Foundations

This has been the most exasperating chapter for us to write. Both of us had done a lot of conventional construction prior to getting involved with earthbag building. Maybe it's just us, but we both dread the idea of building a typical concrete foundation system. To us they are boring and tedious to construct. They are expensive and use up godly amounts of natural resources while pumping the atmosphere full of ungodly amounts of pollutants. Plus, they don't last very long. A typical residential concrete foundation has an average life span of 100 years. The cement eventually dissolves from efflorescence, the steel rusts, and the whole thing sucks up moisture like a sponge ... but they are sanctioned by building codes!

Our personal experience with building foundations is strongly influenced by living in a dry climate. We get sub-zero temperatures, but less than eight inches (20 cm) of annual rainfall. Since the primary foe of foundations is frost and moisture damage, our focus has been on providing excellent drainage. We are learning to adapt earthbag architecture to a moister and colder climate than our own. This chapter is designed as an informal exchange of information based on what we have experienced and what we are in the process of learning. What we offer are some examples of

4.1: 1,000-year-old Anasazi dwelling on an exposed bedrock foundation in Hovenweep National Monument.

alternative foundation options that we can mix and match to suit our various needs and esthetic aspirations. We'll start with a brief description of a conventional foundation system and then move on to alternative adaptations.

Conventional Concrete Foundation System

Poured concrete is the most popular foundation system used in conventional construction practices. The standard procedure has been to dig a trench down to the prescribed frost line, then pour a concrete footer wider than the width of the wall to provide a stable base. Within this footer, steel rebar is suspended to provide tensile strength, as concrete alone is brittle. On top of the footer, a stem wall is poured (with additional rebar for reinforcement) equal to the width of the finished wall of the structure and tall enough above grade to keep the wall dry (Fig. 4.2).

Frost depths vary with the climate from non-existent to permanent. The average depth that frost enters the ground here in Moab, Utah is 20 inches (50 cm). The average frost depth in Vermont is four feet (120

cm). The logic is that by beginning the foundation below the frost depth, the foundation rests on a stable base, free from the forces of expansion and contraction occurring from freeze/ thaw cycles commonly called frost heaving Moisture freezing below an insufficient foundation depth can result in upheaval of the structure causing cracking of the walls or annoying things like forcing all the doors and windows out of alignment. Conventional construction logic believes that deep frost lines require deep foundations.

This logic poses a challenge to most alternative architecture, as new energy priorities have increased the width of the walls to as much as two to three feet (60-90 cm) thick. To adapt conventional poured concrete foundations to thick earthen walls would defeat the resource and cost effectiveness of building them in the first place. So it makes sense to innovate a foundation system suited to thicker walls.

One suitable foundation system is inspired by a 1950's design by Frank Lloyd Wright, devised as a way to lower construction costs for foundations built in

A FOUNDATION PERFORMS SEVERAL FUNCTIONS:

• A solid footing to distribute the perimeter weight of the structure evenly over the surface of the ground.

• A stable base that defies the up-heaving and settling forces caused by freeze/thaw cycles in cold, wet climates.

• A protective perimeter that guards the lower portion of the walls from erosion and moisture damage.

• A means to anchor a structure to the ground in response to severe weather conditions such as high winds, flooding, earthquakes, etc.

1/2" vertical rebar in

'Fa'

stemwall bent & wired around horizontal rebar in

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footer

. i * *

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!

Lv * J^1 'îl* j

4.2: Cross-section view of a typical concrete foundation and stem wall.

10" high concrete "Grade Beam"

Packed gravel below frost line

10" high concrete "Grade Beam"

Packed gravel below frost line

Frank Lloyd Wrights "Floating Footer Foundation"

4.3: Floating footer on a rubble trench foundation.

deep frost climates. It is called the rubble trench foundation with a floating footer. Instead of filling the frost depth of the foundation trench with poured concrete, he used rubble stone and packed gravel as the supportive base and limited the concrete work to a ten-inch (25 cm) high concrete, steel reinforced grade beam that rests on the surface of the rubble trench. To avoid frost heaving, the rubble stone allows drainage below the frost line where water can continue to percolate. This combination rubble trench/grade beam acts as the footer and the stem wall. Expenses and resources are reduced with the substitution of packed rock for an entire poured concrete foundation (Fig. 4.3).

