Because Cast Earth is new construction, energy efficient design principles can be inherently incorporated at maximum efficiency, enabling the greatest energy savings at negligible additional cost. In many climates, the properties of earth walls, (details below) are all that is necessary to achieve excellent low energy design.
When climatic conditions are so severe that insulation is desirable, Cast Earth is uniquely able to allow inexpensive, practical incorporation of integral insulation by casting a thermal barrier directly into the wall as it is poured.
In other earthbuilding methods, insulation is not easily included inside the walls themselves. It is practically impossible to incorporate insulation within rammed earth; placing a thermal barrier inside adobe requires building two separate walls on either side of the insulation. Instead, if insulation is necessary, it is only practical to apply it as an external sheath after the walls are up. Obviously, this precludes leaving the exterior "natural". The esthetics of unfinished earth walls are destroyed when exterior insulation is applied.
With Cast Earth, insulation is simply inserted in the forms and positioned about six inches from the exterior. Then the Cast Earth mix is poured on both sides of the insulation as the forms are filled. This extra step adds almost negligible cost or effort. The process is illustrated in the adjacent image, which shows a styrofoam insulation panel in place between the forms, with Cast Earth partially filling the void volume.
Inserting a thermal barrier eliminates the wall's role as a solar collector, but preserves its heat sink properties for the interior mass. This technique would be very useful for North and East walls in a very cold climate, or for South and West walls in a very hot one.
A view of Insteel insulation (styrofoam in a steel wire tie-through mesh), this time in a wall three feet thick. The insulation is contained in the outer face of the wall to leave a large mass for thermal storage on the interior.
If the exterior is to be stuccoed or otherwise clad, insulation would be placed directly on the exterior surface instead of slightly within the wall. In many cases, it will be preferred to leave the beautiful exterior unfinished, so internal insulation will be employed -- again, only possible at low expense when using Cast Earth.
The Heat Sink Effect:
First, a wall of earth (or other masonry) acts as a "heat sink". Consider a day's cycle in a warm climate: after cooling at night, the walls begin to pick up heat as the outside air temperature increases. The thick walls store this heat, from outside to inside as the day progresses. Because the interior surface of the wall remains cool during the day, so does the building and its occupants (assuming there is low internal heat generation, as in a residence).
Earth homes in hot climates have always been known to have cool interiors in the summer, and this is due to the heat sink (storage) effect. By the time evening arrives, the wall has stored about all the heat it has capacity for, and heat begins radiating into the interior. At the same time, outside temperatures are dropping, and some of the heat is radiating back to the outside air. If the wall thickness is sufficient in a given climate, heat release into the interior can be delayed until early morning hours, when ventilation with outside air is at its coolest.
At this time of day, some form of cooling will become necessary, even if it has not been utilized during the daytime. In higher elevations (above about 2,000 - 3,000 feet), this can simply be natural or forced circulation of cool, exterior night air to convey outdoors the heat released to the interior. In midsummer in a low elevation, the outside air may not be sufficiently cool, especially in early evening, and some artificial cooling is employed.
In arid climates, this cooling may be evaporative air conditioning, which is much cheaper to install and run than refrigeration, though refrigeration may be utilized and will have a lower total load because of the mass of the wall. In fact, the moderating effect of earth walls may make evaporative cooling very comfortable, whereas it might be marginal in lightweight insulated structures. An ancillary benefit of evaporative cooling is that a constant stream of fresh, cool air is being injected into the building. Windows and doors may be left partly open. The building is not "sealed up" in the cooling season, as is necessary with refrigeration.
From an electric utility's standpoint, whether evaporative or refrigeration cooling is used, the effect is one of "peak shaving", because the cooling load is transferred from the hottest part of the day to a time of lower load. The result is a reduced need for peak electrical generation capacity and less overall electrical consumption, with short and long term power cost reduction to the utility and the homeowner.
The Solar Collector Effect:
Switching now to the cold season, the mass walls have the potential for the opposite effect: they can actually transfer the sun's energy into the house, reducing overall heating needs. For this reason, earth buildings are also known as being warm in winter.
If the earth wall is exposed to the winter sun, it will soak up a substantial amount of energy during the daytime. If the house is not ventilated to the outdoors to any great extent, some of this energy will be radiated into the living space during the late afternoon and evening hours -- the walls will actually start transferring energy into the building at the time it is needed. Rather than being a simple insulator, the mass wall becomes a solar collector with a properly timed release of heat.
While earth is not as effective as an insulator per inch of thickness as fiber insulation pads, foam or the like, its use in a mass wall produces an effect which insulation is incapable of. Because of this unique capability of mass walls, comparison of earth walls on a simple "R-Value" basis is not meaningful. An understanding of solar principles and the thermodynamics of earth walls allows design of earth structures which are inherently energy efficient, without a brute force application of insulation.
Finally, in cases of climactic extremes, it is feasible and often customary to insulate the exterior of earth walls, yielding some of the benefits of both technologies. In these cases, the mass walls are primarily an energy leveling device through their heat sink capability, allowing recovery of energy expended on daytime heating or cooling to be recovered at night when artificial climate control is turned to a lower setting during sleeping hours.
Passive Solar Design:
There are many elements to the principles of passive solar architecture. Orientation of both house and lot, as well as geographical location and altitude, are some of the influential factors. Wall thickness is another variable.
A key principle is that if roof overhangs and window overhangs or awnings are properly designed, taking into account the orientation of the wall and the latitude, it is possible to shield all or much of the wall or window (especially the Southern and Western exposures) from intense mid day summer sunshine of in order to minimize heat absorption when the sun is highest. This results in low solar energy transfer into a passive solar building in summer.
However, in winter, the sun is much lower, and its direct rays will shine under these overhangs and impinge on the walls to a greater extent. During this part of the year, collection of solar energy by the wall will contribute to interior warmth. While compromise is a part of passive solar design, it is possible to have most of the best of both summer and winter solar minimization and maximization, respectively.
Obviously, window orientation and quantity are an important part of passive solar design. A deeper discussion of these matters is properly undertaken with an engineer or architect versed in passive solar technique. The principles are applicable to both mass walls and lightweight insulated walls, but in the case of mass walls, they allow the solar collector effect to work at its least effective in summer and most effective in winter. With proper design, the sun's effect is automatically regulated without intervention by the building's occupants.
Sources of solar angle versus time of day and time of year at various latitudes are available to aid in solar design. One such source is the Sun Angle Main Menu.