I’ve read, and reread your net-zero home article (“Net-Zero Performance” in HP150) many times. I am going to break ground on an earth tube system near Provo, Utah, modeled upon the one in the article, and I have some questions.
Is antimicrobial trade-name pipe better than stock pipe? What evidence exists showing bacterial overgrowth in well-designed earth tube systems, and mitigation with the more expensive product?
Costs depend greatly on trench depth. In rocky, boulder-strewn ground like mine, I’ll have to strike a balance between trench depth and cost. How about placing polystyrene insulation on top of the pipe at a depth of 3 to 5 feet and then backfilling, instead of digging 10 feet deep?
Chris Anderson • via email
The system I used— the only one I found with an antimicrobial coating in the tubes—is Ecoair by Rehau. Their U.S. division is Amvic (amvicsystem.com).
I read about the issue of mold in a handful of articles by folks who installed earth tubes, but typically theirs were installed in more humid climates than my climate zone 5B in central Colorado. I found as much anecdotal evidence of systems that had no mold issues. While I’m not sure what all the differentiating factors are, I suspect:
Still, I did not want to gamble with my family’s health, so I went with the Ecoair. The systems are costly, though. For the roughly 4.5 kBtu per hour heating capacity we gain from the earth tube, we will have a very long payback time to recoup the $6,500 system cost, including trenching, labor, and parts. I knew this when we designed the house, but this was one of about four areas where my wife and I opted for the moral imperative of reducing our ecological footprint—we intentionally did not select the most cost-effective course of action.
As for trench depth, I believe you will be compromising the system’s performance if you don’t bury the pipe deep enough. In the Provo, Utah, area, at a depth of 3 to 5 feet, pipes will barely be below the frost line and ground temperature will fluctuate. At your latitude, the soil temperature will be 50°F to 52°F at approximately 8 feet of depth. By insulating above the pipe, you would reduce part of the surface area available for heat transfer. However, if trench depth is a insurmountable limitation, using insulation would be better than not using insulation. I would use R-10, 2-inch extruded polystyrene foam, and extend the foam 4 feet on either side of the tube. But overall, the deeper the tube, the better. Data from the geothermal heat-pump industry indicates that at depths of 6 meters and greater, the soil temperature equals the mean annual air temperature. At 4 meters below the surface, the soil temperatures vary approximately 3°F; at two meters, about 10°F.
Some installers run a cable the full length of the earth tube during construction, which is used to pull a cleaning rag or plug through the tube once per year. This seems like a good, low-tech way to reduce the risk of mold.
Jim Riggins • EnerSmart Energy Solutions; enersmartenergy.com