Another successful foundation system being utilized by HUD (Housing and Urban Development) is called a shallow, frost-protected foundation. Since one of the main objectives of a foundation is to defy freeze/thaw action, another solution is to inhibit frost penetration from entering the foundation with buried exterior insulation. An insulated foundation also helps reduce heating costs, as an average of 17 percent of a home's warmth can escape through the foundation (Fig. 4.4). You can visit HUD's website for more detailed information and to find out the specs on using an insulated frost-protected foundation system for your climate. Refer to the Resource Guide for the website location of HUD.

Of course, conventional concrete foundations and stem walls work with earthbag walls as well.

Earthbag Rubble Trench Foundation Systems

For the sake of simplicity and function, we use a streamlined version of the rubble trench foundation system for building freestanding garden walls, and a simplified, low-tech version of Frank Lloyd Wright's floating footer system for a house. Either system can be appropriate for an earthbag dwelling depending on the climate. The main difference between a conventional poured concrete foundation and the foundation systems we have adopted for earthbag walls is that our systems are built of individual units rather than a continuous beam.

Vertical and horizontal rigid form insulation on exterior of stemwall/ foundation

Vertical and horizontal rigid form insulation on exterior of stemwall/ foundation

. Packed gravel trench

4.4: Shallow, frost-protected foundation.

A continuous concrete foundation is a fairly recent invention. Humans have been building "alternative" foundations for 10,000 years. When we observe history, the oldest surviving structures throughout the world are sitting on individual stone blocks and packed sand and gravel. Some are "cemented" together with a mud mortar like the 800 -1,200 year old Anasazi ruins found in the Four-Corners region of the Southwestern United States. The Romans and Greeks built whole empires with rubble rock held together with the glue of lime. In the Northeastern United States, 200-year-old Vermont timber frame barns are still sitting on dry-stack stone foundations. Neither the 600-year-old stone Trulli villages in Italy, nor the 300-year-old cob cottages off the coast of Wales have a lick of steel or cement in them.

The other difference is that we use gravity as our anchor to the foundation/stem wall rather than bolts or impaling the bags with rebar (a common choice for attaching strawbale walls to a concrete stem wall). Instead, we rely on the massive weight and strategic design of the walls to keep the building stable. (Refer to Chapter 5).

Our Basic Rubble Trench

For garden and privacy walls in our dry climate, we dig a trench about four to six inches (10-15 cm) wider than the width of the proposed finished wall, and

about 12 inches (30 cm) deep (about one-half of our average frost depth). In wetter, colder climates it may be necessary to dig below the frost line. The trench is filled with coarse rock progressing to smaller gravel toward the top of the trench. Any sand in the mix should be clean and coarse (avoid silty and clay-rich soils). Whatever we can get that compacts well and still provides good drainage will work. Spray with water during installation to help the gravel and any sands to compact better. This is the basis of the rubble trench foundation (Fig. 4.5a & b).

Concrete Earthbag Stem Wall

Our idea of a concrete stem wall for an earthbag wall is to fill the first two to three rows of bags on top of a rubble trench with concrete. Marty Grupp built a "fast-food mentality" concrete earthbag stem wall for two serpentine walls in front of a small apartment complex. He laid a row of Quick-crete bags, perforated each bag and soaked them with water. He laid down two strands of barbed wire and another row of Quick-crete bags, perforated and sprayed them with water and called it done. He figured that by the time he ordered all the materials and the mixer and the extra help to pour all the concrete, he could lay prepackaged bags of Quick-crete himself for just about the same amount of money; and he did! (Fig. 4.6).

