This first solar project I did was in the late 70’s. It was an addition / remodel of an old stone farmhouse. We enclosed it inside a solar envelope design. With this much mass the night day swings were small for a passive solar house.
It’s well understood that large amounts of thermal mass store the suns energy to keep the house warm at night and through cloudy days, and it’s often expensive. The stone house was already there so enclosing it was more cost effective.
Water is an excellent heat medium but takes up space and the potential leaks can be avoided by using solid materials. Adobe is a good choice, especially in the southwest. It has low embodied energy (if un-stabilized) and performs well as a heat sink though not nearly as well as stone or concrete. It holds almost as much heat but moves it much more slowly. To get a thermal performance comparable to concrete, an adobe building would need about twice as much surface area of exposed mass. Adobe has some advantages. It balances the humidity and softens sound, but is expensive compared to wood framing so using it only on inside walls (exposed on both sides to the inside air) would be more cost effective.
Mass wall with radiant heat tubing
Another variable to be considered when designing mass into a solar building is getting the heat into and out of the storage medium. If the sun’s direct rays strike a massive surface there will be good heat transfer. If the mass is in the same room with the sunshine but only gets indirect radiation, about 40% is absorbed into the exposed mass. One strategy to get deeper penetration of direct solar gain into a house is to have an internal mass wall struck on one side by the sun. Again mass walls are expensive.
Some years ago I started to look at thicker mass floors to save some expense and retain the solar thermal performance of a high mass building. This was done by excavating some extra soil below where the slab would be poured, placing the insulation deeper, and compacting several inches of gravel and fines below the slab. This is a cost effective way of containing more mass within the insulated envelope. As the solar heated fluid raises the temperature of the concrete the heat also migrates into the gravel below adding to the stored heat in the floor. Data loggers placed at the bottom and top of the mass have shown the daily pulses of heat and also when clouds have blocked the heat gain.
As time went on I increased the thickness of the mass to 16” with 3” of insulation below. A newer project has 32” of mass over load bearing insulation that is continuous under the footing, to eliminate the thermal leak. Four inches of perimeter foundation insulation is used as the temperature of the ground at the edge is much colder in winter than under the house. There is a second set of radiant tubes placed in the gravel 8” above the insulation. They are also fed by the solar collectors as a surplus dump. This lower set of tubes will continue to be fed slightly during the summer to maintain the 270 tons of mass inside the insulated envelope at about 70 degrees. As long as the floor surface temperature is kept below the room temperature no heat will be transferred to the living space. Temperatures are monitored by data loggers at 4 different levels. One was placed below the insulation, one just above the insulation, another in the middle of the mass and one in the slab. The heat transfer through the mass is working as predicted, allowing the stored heat to migrate to the slab during cloudy spells.
A heat recovery ventilator is a vital part of an energy efficient home. Heat recovery ventilators have a minor problem in this cold climate. They tend to frost up below 12 degrees and use a 1000 watt heater to prevent frost. In this climate where temperatures often go below this at night, there is a fair amount of energy consumed. To prevent this, a 6” dia. pre-heater is cast in the mass below the slab.
Much of the thermal performance of this house is below the floor and the energy conserving elements above complete the package. Compressed earth block is used for most of the walls both inside and out. Closed cell spray foam is used on the exterior walls and is the only efficient option for insulating an external mass wall. Twenty inches of cellulose is used in the roof and R-9 quad windows and doors are used throughout.