Thermal Performance Concepts

Overview

A proper understanding of thermal performance concepts forces a consideration of all relevant elements of the building envelope. Design, building orientation. shading, roofing insulation, windows and doors, heating, ventilation, air conditioning, and the exterior wall construction all affect the total building energy efficiency. Omni Block’s exterior wall system utilizes age-old proven concepts, such as, the effective use of exposed thermal mass or the Adobe Principle, thermal lag and thermal damping, and combines them with a unique block and foam insulation design. It is this combination that provides the opportunity for superior thermal performance.

Thermal Terms Defined

R-Value represents Resistance to heat transfer. The wide variety of building materials have differing R-Values. The various assemblies of these building materials should be closely investigated because in most cases there are “weak links” that diminish overall R-Values. For an in depth discussion, please visit the U.S. Department of Energy’s website, specifically the article found at www.ornl.gov/sci/roofs+walls/articles/wallratings/index.html (Note that this link takes you out of Omni Block’s website).

Thermal Mass refers to the quantity of matter in a material. Technically, mass is not a resistor. but rather a very slow conductor. Like water, concrete block consists of a great deal of thermal mass. Thermal mass has the capacity to absorb and store heat or lack of heat.

Thermal Lag is the time it takes the temperature on one side of a wall to be detected on the other side.

Thermal Flywheel is a feature that is not solely unique to Omni Block but Omni Block’s block design maximizes the thermal flywheel effect on interior room temperatures. The illustration below (Courtesy of Energy Mass Wall Systems at www.energymasswall.com) does not depict Omni Block, but the principal and effect are the same.As the sun’s rays hit the exterior wall its energy begins to move through the block. But as the day progresses, the sun’s rays move off the wall. The sun has heated and penetrated a portion of the wall, but its energy does not reach the interior side because of the extended thermal path created by offset and constricted cross webs, a middle lineal wall, and two layers of foam insulation for the 8″ block and two middle lineal walls and three layers of foam insulation for the 12″ block. During the nighttime, the block cools, preparing for the next day. Passive solarists have labeled this the “thermal flywheel effect”. The same effect in the winter time is accomplished by the interior wall surface absorbing a layer of heat, storing it on the surface, and then releasing it back into the room. This passive process creates comfortable, “draft-free” interiors.

Thermal Damping is the term used when a leveling of energy consumption occurs. Damping is important in that it reduces the energy required to maintain a constant internal temperature and it also delays the load requirements to a later time when energy costs are lower. The graph compares an insulated stud wall to a standard CMU wall of comparable insulation value on a typical sunny day. It illustrates when the heat gains and losses for the interior space are highest. The stud wall has a peak gain two hours after the outdoor temperature peak. The masonry wall has a lag of six hours, with the peak occurring at 9 p.m. There is also considerable temperature damping with the masonry wall as the peak energy flows are only about one-quarter of those of the stud wall. Since the R-Value of the 8″ Omni Block is 6 times (R-2.2 compared to R-13.6) that of standard CMU, the thermal lag of Omni Block would be greater and therefore it is reasonable to conclude that the heat gain would be greatly reduced resulting in a more energy efficient wall.

Interesting Fact

It would take approximately 9 hollow core 8″ standard CMU butted together to achieve a thermal resistance of R-19. However, the sun’s direct energy hitting on the exterior or “captured” heat from forced air on the interior would never penetrate through the 9 block (Masonry Structures: Behavior and Design / Drysdale, Hamid, Baher, Chapter 14, Application of Building Science for Environmental Loads, ppg 557, 558). The illustration below shows 5 block abutted next to each other to equal an R-11. Again, this begs the logical question, “would heat ever move completely through (in or out) the cave-like structure that is made up of 5 block just to achieve an R-19?” This illustration and the application of logic demonstrates that there is more to energy efficiency than simply R-value.

Interesting Fact

The “Law of Diminishing Returns” shown in this graph illustrates that as wall R-Value increases, the incremental energy savings diminish (ASHRAE, Handbook of Fundamentals). Therefore, beyond a certain point, energy savings realized are trumped by the extra costs of increasing the wall R-Value. The exact point where this happens is dependent upon building design, climatic location, insulation costs, and of course, actual Btu costs.

Interesting Fact

Only 11% of a residential building’s heat gain or loss is through the walls; 39% is attributed to the windows and doors; 31% the ceiling; while the rest is attributed to “leakage” (U.S. Department of Energy). This point begs the question, then why Omni Block? From this information we learn that there is 89% heat gain or loss through other components than the walls. Regarding heat gain, Omni Block’s exposed thermal mass will absorb heat from the building due to these other sources, including those not mentioned, such as, lighting, cooking, showers, to name a few. Regarding heat loss, Omni Block’s exposed thermal mass again absorbs heat from the building and then as heat is lost due to the other sources, it releases heat into the room evenly. In both cases, exposed thermal mass becomes “passively active”.

Conclusions

The current R-Value rating of a building material alone is not an accurate measurement when assessing masonry thermal performance. The building science industry has taken major strides in determining what works and what doesn’t when it comes to a building’s thermal efficiency. However, it still requires a “resistance” measurement on a “conductive” material. This is a fundamental problem because mass is a very slow conductor. The other principles of thermal mass, thermal lag, thermal flywheel, thermal damping, and air tightness must be considered into the overall “energy efficiency puzzle”.

Due to the mass properties of Omni Block, it can be “charged” (made cooler or warmer). As discussed, Omni Block can store energy. Block has an innate or natural temperature of approximately 56°F which attracts heat (heat always chases cool). Since Omni Block walls are not covered (furred-out and drywalled) there is a tremendous amount of wall surface area (thermal mass) exposed to the interior of the building. In the wintertime, the heating of a structure’s interior all heat to be stored on Omni Block’s surface, only to be released, if and when, the room temperature cools, thus moderating ambient air temperatures. In the summertime, any heat that enters into the structure (through windows, doors, leakage, or the ceiling) seeks to expend itself by moving to the cooler masonry. The result is a significant reduction in cooling costs; a more even ambient interior temperature throughout the building; and a cleaner, healthier interior environment.