Our biggest challenge has been coming up with an entirely cement-free foundation/stem wall. As far as architectural time scales go,"portland cement" is a fairly recent development. In 1824, Joseph Aspdin, an English bricklayer, patented a process for making what he called portland cement, with properties superior to earlier varieties. This is the cement used in most modern construction. The use of cement has greatly increased in modern life. It's used for foundations, walls, plasters, blocks, floors, roofs, high rises, bridges, freeway overpasses, highways, sidewalks, swimming pools, canals, locks, piers, boat slips, runways, underground tunnels, and dams ... to name a few.

4.5a, 4.5b: A simple rubble trench showing (A) a rubble rock base, (B) topped off with clean, well-tamped gravel.

It takes a lot of produced power (embodied energy) to produce cement. According to The Adobe Story by Paul G. McHenry, in the US it takes "four gallons of gasoline or diesel fuel to produce one bag of cement, while contributing over 8 percent of the total carbon dioxide released into the atmosphere." Worldwide, cement production accounts for 12 percent of that total, or one ton of carbon dioxide for every ton of cement produced. It is in our best interest as a species to learn to minimize our dependence on cement. With these cheery thoughts in mind, on our way to cement free foundations, let's look at some reduced-use, cement stabilized options.

Stabilized Earth Stem Walls

An alternative to using full strength concrete is to fill the first two or three rows of bags (the stem wall bags) with a stabilized earth mix. Stabilized earth is a method of making a soil resistant to the effects of moisture by adding a percentage of a stabilizing agent. As Joe Tibbets states in his excellent reference, The Earthbuilders' Encyclopedia, "The advantage of using a stabilized earth ... is that we can use the soil we already have available for our walls instead of importing washed concrete sand and gravel. As is the case of cement, it uses a lower percentage than for full strength concrete reducing cost of materials." Common stabilizing agents are cement, asphalt emulsion, and lime. Generally most soils suitable for stabilization are coarse, sandy soils. There are exceptions and all three stabilizing agents function in different ways.

The following information on cement and asphalt stabilization is adapted from Joe Tibbet's Earthbuilders' Encyclopedia.

Cement acts as a binder, literally gluing the particles together. Cement provides adhesion as well as additional compression strength. By adding a predetermined amount of cement (anywhere from 6 percent-15 percent depending on the particular soil), cement fully-stabilized earth (aka: soil cement) can be used as a way to minimize the use of cement while making use of cement's ability to remain stable when it comes in contact with water. Cement is more effective

4.6: Marty Grupp's Quick-crete stem wall.

4.7: Because of the lack of tensile strength (e.g. rebar), a continuous tube is more likely to crack in a cold, wet climate, so using tubes would probably work best in a dry or frost-free locale.

with soils low in clay content with a coarse, sandy character. Cement mixed with soils high in expansive clay is less effective as the two tend to oppose each other, thereby compromising their bonding strength. Using tubes filled with cement stabilized earth for foundation/ stem walls creates an interesting effect because there are fewer seams (Fig. 4.7). Once cured, the fabric can be removed to reveal a sculpted stone appearance. This looks particularly exotic in a serpentine wall. The exposed soil cement can be stained or lime-washed any color desired.

Two advantages of using a soil-cement are: reduced ratio of cement to aggregate and (if the soil is suitable) cost savings from not having to purchase more expensive washed gravel and sand needed for standard concrete.

Asphalt emulsion provides a physical barrier to the passage of water. A properly prepared asphalt stabilized earth mix will not absorb more than 2.5 percent of its own weight in water. It functions by surrounding the particles of clay-bound sand clusters with a water-resistant film. The percentage of asphalt emulsion added to soil for stabilization ranges from about 3 percent to a maximum of 6 percent. Any more than that jeopardizes the integrity of the soil. Because it comes in a liquid form, asphalt emulsion makes the mix wetter and may require more time to set up before continuing the earthbag wall system. This is our least favorite form of stabilization. Other than some early experimentation, we do not use it. Asphalt emulsion is a carbon-based fossil fuel by-product, and a known carcinogen.

