An earth tube is a buried pipe, 15-30 m long, 1.5-2.5 m deep and 15-30 cm across, that draws outdoor air through the cooler or warmer soil before it enters the building. Ground temperature does not become constant at 2 m: with a typical soil diffusivity the yearly swing at 2 m is still about 41% of the swing at the surface (for a soil diffusivity of 5 x 10^-7 square metres per second), and the soil temperature there lags the surface by roughly 52 days.
Part of Passive Solar Building Design
Using the soil’s stored heat to pre-cool or pre-warm ventilation air, how the ground’s yearly temperature wave behaves, and the drainage, radon and trench hazards.
The Ground Is a Slow, Damped Temperature Wave
The carrier says that below about 2 meters, ground temperature remains nearly constant year-round, roughly equal to the annual average air temperature. That is the right idea with one correction: the yearly wave in the ground is damped and delayed, not removed. The same conduction equation as the daily one (see Heat Absorption & Release Cycles) applies with a period of one year, P = 365.25 days = 31.6 million seconds.
Characteristic depth: d = sqrt(a x P / pi), with soil thermal diffusivity a typically about 3 to 8 x 10^-7 m2/s (a handbook range; it depends on soil type and moisture). For a = 5 x 10^-7 m2/s:
- d = sqrt(5 x 10^-7 x 31.6 x 10^6 / pi) = sqrt(5.0) = 2.24 m.
- Fraction of the surface’s yearly swing remaining at depth x is exp(-x/d): 51% at 1.5 m, 41% at 2 m, 26% at 3 m, 17% at 4 m.
- Lag at depth x is (x/d) x 365.25 / 2 pi days: 52 days at 2 m.
- Across the range a = 3 to 8 x 10^-7, the fraction at 2 m runs from 32% to 49% and the lag from 67 to 41 days.
Worked on a surface swing assumed to be 12 K (not a measurement), the swing at 2 m is about 12 x 0.41 = 4.9 K. In Northern Hemisphere seasons (shift by six months south of the equator), the soil at 2 m is warmest in autumn, not summer, and coldest in late winter or early spring, so a tube is most useful in midsummer (soil still cool from spring) and midwinter (soil still warm from autumn). Mid-spring tube air can be cold.
What You Need
- Pipe — salvaged PVC, clay pipe or stone-lined channels, as the carrier lists; 15-30 cm across.
- A trench — 1.5-2.5 m deep, 15-30 m long, with a fall (see below).
- An intake tower — screened against rodents and insects.
- A drain — at the lowest point.
Method: Lay Out the Tube
Step 1: Choose the inlet Take air from a shaded, clean spot above any flood or snow line, away from smoke, animal pens and latrines.
Step 2: Dig and lay the pipe The longer the tube, the closer the air gets to ground temperature (carrier). Slower air also approaches it more closely. Keep depth at 1.5-2.5 m.
Step 3: Slope it so water runs out A fall of about 1% (0.25 m over 25 m, a commonly used figure) is enough if the water has somewhere to go.
Step 4: Seal and screen Seal every joint so soil gas cannot leak in; screen the intake with fine mesh.
Slope direction. The carrier says to slope the tube toward the building for condensation drainage. This page recommends the opposite: put the lowest point outside the building, with a drain or soak pit, and slope the tube toward it. If the low point is inside, every drop of condensation and any water entering the tube runs into the living space. Summer air entering a cool tube condenses water; expect it.
Worked Example: Air Through a Tube
A tube 25 m long and 0.2 m across (inside) carrying 0.05 m3/s.
- Cross-section: pi x 0.1 x 0.1 = 0.0314 m2.
- Speed: 0.05 / 0.0314 = 1.6 m/s.
- Time in the tube: 25 / 1.6 = 15.7 s.
- Air delivered: 0.05 x 3,600 = 180 m3/h = 3.5 air changes per hour in a 52 m3 room.
This ventilates, not cools by itself: how close the air gets to ground temperature in 16 seconds depends on pipe material, soil and air speed, which this page does not compute. Treat tube air as a modest tempering of fresh-air supply, not as air conditioning.
Common Mistakes
- Treating 2 m as constant temperature — it swings, and it lags.
- Short, wide tube with fast flow — air has no time to exchange heat.
- Low point inside the house — condensation floods the room.
- No clean-out access — a tube you cannot inspect becomes a mould duct.
Safety
Radon, mould and trench collapse
Radon: soil gas from some rocks and soils is radioactive and enters buried pipes through leaking joints. The US EPA action level is 4 picocuries per litre (148 becquerels per cubic metre) and the WHO reference level is 100 becquerels per cubic metre. Seal joints, test indoor air where you can, and do not use a tube in ground where radon is known to be high. Mould and bacteria: damp tubes grow them. Keep the tube draining and dry, with access to clean it, and shut it if the air smells musty. Trenches: a trench 1.5 m or deeper can collapse without warning; US workplace rules (OSHA) require protection for excavations of 5 feet (1.5 m) or deeper. Shore or slope the sides, keep spoil back from the edge and never work in a trench alone.
What’s Next
- Learn next: Passive Solar + Earth-Sheltered — using the ground as the whole envelope
- Combine with: Stack Effect & Thermal Chimneys — drawing air through the tube