- Posted December 16, 2010 by
This iReport is part of an assignment:
Home improvement success
DIY Passive Geothermal Home Heating
I have a small old house, but it's mine, bought and paid for.
The best machine (device) has *no* moving parts. The further you get from that ideal, the less joy you will have with it. In any existing subdivision, there are fairly limited options about what you can do realistically, in terms of alternative energy.
I live in northern Utah, and months of sub-freezing weather would be a normal winter here. My "basement" is finished living space, but it was very expensive to keep it well-heated. I put in a passive geo-thermal home heating system, to reduce my heating bills. Basically, I took the bottom half of the house from Northern Utah to South Carolina.
The bottom line:
My home heating bills dropped by 35%~45%, so I broke even for the pay-back on the project in less than one winter. Most importantly, no neighbors have objected to the appearance of the house.
It cost me $125 for a dumptruck-plus-a-pony-load of dirt, and $40 for plastic sheeting. The grunt-work was done in half a day, by a Bobcat front-loader with operator, for $100. I had a borrowed Ditch-Witch trencher (used to put in an irrigation system, later in the week).
See the cross-section picture, attached.
My house foundation is about four to five feet exposed, above the ground line. Above the foundation, the house is brick. I built a "Dry Dirt" berm all the way around the house, one foot below the brickwork, and at least a foot wide across the top, away from the house. This berm then slopes at 40 degrees, down to the existing ground level. I used 2-mil sheet plastic (the type which never degrades in a landfill). I taped the sheet plastic to the top of the foundation, and ran the plastic down the wall, across the berm, and down the slope. At the toe of the slope, I used the trencher to make a six-inch slit trench, as deep as the Ditch-Witch could reach (about six feet down). You want the slit trench to be as deep as possible, to get the most benefit from the system. I used 1x2 slats to push the "skirt" of the sheet plastic all the way down to the bottom of the trench, before the trench could collapse behind the trencher.
The Bobcat then covered the plastic sheet (and filled the slit trench), adding a one foot depth of Planting Dirt to the original berm, so that ground-covering landscaping plants could be added. No plastic sheet is exposed, at any location. With the expected minor settling, my berms start just below the brickwork line, all around the house. We were done, including the sweeping, by sundown.
Why it works:
As you dig deeper, the ground gets warmer. Below about 2'~3' the earth never freezes, in my location. At the bottom of the slit trench, a thermometer inserted into the dirt would read about 50 degrees F, all year long. The dirt beneath the plastic sheet soon loses ALL moisture, and this Dry Dirt becomes very efficient insulation, both as a convection barrier (like fiberglass insulation) and as thermal mass (it's hard to cool any big mass, by applying cold temperatures to only one side). It is the moisture content of normal soil which normally makes dirt act as a poor insulator. In short order, the entire Dry Dirt berm gets to the same temperature that you found at the bottom of the slit trench. The interior walls in the basement now do not get cold, like before. I believe this project would also reduce or eliminate any basement seepage, if moisture was a minor problem before.
An insulated interior basement wall would have been the conventional approach or remedy, for my cold basement. For the same cost as my project, I figure (doing all the work myself) I might have built an interior insulated wall about eight feet long, inside the foundation (not much help, there). The total cost of materials alone to *complete* such a pricey project would have been well over five thousand dollars, with a decades-long pay-back (break-even) in heating bills as a result. I would have had to rip out all of the tongue-and-groove pine paneling (which I like), and made each room smaller as a result. I have neatly avoided all of these issues, with my berm project.
I like the benefits of lower heating bills, *and* putting less CO2 into the air. If you look at the pictures attached, I would hope the word that pops to mind is "unobtrusive." I believe this project will last as long as the house, with no maintenance issues. Remember that without the trick berming, the foundation would have been frozen concrete below the brickwork, and for several feet more, underground.