Lime Stabilization. Of the three common stabilizers, lime is the one most compatible with clay-rich soils. When we speak of lime, we are referring to building lime and not agricultural lime. The most familiar form of building lime in North America is Type S -Hydrated Lime that comes in a dry powdered form in 50-lb. (22.2 kg) bags. It is most commonly used as an additive to cement stucco and cement-based mortars to enhance workability and inhibit moisture migration.

Lime is made by firing limestone to produce calcium oxide by burning off the carbon. This calcium oxide (also referred to as quicklime) is then reacted with specific amounts of water to produce building lime. (Agricultural lime is simply powdered limestone in its natural, unfired state.) The complexities of lime are fascinating and worth taking the time to research and learn about. (See the Resource Guide for suggested reading about lime).

For the purpose of soil stabilization we will focus on the use of Type-S lime hydrate available in the US at most lumber yards, building supply warehouses, and wherever cement products are sold. One critical factor worth mentioning is the need to acquire lime in as fresh a state as possible as it weakens with exposure to moisture from the air over time. Wrapped in plastic, fresh off the pallet from the lumber yard, it ought to still have some life to it. Purchasing lime directly from the manufacturer and sealing the bags in plastic garbage bags until needed helps ensure the lime's potency.

Interestingly enough, lime used as a soil stabilizer for road work was pioneered in the US in the 1920's. Thousands of miles of roads have been constructed on top of lime stabilized soils. As noted by Hugo Houben and Hubert Guillaud in their book, Earth Construction, "the Dallas-Ft. Worth airport was constructed over 70 square kilometers using lime as a base soil stabilizer."

Lime interests us as a soil stabilizer for several reasons. Ton per ton, it takes one-third the embodied energy to produce lime than it does cement. During lime's curing process, it reabsorbs the carbon dioxide it gave off when it was fired. In a way, in cleans up after itself. Cement, on the other hand, pumps a ton of carbon dioxide per ton of cement produced into the atmosphere and leaves it there. Lime is the lower impact choice for stabilizing a soil.

Here is a simplified explanation of how lime stabilization works. Lime reacts with clay in two significant ways. First, it agglomerates the fine clay particles into coarse, friable particles (silt and sand-sized) through a process called Base Exchange. Next, it reacts chemically with available silica and aluminum in the raw soil to produce a hardening action that literally glues all the particles together. This alchemical process is known as a pozzolanic reaction. Other additives that cause this chemical reaction with lime are also called pozzolans. The origin of the term pozzolan comes from the early discovery of a volcanic ash mined near Pozzolano, Italy that was used as a catalyst with lime to produce Roman concrete. Venice,

Italy is still held together by the glue of lime reacted with a pozzolan. In addition to volcanic sands and ash, other pozzolans include pumice, scoria, low-fired brick fines, rice hull ash, etc. Any of these can be added to fortify a lime stabilized earth.

Lime reacts best with montmorillonite clay soils. For stabilizing stem walls, the optimum soil is strongest with a fair to high clay content of 10 percent-30 percent and the balance made up of well-graded sands and gravel to provide compressive strength. Often the material available as "road base" or reject sand at gravel yards is suitable for lime stabilization.

Another distinctive advantage of lime is that lime stabilized soil forms a water resistant barrier by inhibiting penetration of water from above (rain) as well as capillary moisture from below. This indicates that lime stabilized earth is less likely to require a significant capillary break built in between a raw earth wall and a lime stabilized stem wall (more about capillary break later in this chapter).

Experimentation will determine whether the soil is compatible with lime and what appropriate ratio of lime hydrate to soil will be needed to achieve optimum results. Every dirt will have its favorite ratio of lime to soil. In general, full stabilization occurs with the addition of anywhere from 10 percent-20 percent (dry volume) lime hydrate to dry soil, depending on the soil type. Adding 5 percent-25 percent of a pozzolan (determined by tests) provides superior compression strength and water resistance.

