Natural Insulation

-- Regulating Heat and Cold

One of the best low-cost insulating materials is clay-coated straw (or other lightweight plant materials). A light coating of clay acts as both a binder and preservative. Clay-coated straw has been shown to last over 700 years as a non-deteriorating insulation!! It has an R factor of about 3+ per inch. A "typical" 12 inch wall using this material would have about an R-40 insulation factor. As the clay dries, it binds the straw together in a surprisingly rigid mass. It's a "natural styrofoam". Other excellent insulating materials include expanded natural ceramics, such as expanded clay, slate, shale. These materials are sold as perlite, vermiculite, and other expanded products and cost about $50 per ton purchased in bulk. Other insulators include cellulose in many forms such as balled newspaper, "fluffed" newspaper (processed through a hammer mill), or any light, dry plant material such as rice hulls, or other agricultural waste. Coat with a light clay slurry or soak in a dilute boric acid solution to keep from deteriorating. Basically, insulation is just trapped air. Fibrous composites (papercrete, fibercrete, etc.), can also be very insulating, depending upon their design density, but can have structural capabilities as well.

Insulating materials are used in housing, greenhouses, inertial refrigerators, and other innovations described in the Life Supporting Technologies section. For illustration purposes we will describe the straw-clay system since fewer people are familiar with this technology.


Any stiff agricultural waste similar to straw will work. Hay is too flimsy and has seeds, so it doesn't work very well. Barley straw, wheat straw, and other grain straws work well.

Clay can be gotten from the earth. Many subsoils are primarily clay. River bottoms and river banks are usually clay. Clay is also used by brick and tile manufacturers and can be bought from them cheaply. (in our area, about $16 per ton)

Even soil which has a moderate amount of clay such as commonly used for adobe, about 35-50% clay, will work. The slurry is not as sticky, compared with pure clay, but even ordinary mud works well enough. This is not rocket science. Use a dry wall stirring paddle and electric drill to mix the clay or mix in any kind of mixer. Mud mixed in a box with a hoe works.


  1. Break the clay into small particles so that it will mix with water easily (factories use a stone wheel to press dry clay through a grate, crushing it into a powder).
  2. Make up a slurry of clay and water. Any soil that is mostly clay will also work. The consistency should be like cream or a thin milk shake.
  3. Spread the straw out on the ground. Dampen the straw with a spray nozzle if available.
  4. Pour (drizzle) the slurry over the straw, then toss and mix the straw so that it becomes lightly coated. Ordinary garden rakes work well. The clay should only very lightly coat the straw. This is NOT adobe. Maybe 5-10% clay, 90-95% straw. When dried in the wall, you can hardly see the clay, but it binds the straw together very well.


In addition to being an insulator, it can be used as a wall forming material. In the middle ages, even up the the present time, the method works like this:
  1. A post and beam structure is first built.
  2. Two boards are temporarily nailed to the posts, one on each side.
  3. The resulting cavity is filled with straw-clay.
  4. The material is tamped down (a 2x4, 4x4, or small post will do). The idea is not to compact it into a solid mass, you couldn't do it easily anyway because the straw will remain springy until it dries.
  5. The two side boards are moved up immediately and stuffed again and again until the wall is as high as desired. No need to wait for the straw-clay to dry before moving the boards up. (A moveable, sliding form could also be used to make walls.)
  6. A saw is used to cut out windows, or window frames are placed first.
  7. The wall is allowed to dry and is hand plastered inside and out. The soft undulating plastering adds a charm that cannot be found in modern buildings.
The straw-clay can also be pressed against forms to create a structure. Then the structure can be plastered and waterproofed. This method is talked about in the Design Ideas section. Or it can be used as an insulated fill material for the honeycomb construction method as described in the Design section.

The important concept here is that insulation materials protect us from extremes of heat and cold. The best insulators are ones that are non-toxic, renewable, and widely available. Low cost insulation materials such as straw or expanded aggregates such as perlite, vermiculite, and other expanded ceramics make excellent non-toxic insulation. In developed countries with the requisite technologies, the perlite type materials may be preferrable to straw-clay. Fiber composites can be both structural and insulating.

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