Humidity Control Issues
Humidity levels vary from summer months to winter months, as well as with location. Earth blocks play an integral part in the stabilization of these
variations. The way the earth bricks work with humidity is that they absorb cool air and moisture at night and then release that air into the interior
during the warmth of the day. As the cool air is released, the warm daytime air is absorbed and stored, which will be released in the coolness of the
night. Many “tricks” have been preformed by earth block homeowners to enhance this effect. During the night hours, it has been found helpful to leave
windows open to promote airflow and to allow the heat from the bricks to be driven out through circulation. If the night time air is still warm, one can
even use the air conditioner for a short time to load the walls with cool air.An example of this can be found in Cairo, Egypt, where the thermal conditions
of two house of the same size were compared. One house was constructed of 50cm thick walls of earth and the other was made from 10cm thick walls of
pre-cast concrete. On a day were the outside temperature varied by 13 degrees C, the concrete house had a 16-degree variation and the earth block home only
varied by 4 degrees. Furthermore, at 4 p.m., the concrete house had an indoor temperature that was 5 degrees C warmer than the exterior and the earth
house’s temperature was 5 degrees C cooler than that of the outside.Adding insulation within 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 a
environment in an area that is to humid and desorb water into a 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 negativeeffect, such as cracking and erosion. The very component that allows for such
a great thermal lag, clay, also is the primary source of cracking. When first constructed, the block is left to cure for a set amount of time. As the
drying process progresses, water is evaporated from the block, which causes the clay to shrink. This shrinkage may cause small cracks both within the
interior and exterior of the block. This can be harmful in two ways; one, wind driven rain and other forms of water are allowed to enter the brick at a
rate that is not natural for the brick. With a sudden increase of water, the brick may become in stable and begin to expand, reversing the process it under
went while drying. This expansion not only causes additional cracks to form, but may also cause bricks to explode from the surrounding pressure. This in
turn could cause the entire wall section to fail. The second effect has to do with the natural water absorbing ability of the block. Normally, the exterior
and interior environments of the building will differ, one will be hot and one 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 are can hold more water. 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 thiscondensation 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.

Earth blocks offer numerous advantages in the construction of homes. Disadvantages also exist, and must be designed for from the beginning.
Humidity plays an integral part in the performance of the wall and must not go unnoticed in the conception of the design.

Resources:
Gernot, Minke. Earth Construction Handbook. Wit Press. Boston, MS; 2000.
http://www.greenbuilder.com/sourcebook/EarthGuidelines.html
http://www.natmus.dk/cons/tp/wallbuff/wallbuff.htm
http://askeric.org/Virtual/Lessons/Science/Meterology/MET0007.html
http://renewable.greenhouse.gov.au/home/passive_solar.html