Energy Benefit of Brick Used on Buildings

Words: Steven Judd

PART 1
Item 1: Brick Veneer Cavity Wall Over Stud Framing
There is a large contingent in the marketplace that will suggest that brick veneer over stud framing—the typical cavity wall construction (considered to be the “perfect wall”)—has little or no impact on the energy-saving aspects of an exterior wall. The unsubstantiated presumption is that since the air cavity behind the brick is vented (via weeps at the bottom of the wall section) or ventilated (via weeps at the bottom of the wall section and vents near the top of the wall section), the brick is thermally isolated from the wall behind the air cavity and therefore has no thermal value. This happens to be a completely false assumption for any place where the sun rises and sets daily, and the temperature routinely changes from night to day—which is everywhere in most of the northern hemisphere—where we call “home.”

Published research papers based on thermal studies of brick veneer wall performance undertaken at the National Brick Research Center (NBRC), associated with Clemson University, have proven that the air chamber behind the brick veneer is a reasonably stagnant air chamber. It does have some minor convection currents as the hot air rises and the colder air sinks; however, the weeps and vents significantly restrict any large volume changes of the air. The studies also included wall performance under simulated wind conditions, which did not statistically change the conclusions. The air chamber in the typical brick cavity wall construction acts as a buffer, reducing (or regulating) the outside temperature changes that impact the wall framing, which is where the code-prescribed insulation is generally located—at the back of the air chamber and/or in the wall framing.

It takes time for the brick to heat up during the day and time for the brick to cool off during the night, to the effect that it acts like a thermal shock absorber because of the mass of the brick. You can imagine that if you place your hand on the back of a brick and take a blow torch to the front of the brick, the back face of the brick will gradually get warmer and eventually get hot if the heat source remains in place. But the back face doesn’t get hot immediately—it gets hot eventually. Likewise, the brick doesn’t cool off immediately either; it will remain warm for quite some time after the heat source is removed. The brick also acts like a battery, in a sense, as it absorbs and stores heat due to its mass. Consequently, the backside of the brick rarely gets as hot as the face of the brick during the heat of the day, and the backside rarely gets as cool as the outside air temperature during the night. This means that the air chamber behind the brick experiences a moderated temperature swing. The air chamber rarely experiences the maximum or minimum temperatures that occur at the exterior surface of the brick—the daily highs and lows. This is the advantage of having a mass wall—the mass of the wall regulates the temperatures that the framed wall experiences. If you have ever entered a large cavern or a historic masonry castle, you may have noticed that the inside air temperature stays at roughly 40 degrees Fahrenheit, regardless of the air temperature outside, which is roughly the average annual outside air temperature. The mass of the walls of a castle and the mass of earth surrounding a cavern don’t allow the interior temperatures to change much, if at all. The interior air temperature is moderated by the mass of material surrounding the interior space(s).

Since the wall framing behind the brick and air cavity only experiences these reduced/moderated temperature changes, the insulation demand on the wall is reduced—the insulation doesn’t have to work as hard. This phenomenon is recognized in the code as a “mass wall.” A mass wall is a wall that has sufficient mass (heat capacity, actually) in the assembly to regulate (moderate) the effects of the temperature swings occurring in the outside air. Both the International Energy Conservation Code (IECC) and the ASHRAE 90.1 (90.1) code recognize the benefits of a mass wall. The IECC is generally used for residential construction, and the 90.1 code applies to commercial construction. Both have allowances for mass walls regarding the minimum amount of insulation required for energy conservation requirements for exterior walls. These codes dictate how much insulation is required to satisfy the energy performance of various enclosure systems, including exterior walls, both above-grade walls and below-grade walls. For the following exercise, we will be looking at above-grade walls.

Here is the comparison: For a steel stud-framed wall in Climate Zone 5, used for commercial construction, the insulation needed to comply with 90.1 prescriptive requirements is R-13 within the wall framing plus R-10 continuous insulation (c.i.) [c.i. is usually applied outboard of the exterior sheathing and water-resistive barrier (WRB)], for a total of R-23. (The R-value is a measurement of the resistance to heat flow, the performance of the insulation: the higher the number, the more resistance.) If one were to add brick veneer to this same wall assembly, the mass of the brick generally suffices to qualify the wall assembly as a mass wall (having sufficient heat capacity to meet the code requirements for a mass wall) and thus requires only R-11.4 c.i. (usually applied outboard of the exterior sheathing and WRB). So, for the same energy performance, the brick veneer wall requires less than half of the insulation of the steel stud-framed wall without the brick veneer. This is a significant cost saving for equivalent energy performance of the wall. The additional bonus using this approach is that by removing the R-13 within the wall framing, one can almost guarantee that there will be no condensation in the wall framing, which reduces the potential for the rusting of the steel studs or mold and rot for a wood-framed wall. The insulation comparison for a wood-framed wall is R-13 + R-7.5 c.i. (R-20.5 total) or R-19 + R-5 c.i. (R-24 total), for a 2x4 and 2x6 framed assembly, respectively, versus R-11.4 for a mass wall.

