Thermal mass is the property of a material to resist a change of rate in thermal conductivity. The resistance to a change of rate in thermal conductivity results in a slow variance of temperatures and can be seen as a delay in reaction to change of temperatures. As the extreme differences in temperature happen, the structure maintains an average temperature . How does Thermal Mass work? Controlling/maintaining temperature, providing a comfortable zone and protecting from the exterior is the essence of a house. Thermal mass has been at the heart of housing in arid and hot environment since mankind has built. It is also still used today. For example in Earthships, which combine thermal mass using soil as a long term heat or coolant absorber with short term temperature control systems such as ventilation, temperature buffer zones, orientation according to natural inputs of energy, and controlling sunlight penetration according to season through the angle of the roof . All of this helps to control and maintain a comfortable temperature, but it is the mass of the building that ultimately defines its reaction to temperature.
Watch this video for an example of how Thermal Mass works. This video illustrates how a source of thermal mass such as water can be used to prevent a home from heating or cooling too much using two model homes, Xplorer GLX, and temperature probes (text from the YouTube description).
Past examples of Thermal Mass in Texas:
Vernacular architecture in Texas has had different forms. Before colonization, the Caddo Indians used two forms of housing; one for summer and one for winter. The summer house consisted in elevated flooring combined with a roof, which effectively creating a cooler area in the hot and humid weather. The winter house consisted of a thatched hut devoid of windows creating a highly insulated zone due to the air that is trapped in the thatch . After colonization, a form of the shotgun house, the dog-trot, was often used by settlers. The dog-trot shotgun house can be described as an elongated house, one room in width, with an open covered area in the middle. This middle area provides a cool workspace for the summers while the enclosing rooms provides an insulated zone for winters. The middle also provides ventilation for the house . One could make a connection with the dog-trot form of the shotgun house with the Caddo Indian strategy as combining the outdoor summer house and the closed winter house.
Sketch of Caddo's Winter House. 
Sketch of Dog-Trot Cabin House. 
Texas Today: Insulation or Thermal Mass?
In Texas, the conventional house is a timber frame with insulation within the structure. As wood contains many bubbles of air, it is a very effective insulator and will not act as a thermal bridge. Therefore, a conventional house in North Texas relies on insulation rather than thermal mass. The issue with conventional U.S. housing comes from the joinery between structure and fenestration which drastically reduces the effectiveness of the insulation . Temperature control is often maintained by HVAC systems which lose their efficiency if the building is not properly sealed, as more energy is needed to compensate for the thermal bridges with the exterior environment.
How can Thermal Mass be used in Texas?
If we consider that thermal mass is more appropriate for areas with high variance in temperatures, then the conventional U.S. house does not score highly in Texas. If we look at vernacular architecture in the area, thermal mass has not traditionally found its place in Texas. This is surely due to the availability of certain resources over others. Using thermal mass is not common in contemporary architecture because it relies on abundance of matter, for example soil or stone. Earthships show that thermal mass has a place in the southwest, basing itself on the availability of used tires as the main material. Michael Reynolds, with his earthships, shows that there is a place for thermal mass in contemporary housing. Furthermore, the earthships design also shows that thermal mass can be applied to Texas to greatly decrease the use of energy coming from a power plant .
Earthship Principles - Click for more information on earthships
Case Study: Can Thermal Mass lower costs?
A tool was developed that allows evaluation of thermal mass control strategies using HVAC utility costs as the baseline for comparison. Inverse models are used to represent the behavior of the building, cooling plant, and air distribution system. Inverse models use measured data to “learn” system behavior and provide relatively accurate site-specific performance predictions. Based on weather and solar inputs, as well as occupancy and internal gains schedules and utility rates, the evaluation tool predicts the total HVAC utility cost for a specified control strategy. Intelligent thermal mass control strategies can then be identified in a simulation environment using this analysis tool. The evaluation tool was validated using data collected from a field site located near Chicago, Illinois. The tool predicted HVAC utility costs for a summer month billing period that were within approximately 5% of actual costs. Additional studies were performed to examine the utility savings potential for summertime operations at the field site using various thermal mass control strategies. The best strategy resulted in approximately a 40% reduction in total cooling costs as compared with night setup control. Simulation studies were also used to analyze the overall impact of location on the savings potential for use of building thermal mass. Representative utility rates for five locations (Boston, Chicago, Miami, Phoenix, and Seattle) were used along with the models obtained for the field site. Significant savings were achieved in all locations except Seattle .