Understanding Stabilized Earth Construction: Building with Humidity in Mind
Aaron Harte and Angie Stutte
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Humidity Control Issue The addition of insulation within the north and east walls is helpful in cold climates while insulating within the west wall would be advantageous in a hot climate. Also, the external walls have a low resistive insulation but a high thermal lag of nine to 11 hours. This provides a "thermal flywheel" effect. If using mud plaster as a permeable finish, the wall will be allowed to absorb and give off airborne moisture more easily than if cement stucco is used as a finish. Wall thicknesses from 12 to 14 inches are considered optimum for thermal mass performance. Also, double wall construction can enhance the insulation factor. The very same thermal benefits that work to promote earth block construction can also offer a disadvantage to this type of construction. The interior of an earth block home offers a controlled environment due to the time lag of thermal storage. The ability for a mud wall to absorb water from an environment in an area that is too humid and desorb water into an area that is to dry is considered a virtue when speaking of comfort levels within a given space. But the constant presence of water within a wall can have a negative effect, such as cracking and erosion. The very component that allows for such a great thermal lag, clay, also is the primary source of cracking. Normally, the exterior and interior environments of the building wall
differ, one will be hot and the other will be cold. The earth brick offers
a medium for the transformation from the exterior to the interior. If
the temperatures are right, somewhere in the middle of the wall the temperature
will meet the dew point. The dew point can be explained as "the temperature
to which air must be cooled to become saturated." Warmer air can
hold more water, but as air becomes cooler, the amount of water that is
necessary to saturate it is lowered. As the temperature of the air is
cooled below the dew point, water is released from the air in the form
of water vapor, or condensation. Although this condensation is not enough
to actually expand the block, this moisture can settle in existing cracks
formed in the drying process and slowly begin to erode the block. The
presence of condensation also works to hinder the thermal insulation of
the block and must be removed through capillary action. This is the process
of the wall breathing, which allows for the removal of this moisture in
a relatively quick fashion. Linseed oil is used to preserve concrete surfaces, but can also be used to protect the wear on the earth brick. A coating placed on the exterior wall would prevent destructive water and salts from penetrating into the earth block. Clay boards can be used as an alternative to gypsum plasterboard which
would not only can hold pictures, wallpaper, and paints, but also absorbs
odors and keeps the stabilized earth blocks able to breath. Clay board
also absorb and diffuse water vapor like the earth bricks do and is a
good substitute since the earth walls need to breath. Bentonite is formed from clay and is used as an effective bond and stabilizer.
It is resistant to water and some acidic substances. Bentonite can be
used as a thickening gel that is stable under high temperatures and mechanical
movements. Bentonite is very sturdy when mixed with clayey soils to produce
an effective, strong earth brick. Portland cement is often used to reinforce earth blocks, but only a small amount should be added to reinforce the brick. Too much cement makes the brick heavy and reduces moisture movement. Flyash is derived from burning coal and is an additive to concrete and blends easily into mixtures. It increases strength when combined with lime and decreases permeability. It increases durability while at the same time reduces efflorescence and shrinkage. Different Insulations and Finishes for Earth Walls Insulation has many different opportunities in an earth brick home. Sometimes insulation is only on the exterior wall after the waterproofing material is applied. In this case it is to keep the heat generated, collected, and absorbed inside the interior volume. This, however, reduces the summer cooling effect of the walls, but prevents the walls from cooling down to earth temperature. To solve this humidity issue, mechanical air conditioning or dehumidifiers are required. A protective layer of board should be added if insulating the exterior to keep it from contacting the earth wall. Depending on the type of structure (wood, masonry, concrete, or steel) insulation may be placed on the inside of the walls before waterproofing material is applied. A list of some impermeable materials, semi-permeable materials, and permeable materials can be found on p.266 of Builder’s Guide to Cold Climates by Joseph Lstiburek. These help to understand which materials should be used for the earth wall to be able to absorb and give off moisture while still being able to allow air flow as necessary. There are two finishes to earth walls, breathable or waterproof finishes.
Permeable finishes are less expensive, less durable and allow for the
wall to give off and absorb moisture. One such permeable finish, mud plaster,
is usually applied in two coats to interior and exterior walls with straw
mixed in to reinforce plaster and decrease tendency for cracking. High
clay content will result in a poor bond of plaster to the wall. Stabilizers
can also be added just like in the earth blocks to stabilize the mixture.
Another finish which is more expensive initially and more permanent is
stucco which can be used as a low-maintenance finish, but doesn’t
accommodate for the breathing of the walls as much as mud plaster. Instead,
stucco contains and traps the moisture instead of releasing it at a later
time, which leads to problematic issues. When using stucco, it is necessary
to using netting in order accommodate for the settling and cracking of
the stucco. Resources and helpful websites: |