Part VI: Staying CoolChapter 10. Passive and Low-Energy Cooling
Earth Coupling and Ground Temperatures
How deep soil temperature lags the air, and the cautious ways people couple a house to it.
- Difficulty
- Intermediate
- Cost
- Free
A few feet down, the soil does not follow today's weather. It lags the seasons. Deeper still, the temperature sits near the annual average of the air, a little warmer or cooler depending on the site, and barely swings. In a climate whose average is near 50 or 55 degrees F, that deep soil is a cooling source in July and a mild neighbor in January. In a hot climate the average itself is warm, and "underground" is not automatically comfortable.
The cautious uses of that fact are the ones the house already has: a basement, a slab, a berm against a foundation. The tempting misuse is a buried pipe that is supposed to cool outdoor air on the way indoors. Warm humid air in a cool pipe condenses. The water sits. The next thing the fan does is blow that wet, dirty pipe into the house. Radon and soil gas have a path they did not have before. This page will not size that pipe, because the usual homemade version is a mold machine.
Difficulty
- Level
- Intermediate
- Time
- An evening of looking up your ground temperature, not a trench
- Skills
- Separating a soil fact from a duct project
Cost estimate
Free
A ground-temperature table for your region is a lookup. A ground-source heat pump is a different machine and a large purchase, with refrigerant handled by a licensed technician. A buried air duct is the option this page refuses to price, because it should not be your next project.
Materials
| Item | Qty | Notes |
|---|---|---|
| A local ground-temperature reference | 1 | Annual average air temperature is only a hint. Use a regional table. Do not borrow 55 degrees F from a book about somewhere else. |
| The basement or slab you already have | If any | That is the coupling. Measure it before you invent a new one. |
| A radon test | If you live over soil | Any plan that opens a path from the soil to the air you breathe starts with this, and often ends with a mitigation system instead of a clever duct. |
What the temperature is doing
Near the surface, the soil swings with the weather, delayed by days. A few feet down, the swing shrinks and the peak arrives months late. Deep enough, you mostly feel the yearly average. That is why a basement can be the coolest room in July and a reasonable place in winter, and why it can also be damp: cool surfaces and moist soil air are a condensation pair even when nobody buried a duct.
A slab on grade couples the floor to shallow soil, not to deep earth. It helps a little. It is not an air conditioner. Carpet disconnects it, which you may want in a cold winter and not want in a hot summer. An earth berm against a wall cuts the wind and the temperature swing on that wall. It also needs waterproofing that actually works, or the wall behind it stays wet.
A ground-source heat pump uses the ground as a reservoir for a refrigerant machine. It is not a fan and a culvert. The loop is closed, the refrigerant stays in the machine, and the people who install it are licensed for that work. If a contractor describes an open air pipe as "like geothermal," they are describing a different and worse idea.
Why the air pipe fails
Take a July afternoon at 90 degrees F and high humidity. Push that air through a pipe at 55 degrees F. The air cools, which was the hope, and it crosses the temperature where it can no longer hold its moisture. Water films the pipe. Dust sticks. A fan later blows across that film into bedrooms. A drain at the low point removes liquid water and does not clean the film off the walls. Filters clog and then channel. In winter the same pipe can be a cold surface in a different way, and soil gas, including radon, can move toward the fan whenever the house is at a lower pressure.
People have built careful versions with smooth pipe, a reliable slope, access for cleaning, and a radon strategy. Those are engineered ventilation projects. They are not a weekend with corrugated drain pipe. This book does not contain the dimensions, because copying a diameter and a length from a general page is how the bad version gets built. If you need cooling, shade the glass, night-flush when the air is dry and cool, and use a real air conditioner when the weather is dangerous. If you need a basement that is dry, waterproof it and dehumidify it. Test for radon either way.
Look up the ground temperature where you live
Use a regional table and note the depth. Shallow soil is not deep soil.
Measure the basement or the slab you already have
Summer temperature and whether it smells damp. A dehumidifier and a water problem are more useful than a new trench.
Test for radon before you open any soil path
Follow the test instructions. Mitigate if the result says to. Do not add a fan that pulls on the soil.
Keep cooling plans above ground
Shade, a cooler roof if the attic is thin, and night air when it is actually cool. Leave refrigerant loops to licensed installers.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Cooling/passive_cooling.htm
- https://www.builditsolar.com/Projects/Cooling/Shading/Shading.htm
- https://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
- https://www.builditsolar.com/Projects/Cooling/NoAC/ComfortWithNoAC.htm
- https://www.builditsolar.com/Experimental/RoofCooling.htm
- https://www.builditsolar.com/Experimental/RadiationCool/EvaproRad.htm
- https://www.builditsolar.com/Experimental/VentSize/
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.