To facilitate proper curing, lime stabilized earth must be kept moist over a period of at least two weeks — three is better. The longer it is kept moist the stronger it sets. For use in earthbag stem walls, this is easy to achieve by covering the rows with a plastic tarp and spritzing them occasionally with water. Double-bagging the stem wall bags will also help to retain moisture longer. The moisture curing period is essential for creating the environment necessary to foster the pozzolanic alchemy that will result in a fully stabilized soil. A fully stabilized soil is unaffected by water and will remain stable even when fully immersed. It's worth the effort to achieve especially if you are building in a wet climate.

Mixing procedure for cement or lime stabilized soil.

Pre-mix the soil and cement or lime in a dry or semi-damp state. Mixing them dry achieves complete mixing of the two materials. Mixing can be done by hand in a wheelbarrow or in a powered cement or mortar mixer. Once all of the dry ingredients are thoroughly integrated, water can be added. A slightly moister mix than that of a typical rammed earth mix is needed. Add enough water to achieve about 20 percent moisture, or enough water that the moisture will slightly "weep" through the weave of the earthbags when tamped. Keep damp for as long as possible to cure properly.

Moisture Barriers, Vapor Barriers, and Capillary Breaks

To avoid any confusion, we want to explain the difference between moisture barriers and vapor barriers before we get into how they are used. A moisture barrier inhibits moisture penetration (water in the liquid state), but allows vapor (water in the gaseous state) to transpire. Moisture barriers are generally used in an external application, such as "Tyvek wrap." A vapor barrier impedes water migration in both the liquid and gaseous states. A vapor barrier is also referred to as a waterproof membrane. A good example of a vapor barrier is plastic sheeting.

Although cement will remain stable when in contact with water, it has a notorious habit of wicking moisture up from the ground resulting in water migration into the earthen wall it is designed to protect. Many earthen walls have succumbed to failure due to well-meaning yet incompatible restoration repair jobs using cement to stabilize historic adobe missions throughout the Southwestern United States. Earthen walls retain their integrity as long as they stay dry or can dry quickly when they do get wet.

Cement wicks moisture as well as inhibits evaporation. Water goes in but is slow to come out. Water likes to travel and will search for an outlet even if it means defying gravity by migrating into the more porous, raw earthen wall above. Water's rising by

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Barrier

Barrier

GRADE

4.8: Capillary action: Without a capillary break, moisture in the ground is wicked up by the concrete and absorbed into the more porous earthen wall above.

4.9: The waterproof membrane prevents capillary action, inhibiting water absorption by imposing a barrier.

absorption into a more porous substrate above is described as capillary action, much like how a sponge soaks up water.

Conventional wood frame construction is required by code to install a vapor barrier in between the top of the concrete stem wall and the wooden sill plate that the stud wall framework is attached to. It can be a roll of one-eighth-inch (0.3 cm) closed cell foam, heavy gauge plastic sheeting, tarpaper, or a non-toxic alternative liquid sealer, like DynoSeal made by AFM products, slathered on top of the surface of the concrete stem wall (Fig. 4.8 & 4.9).

However, a waterproof membrane (vapor barrier) designed to inhibit water absorption from below can also prevent drainage from above. If for some reason water was to enter the wall from above the foundation (from leaky roofs, windows, or plumbing) it could

CAPILLARY BREAK [Two rows dry stack stone in between stabilized stem wall and rawearthbag wall]

Packed grajvei trench foundation '

ft v

CAPILLARY BREAK [Two rows dry stack stone in between stabilized stem wall and rawearthbag wall]

Packed grajvei trench foundation '

Plaster down to J-Metal Weep Screed [see detail]

Exposed stabilized stem wall

Plaster down to J-Metal Weep Screed [see detail]

Exposed stabilized stem wall ft v

4.10a: Dry-stacked, flat rock capillary break. Weep screed between earth plaster and stabilized stem wall.

dribble down onto the surface of the vapor barrier, pool up and be wicked into the wall it was designed to protect. There is a lot of debate about the use of vapor barriers and cement in general as both materials have shown evidence of retaining or diverting dampness to organic building materials, causing moisture problems.

