techniques outside the established construction used by builders, and the end product isn't one familiar to the tastes marketers and culture have instilled in home buyers over time," Taylor says. "That doesn't mean individuals shouldn't build one if they want to. There are all sorts of alternate types of construction, like log houses, rammed-earth, straw bale, etc., that are easy to dismiss because of their lack of general adaptability which make their individual occupants very happy."
Our expert's opinion
Here's what GBA technical director Peter Yost had to add:
Having toured the Terra-Dome website, I have to admit that some of their homes in the photo gallery are pretty intriguing. That’s the Hobbit in me, and I don’t say that sarcastically; there is something comforting and cozy about any earth-bermed home. And in some ways, depending on the specific design of the home, you are essentially building a walk-out basement home.
But the builder and building scientist in me cringes, based on these points:
Ground-coupling. This approach — inherently cooler in summer and warmer in winter — just sort of rolls off the tongue. But it’s damn near impossible to accurately model this to get insulation levels well-tuned. Overheating and underheating when diurnal cycles and the sun and clouds don’t cooperate may well end up being the rule and not the exception. And whether or not we can accurately model ground-coupling, installing a radiant floor heating system that is ground-coupled (with no insulation underneath the coils) will heat the ground below more efficiently than the living space above.
The need to be mistake-proof. With so many complex waterproofing details buried underground, you pretty much get just one chance to get it right. Any home benefits from quality of design and construction, but this approach seems to me to require it.
Durable but inflexible design. Depending on the design — just how much of the roof is actually underground and how many interior walls are structural concrete — you had better get the size of your home and family perfectly matched, and you had better anticipate all the intended uses. Retrofit and addition options will be more than just limited.
Cost. I took a quick look at the pricing sheet for Terra-Dome. It does not start off well: “…Insulation, waterproofing, drainage, and backfill are not included in the construction costs.” It scares me that while other per-component costs are included in the price sheet, something as essential to the design as moisture management is not included, even in a per-square-foot range of cost.
Warranty. With this type of design and construction I would expect a warranty to be front and center on the website. I searched around quite a bit and found no discussion at all of system warranties.
Here is the good news: This GBA Q&A Spotlight blog and the Q&A thread that inspired the blog mean that Laurel Davison has a ton of information and insights to make her eventual design and construction much more successful.
- John Hait
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