Energy performance on most buildings can be improved with insulation, including those made of earth such as adobe and earthbag structures. Although most earth structures are located in hot, dry climates, there is increasing demand for low-cost, eco-friendly earth building techniques in cold climates. This article explores three innovative methods for insulating earthbag buildings, which extends their building range to cold regions.
Most earthbag buildings use polypropylene grain bags filled with soil. The bags are filled, stacked in level courses and then tamped solid. One or two strands of barbed wire between courses bond the bags to each other and add tensile strength. The building process using earthbags filled with insulation as described here would be nearly the same, although the bags would weigh significantly less and speed construction considerably.
Unlike other earth building methods, earthbag building has the unique advantage of providing either thermal mass or insulation, and therefore can be adapted for cold climates with an insulated fill material. Scoria, pumice, perlite, vermiculite or rice hulls are all suitable insulating materials. These materials are natural, lightweight, easy to work with and non-toxic. They will not burn or rot and do not attract insects or vermin. In addition, scoria, pumice, perlite, vermiculite are not adversely affected by moisture and can be used as part of earth-bermed structures.
The table below compares the approximate R-values of three sustainable insulating materials that could be used in earthbags. (The first column in the table is the insulative value per inch; the second column shows the R-value for a typical 15″ thick earthbag wall.)
Material — R-value/inch — R-value/15″
Rice hulls — R-3 — R-45
Perlite — R-2.7 — R-40
Vermiculite — R-2.13 — R-32 to 36
(Source: Wikipedia Encyclopedia.) http://en.wikipedia.org/wiki/R-value_(insulation)
1) The first method for insulating earthbag buildings uses bags filled entirely with insulation. The main advantage of this method over the other methods described below is ease of construction. Walls are one bag wide and filled entirely with insulation. Thick earth or lime plaster on the interior provides thermal mass to help stabilize indoor temperatures.
A demonstration home using this method was built in Crestone, Colorado with scoria-filled earthbags. Scoria also is known as volcanic rock or lava rock. Due to its volcanic origin, scoria is filled with tiny air spaces, making it a good insulator. Although the R-value of scoria is debatable, the owner claims these earthbag walls are comparable to straw bale walls of around R-26 to R-30. This estimate includes 5″ of papercrete at approximately R-2/inch.
2) Another method for insulating earthbag buildings uses tube sandbags, also called traction tube sand bags, typically used to improve automobile traction on snowy/icy roads. (The bags are sold to add weight for vehicle traction.) This method involves stacking tube sandbags filled with insulation on the exterior of earthbag walls, thereby creating a double wall.
Filled tube sandbags provide about 10″ of insulation, which is perfect for many climates – not too much, not too little. Again, scoria, pumice, perlite, vermiculite or rice hulls could all be used for insulation. Perlite would be my first choice due to its high R-value (R-2.7×10″=R-27) and resistance to moisture damage, although the final decision needs to be weighed against other locally available and inexpensive natural materials.
3) A third possibility is to add a seam lengthwise down earthbags to divide them into two compartments. The outer part could be filled with insulation; the inner part with soil. Like the other systems described here, this would create an insulated wall with thermal mass on the interior. For many situations, especially structures in moderately cold regions, this is an ideal wall system.
The placement of the seam could vary, depending on the climate. In a mild climate like New Mexico, about 4″-5″ of insulation on the outside would suffice. This would provide about R-10 insulation. In a slightly colder climate the seam could go down the middle (50% insulation / 50% soil). In extremely cold or extremely hot climates I would fill the bags with 100% insulation (or all earth in a hot climate if insulation is not available).