An alternative approach is to design a capillary break that prevents moisture rising from below as well as providing drainage from potential moisture invasion from above. This can be achieved by creating large enough air spaces so that water is unable to be absorbed. Double bagging the woven poly bags and filling them with three-quarter-inch (1.9 cm) gravel can make a simple capillary break. Although we feel gravel-filled bags would make an effective capillary break, we have yet to experience how they would hold up over time. Since the gravel is held in place by relying entirely on the bag, in addition to doubling the bags, make sure to keep them well protected from sunlight immediately after installation to ensure the full benefit of their integrity.

Another option for a capillary break is a couple of layers of flat stones arranged on top of the concrete or stabilized stem wall in a way that allows air passage to occur between the rocks. Rock should be of an impermeable nature (rather than a porous type like soft sandstone) that will inhibit moisture migration (Fig. 14.10a & b). The entire stem wall can be built out of stone, dry-stacked directly on top of a rubble trench. A rubble trench is, in itself, a type of capillary break.

Earthbag Foundations

Stabilized earthbag stem wall

CAPILLARY BREAK [two rows dry stack stone]

Plaster down to J-Metal Weep Screed;

Installed onto surface of first raw-earth bag with Galvanized roofing nails

Stabilized earthbag stem wall

CAPILLARY BREAK [two rows dry stack stone]

4.10b: J-Metal weep screed detail.

Plaster down to J-Metal Weep Screed;

Installed onto surface of first raw-earth bag with Galvanized roofing nails

4.11 (above): Plastering down to ground level risks wicking moisture into the walls, causing spalling and other moisture-related damage.

high crown cob cap ifcPto

ter on grajvel trench

'flat top mortared

'flat top mortared

good year tire filled with grajvel on rubble rock

sloping stone veneer

4.12 (below): Traditional and alternative foundation/stem walls.

sloping stone veneer stabalized earth, rebbar anchors, two strands stabalized earth, rebbar anchors, two strands

sloping ro edge sloping ro edge

"urbanite" recycled busted concrete dry stacked or mortared

mortared river rock cap

mortared river rock

Weep Screed (often referred to as "J-metal" in the building trades) is a platform for the wall plaster to come down to. It is a mini-capillary break that protects the plaster from wicking moisture up from the ground or a concrete or stabilized foundation/stem wall (Fig. 4.11).

An alternative to "J-metal" is five-eighths-inch (1.5 cm) or three-quarter-inch (1.9 cm) hose secured in between the stem wall bags and the raw earth bags with either tie wires or long finish nails. We have used both soaker hose and black "poly-tubing" (used commonly for irrigation) as a flexible weep screed for curved walls. J-metal, however, can be successfully "clipped" to conform to curved walls as well.

The reason we mention something intimately related to plaster at this time is to point out the fact that all things are tied to and dependent on each other. The big advantage to prior planning is that it allows you to address a situation early on in the construction process to make a later task easier. Think ahead!

Traditional and Alternative Foundation/Stem Walls

The stem wall is the most vulnerable part of the foundation system since it is the most exposed to the

4.13: Tire foundation/ stem wall used to support an earth-bag wall at Colorado Natural Building Workshop in Rico, Colorado.

elements. This is the area where splash occurs and wet leaves cluster, that grass migrates toward, and where microorganisms in the soil try to munch the wall back into compost. In a really dry climate (less than ten inches [25 cm] of annual rainfall), we can get away with placing the raw, natural (non-stabilized) earth-bags directly on top of the rubble trench with a yearly maintenance of earth plaster. Adding a protective rock veneer as a splashguard on the exterior of the natural earthbags will increase their durability. Other options include dry-stack stone, mortared stone with earthen or lime base mortar, stabilized adobes, fired brick, recycled broken concrete slabs, and gravel-filled or rammed earth tires (Fig. 4.12).

