Effective R Value?

Amory Lovins’ home boasts an “effective R-Value” of 40 for the masonry walls. I was skeptical knowing they were masonry with a 4″ polyurethane core. The core would be about 20 if it was XPS foam, less if EPS foam was used. Masonry is not know for its R Value and solid concrete 4″ thick is about an R value of between 1.5 and 2.5. So the wall would be about R-25 not 40.
But I researched the R-value of concrete and found out there is something called “Effective R Value” for a thermal mass wall that has a lower R value of the materials alone. This effective value is described from hot box tests of various wall compositions at the Oak Ridge Laboratory in the early 90’s. A paper was published (Also found here.) and subsequently an article in the Home Energy magazine.
I also found a California Energy Standards Appendix describing additional energy measures, U-factor, C-factor, and Thermal Mass Data for various wall assemblies. Thermal Mass is identified by HC or Heat Capacity. I am not sure how heat capacity is translated into “effective R value” but I’m glad to know about this concept, since our trombe wall has a very low R value due to its materials. 8″ concrete blocks are about R-1.5 to 2.5 depending on the density of the concrete and solid slag blocks at 4″ are similar to 4″ brick for an additional .44 and what appears to be 4″ concrete blocks on the outside layer.  That makes our wall about R 3.5 at the high end since the slag brick and the concrete are back to back and there is no air cavity or insulating layer. The heating capacity of the wall would take into account its ability to moderate the temperatures in the home as well as gather heat from the solar collector surface.

Trombe Wall Inside Detail

Trombe Wall Inside Detail about 16″ Thick

Trombe Wall Outside Detail

Trombe Wall Outside Detail

This masonry organization mentions the research on thermal mass in their description of masonry R-Value.

“Mass effect is real. High-mass walls really can significantly outperform low-mass walls of comparable steady-stated R-value. However, the mass-enhanced R-Value is only significant when the outdoor temperatures cycle above and below indoor temperatures within a 24-hour period. High mass walls are most beneficial in moderate climates that have high daily temperature swings and nearly all areas with significant cooling loads can benefit from thermal mass in EXTERIOR walls. This is especially true for the sunny Southwest areas of Arizona, New Mexico and Colorado.”

They reference the article from the Oak Ridge Laboratory report in the Home Energy Magazine.

“According to an article written by Jeffrey E. Christian and Jan Kosny titled “Wall R-Values that Tell It Like It Is,” wall systems with significant thermal mass have the potential, depending on climate, to reduce annual heating and cooling energy requirements below those required by standard wood frame construction with similar steady-state R-value.

Masonry products, with mass-enhanced R-value or thermal mass, provide some of the best energy values for homeowners today. They consistently rank higher than steady-stated R-value of wood framed walls. Remember, the overall R-value is not as important as how the home is constructed. Attention to details like the windows you select, like low e-thermal, dual pane windows that are tinted, is just as important as the R-value in the walls. In fact, much heat loss or gain, up to 48%, is through windows, not walls! The most energy efficient building materials for the desert or Southwest climate is 24″ thick adobe, which only has an R-value of less than 7. It is energy efficient because of it other attributes including thermal mass, air tightness, thermal lag and thermal dampening. This proves that R-value is just one piece of the energy puzzle, and often, does not paint a realistic picture of energy efficiency.”

Another reference to masonry and R Value:

“The effect of thermal mass (also known as thermal inertia) on walls is well documented. High thermal inertia walls, such as concrete masonry, have the ability to delay and reduce the impact of outdoor temperature changes on conditioned indoor environments, improving energy efficiency. The International Energy Conservation Code (1994) recognizes most masonry walls that weigh more than 25 lb per square foot as mass walls. For example, this wall weight is attained with a 90 pcf 8″ un-reinforced cmu.”

Green Building Advisers take a dim look at thermal mass in this forum discussion, with most of their experience in cold climates, they discount the thermal performance of mass vs insulation R-Value except in some desert buildings. However, a poster from the front range in Colorado touts its effectiveness.

“I live and build on the Colorado Front Range and am a big believer in low tech Passive Solar, so no “expensive equipment” needed to harvest the “free energy”. Passive solar, moderate thermal mass, super tight insulation and being comfortable from 65 to 75 degrees, can “almost completely” eliminate heating bills and expensive heating & cooling equipment.”

This forum also has a good explanation of capacitive vs. resistive heat transfer and R-Value.

“R-values are for resistive insulation, calculated and measured in steady-state conditions where heat flow is consistently in one direction, from the warm to the cold side of an assembly. Capacitive insulation effects result from non-steady state condition where the heat flow reverses on a regular diurnal cycle. This can only be useful where exterior temperatures cycle significantly above and below desired interior comfort conditions e.g. in hot desert climates – heat starts to move slowly through the wall during the day but changes direction to head out again when the exterior cools dramatically at night. This is why traditional building cultures in those climates frequently make use of extremely thick mud or masonry walls and roofs. Capacitive insulation has virtually no effect in steady-state heat flow, which is when temperatures are relatively constant for an extended period of time on each side of a material.”

I found an excellent article republished here from a LEED AP explaining the difference between thermal mass and R-Value and how “effective R-Value” is calculated.  There are some fancy computer simulations that can do the job based on generic data or this method could be used.

“Performance of the building as a whole is simulated over a year’s worth of weather data. The results is an estimate of the annual heating and cooling energy use of that building. Next, walls of the building model are change to non-mass wall and the wall R-value is increased (to make up of the lack of thermal mass). Computer simulations are rerun with increasing wall R-value until the annual energy use of the frame wall building matches that of the mass building.”

James Plagmann, our architect, brought up this issue when he first looked at the house’s construction. He was initially perplexed by the trombe wall structure, saying the heat gained during the day would just leave the building at night. Later he told me he talked to some solar experts about the wall who said it would be effective. In fact his initial thoughts were somewhat correct, the thermal mass wall is “superheated” during sunny days by the sun due to the collectors on the south-facing front of the thermal wall. The walls heat slowly and release their heat slowly into the space. Someone in the above discussion explained that thermal mass is like a water cistern, it has to be filled to provide insulation properties. It works precisely because Colorado is so sunny and the difference between day and night temperatures are similar to those in any desert, hotter during the day and colder at night.
I believe our LEED evaluators (Energy Logic) use energy modeling software to calculate whole house performance to judge the energy efficiency of the building. I assume this software has some type of thermal mass modeling that will take into account the low R-value but high thermal capacity of the trombe wall.

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