An earth-sheltered passive solar house is buried on three sides and open to the sun on the fourth, so the ground provides mass and steady temperature while a glazed wall admits winter sun. At 40 degrees north the winter noon sun reaches about 4.8 m into a room behind a 2.4 m high glass wall, and the cool earth walls cause condensation whenever outdoor air has a dew point above the wall temperature.
Part of Passive Solar Building Design
Combining earth-sheltered construction with sun-facing glass, and managing the roof load, light depth, humidity, radon and waterproofing that come with it.
The Ground Is the Mass and the Envelope; the Glass Is the Heater
The carrier states it: earth-sheltered buildings use the ground itself as thermal mass and insulation, and a south-facing glazed wall admits winter sun into the earth-mass interior. This page follows the carrier in Northern Hemisphere wording; in the Southern Hemisphere the glazed wall faces north.
The ground is not a perfect insulator and not a constant. Ground temperature is close to the annual average air temperature (carrier), but the yearly wave in the soil is damped and delayed: at 2 m the swing is still about 41% of the surface swing, with a lag of about 52 days for a soil diffusivity of 5 x 10^-7 m2/s (see Earth Tubes & Ground Coupling). Earth-sheltered rooms stay moderate, and they have no sun except through the open side. Dry earth is a poor insulator (R-0.2 per inch, carrier): the ground’s benefit is a steady temperature, not a high R-value, so thin cover or exposed walls still lose heat in a cold climate. The existing earth-sheltered page covers methods and waterproofing: see Earth-Sheltered Building.
How Deep Does the Winter Sun Reach?
Direct sun reaches along the floor a distance of (head height of the glass) / tan(sun altitude). For a full-height glazed wall with the sill at 0.8 m and head at 2.4 m:
- 40 N, 21 December, noon, altitude 26.6 degrees (tan 0.50): the sun reaches from 0.8 / 0.50 = 1.6 m to 2.4 / 0.50 = 4.8 m from the glass.
- 48 N, altitude 18.6 degrees (tan 0.337): 2.4 m to 7.1 m.
Beyond that distance the room gets no direct winter sun. In an earth-sheltered plan the rooms behind the glass are deeper than this, so use a rear skylight or clerestory, or stack the room depth so the mass wall sits within the sun patch. Overhangs shorten the patch: design the glass and the overhang together using Overhang Design for Seasonal Control.
Roof Load: The Number to Check First
The existing earth-sheltered page gives soil as about 1,500-2,000 kg per cubic metre. A layer of earth of depth t on the roof loads it by rho x g x t:
- 0.5 m of earth: 1,500 x 9.81 x 0.5 = 7.4 kPa; 2,000 x 9.81 x 0.5 = 9.8 kPa.
- 1.0 m of earth: 14.7 to 19.6 kPa.
- A roof spanning 4 m with 0.5 m of cover carries 7.4 to 9.8 kPa x 4 m = 29 to 39 kN for each metre of building length (before the roof’s own weight, and saturated soil weighs more).
Saturated soil weighs more than dry soil, so size for the wet case, and use the existing page’s rule of a safety margin (it asks at least three times the expected dead load). Roof failure gives no warning.
Humidity: Cool Walls Condense Warm Air
The walls sit near ground temperature. Take the walls at 12 degrees Celsius (an assumed value) and outdoor air at 28 degrees Celsius and 60% relative humidity. The dew point from the Magnus approximation (a = 17.62, b = 243.12) is: gamma = ln 0.60 + 17.62 x 28 / (243.12 + 28) = -0.511 + 1.820 = 1.309; dew point = 243.12 x 1.309 / (17.62 - 1.309) = 19.5 degrees Celsius.
The dew point is above the wall temperature, so opening the house to that air makes water condense on the walls: mould, rot and dampness follow. In humid summer weather keep ventilation to what is needed and ventilate in the cooler, drier part of the day or night. An earth tube that condenses water out of the air and drains it outside also dries the air that arrives.
What You Need
- Waterproofing and drainage — see Earth-Sheltered Building.
- A glazed wall — sized by Window Placement & Sizing.
- Two ventilation points — one high, one low, on opposite sides.
Method: Design the Sections
Step 1: Section through the sun Draw a section on the winter noon sun line: glass, sun patch, mass floor and wall.
Step 2: Check light depth If rooms lie beyond the patch, add a skylight or clerestory.
Step 3: Size the roof Compute the earth load, plus saturated weight, plus a margin.
Step 4: Plan humidity Plan how moisture leaves: ventilation shafts, tubes that drain outside, and rules for when to open the house in summer.
Common Mistakes
- A deep room with a tiny window — dark and damp.
- Roof sized for dry earth only — it fails in the wettest season.
- Ventilating with warm humid air — condensation.
- Glass without a drained apron — water pools at the base.
Safety
Collapse, radon, low oxygen and no way out
Roof: earth loading collapses without warning if undersized. Radon: soil gas can collect in buried rooms; 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, so ventilate, seal floors and walls and test where possible. Air: closed buried rooms build up carbon dioxide and carbon monoxide from fires. Keep a flue for every fire, with a separate fresh-air path. Exit: provide a second way out, because a single door can be blocked by a fire or flood. Excavation: a trench or pit 1.5 m or deeper can collapse; shore or slope it and never work alone.
What’s Next
- Learn next: Earth-Sheltered Building — structure and waterproofing in detail
- Combine with: Ventilation & Summer Cooling — drying and cooling the buried rooms