A tire stem wall? Mike Reynolds, innovator and designer of the "Recycled Radial Ranchos," or Earthships as they're better known, refers to discarded tires as "indigenous." Old tires can be found just about everywhere, so we might as well make use of them. At the 1999 Colorado Natural Building Workshop in Rico, Colorado, Keith Lindauer (an avid Earthship builder) prepared an impressive terraced rammed tire foundation for us to build an earthbag garden wall onto. For the most part, rammed earth or gravel filled tires are a great way to turn an indigestible man-made artifact into a durable stem wall. Keep in mind that many more options exist for innovating new uses for old materials (Fig. 4.13).

Insulated Earthbag Foundation/Stem Walls

The colder and wetter the climate is, the more a foundation will benefit from the addition of exterior insulation. Insulation is most effective when placed on the exterior of a foundation, as it provides a warm air buffer in between the earthen mass and the outside temperatures. In a cold climate, insulation increases the efficiency of an earthen wall's mass allowing it to retain heat longer and reradiate it back into the living space.

The type of rigid foam we prefer to use is the high density white bead board made from expanded polystyrene (EPS). As of this writing, it is the only rigid foam made entirely without chlorofluorocarbons interior adobe floor 6" a '

«5S&»«—

grajvel trench foundationT

grajvel trench foundationT

(CFC's) or hydro-chlorofluo-rocarbons (HCFC's). It comes in two densities with the higher density the better choice for below ground applications. It has environmental drawbacks, as substantial energy is used to produce it, but can be recycled to a certain extent. High density bead board has a R- value of 4.35 per inch (2.5 cm) of thickness. We reserve its use for perimeter insulation around a foundation (Fig. 4.14). We'll talk later about a more natural (but less available) technique for increasing the insulation of an earthbag. For now, let's look at the advantage of rigid foam insulation.

One advantage to using rigid foam as perimeter insulation, is that by protecting the earthbags from moisture, we can use a raw rammed earth mix and so avoid using any cement in the bags. For below ground applications, high density rigid foam has a fairly high compression strength that is able to resist the lateral loads imposed on it by backfill. The exposed foam insulated stem wall can be sealed with heavy (8 -10 mil) plastic sheeting followed by metal flashing, rock facing, bricks, or packed, sloping gravel to protect the foam from UV deterioration.

For round walls, we apply two layers of one-inch (2.5 cm) rigid foam because it is flexible enough to bend around a curve. Be sure to alternate the seams so there is no direct path for water to migrate (Fig. 4.15). If you are uncertain of the water resistance of your rigid foam, or you want greater water protection, there are several commercially manufactured water resistant membranes available. Heavy gauge polyethylene sheeting or butyl rubber will help inhibit the transport of moisture into your insulated foundation from groundwater, storm runoff, or spring thaw. This is very smart protection for bermed or buried structures where moisture is prominent. Lumberyards, farm co-ops, and catalogues are great sources for heavy 8 -20 mil agricultural grade polyethylene sheeting.

interior adobe floor 6" a '

Standard Brick Dimensions Usa

J-Metal weep galvanized metal fashing

2" rigid foa

J-Metal weep galvanized metal fashing mil plastic sheeting

2" rigid foa

4.14: Insulated earthbag foundation.

For extreme external moisture protection, pond liner material, EPDM (ethylene propylene diene monomers), roof sealer, or a heat-sealed bitumen fabric can be wrapped around the exterior insulation and then back-filled into place. Pond liner is a heavy gauge black butyl rubber material usually reinforced with a woven grid imbedded within it to resist tearing. It has a 50-year (average) lifespan. Protected from the sun, it would probably last much longer.

All raw earthbags

All raw earthbags

over 3/4" pumice ora to daylight over 3/4" pumice ora to daylight

4.15: Curved, insulated foundation with interior sunken floor.

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