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Passive Solar + Earthbag

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Integration with Building Methods
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Passive Solar + Earthbag

A soil-filled earthbag wall is thermal mass, not insulation: using the carrier’s R-0.2 per inch for dry earth, a 40 cm wall is only about R-3, roughly one ninth of an 18-inch straw bale wall. Use it as mass inside an insulated envelope in cold climates, and bare as a daily temperature buffer in hot-dry ones, with the bags plastered to protect them from sunlight.

Part of Passive Solar Building Design

Using earthbag walls as thermal mass in a passive solar house, how much of the wall works, and where insulation has to be added.

Earthbag Is Mass First

The carrier says earthbag walls provide both moderate mass and moderate insulation and advises external insulation in cold climates. Taking the figures at face value the insulation is low. Dry earth is about R-0.2 per inch (carrier, in its list of natural insulation). The earthbag page gives the wall thickness as about 40 cm.

  • 40 cm = 15.75 inches, so R = 15.75 x 0.2 = 3.15 in US units = 3.15 x 0.1761 = 0.55 m2 K/W.
  • An 18-inch bale wall at R-1.5 per inch is R-27. The ratio is 27 / 3.15 = 8.6.

So the soil-filled wall insulates about one eighth to one ninth as well as the bale wall. It is not an insulating wall, which is why the carrier’s advice about external insulation matters.

How Much of the Wall Is Working Mass?

From the diffusion arithmetic (see Heat Absorption & Release Cycles), a daily cycle penetrates about 0.11 m into earth, and the layer within twice that (about 22 cm) holds nearly all the daily swing. In a 40 cm wall the inner 22 cm or so does the daily work if it is exposed to the room; the outer part is long-term storage. That has two consequences:

  1. Put insulation on the outside and leave the inner face exposed (plastered, not panelled), so the inner 22 cm joins the cycle.
  2. Wall thickness above about 22 cm brings little to the daily cycle but stores heat across cloudy days.

Design by Climate

  • Cold climate with winter sun — insulate the outside (straw-clay, bale cladding or an insulated roof and wall skin), glaze the sun side, leave the interior as mass.
  • Hot-dry climate with large day-night swing — a bare, shaded earthbag wall holds the cool of the night through the day. Shade the walls, flush with night air, and do not add insulation inside.
  • Cold and cloudy — mass without sun is a heat sink. Prefer insulation and a tight envelope; use mass only where the sun reaches.
  • Light fill — where light volcanic fill such as scoria or pumice is available, it can replace soil to gain insulation at the cost of mass.

What You Need

Method: Fit the Passive Solar Features

Step 1: Layout Long axis east-west, glazing south (north in the Southern Hemisphere). Domes make orientation harder: put the main window and the sunlit floor on the sun side.

Step 2: Glass and shade Size the glass as in Window Placement & Sizing, and shade with an eave or porch, since a dome has no natural overhang. See Overhang Design for Seasonal Control.

Step 3: Insulate outside in cold climates Wrap with insulation and a weatherproof finish.

Step 4: Expose the inner mass Plaster the inner surface directly on the bags; keep furniture and hangings off the sunlit surfaces.

Worked Example

A 5 m x 5 m single-storey earthbag building with 40 cm walls. The earthbag page puts such a building at 20-30 tonnes. Taking the inner 22 cm of 4 walls of perimeter about 20 m and height 2.2 m: 0.22 x 20 x 2.2 = 9.7 m3. At 1.53 MJ/m3 K (adobe-like earth) and a 5 K swing: 9.7 x 1.53 x 5 = 74 MJ = 21 kWh. The wall alone can hold much of a day’s solar gain if the sun reaches it, but sun rarely reaches all of it; the sun-struck inside surfaces are the ones that charge.

Common Mistakes

  • Counting the wall as insulation — it is R-3 for 40 cm.
  • Insulating the inside face — it cuts mass off from the room.
  • Leaving the bags in sunlight — see below.
  • Bags in shade — the sunlit mass is the useful mass.

Safety

Sunlight destroys the bags; the walls are very heavy

Polypropylene bags degrade in sunlight within weeks to months, so plaster or cover them at once (see Earthbag Building). The walls weigh tens of tonnes: build on a foundation designed for the load, and do not cut openings into finished walls without a lintel or arch.

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