Code exception b provides implied compliance when partially reinforced with insulation in un-grouted cells.



Image 1: Prescriptive R-values and U-factors for Prescriptive Compliance

Conclusion: As shown above, depending on the materials used for the wall framing (wood studs versus steel studs), there can be a significant savings—around half—in the required insulation to meet the prescriptive requirements of the energy code. Additionally, if one were to also consider the heat capacity (mass effects) of all of the wall materials (exterior sheathing, studs, interior gypsum board), one can reach energy code compliance with exterior insulation around R-7.5, which is around one-third of the insulation needed for an exterior wall without brick veneer. This approach—brick veneer used as a mass wall -- has significant positive impact on the construction cost by reducing material and labor and reduces the carbon footprint by reducing the production and transportation of the insulation materials that are not needed to comply with the energy code. Additional secondary savings come from brick as it only needs minimal maintenance over an exceptionally long service life (upwards of 100 years), which further reduces materials, labor, energy use, and carbon footprint.

PART 2
Item #2 – Single Wythe Reinforced Structural Clay Masonry
As noted above, brick veneer (which weighs around 35 psf) has sufficient heat capacity for an assembly utilizing brick veneer to be considered a mass wall. A moderately reinforced single-wythe structural masonry wall—at least 6 inches thick—will also have sufficient mass to meet the heat capacity criteria contained in the codes to be considered a mass wall. Therefore, the prescriptive insulation savings discussed above also apply to single wythe walls. Mass walls require less insulation to meet the energy performance dictated by the energy codes. The reinforced structural masonry walls act like a thermal battery, similar to brick veneer walls, absorbing energy in the heat of the day and cooling off slowly during the cooler parts of the day. The mass walls can shift the peak heating and cooling periods away from the typical occupancy times—maximum interior temperature occurs later in the day (for typical commercial office space), which reduces the energy required to temper the interior space as shown in Image 2.



Image 2: Mass Wall Time Lag and Dampening

Insulation for a single wythe wall can be a bit of a challenge in some climates. In Climate Zone 1, there is no insulation requirement, so that is easy (refer to Images 1 and 3). In Climate Zone 2, there is a code exception that states if single wythe structural masonry mass walls are reinforced with vertical bars at 32 inches o.c. and horizontal bars at 48 inches o.c. and the other cells are filled with insulation (treated perlite, treated vermiculite, or urethane foam, for instance), that wall is defined to comply with the prescriptive energy code requirements. So, nothing else need be done. For climate zones farther north, a portion of the wall can be internally insulated (as previously noted) in addition to some insulation on the inside face near the upper portion of the wall (above the ceiling, for instance) combined with, at times, some additional insulation in the roof. This approach, referred to as the energy trade-off method, is allowed by code and is embedded in the compliance check software packages called COMcheck® (for commercial construction) and REScheck® (for residential construction) that are used by the majority of design professionals. These programs were developed by the Department of Energy for use by design professionals. These code check software packages analyze the input data (the U-factors and heat capacities for mass walls) and determine code compliance, sometimes including roof systems per the user of the software package.



Image 3: ASHRAE 90.1 Climate Zone Map

The energy trade-off approach allows for one to trade off increased insulation in some areas—where there is more insulation than the code bare minimum—with areas that have slightly less than the code minimum insulation in other areas. This trade-off method uses weighted averages to determine compliance with the combined elements. This prescriptive energy trade-off approach uses the averaged U-factor numbers for the various elements (U-factors are approximately the inverse of the R-values: R-10 ≈ U-0.1) and does not require continuous insulation as does the prescriptive R-value approach. The U-factor approach makes design options a bit more flexible from a design perspective. This average U-factor approach can work in Climate Zones 3, 4, and 5 fairly easily, provided the upper portions of exterior walls can receive some amount of interior insulation (as noted, perhaps, above the ceiling), or via dual-purpose panels serving as sound attenuation panels and insulation. These dual-purpose panels are often used in large spaces, like auditoriums, cafeterias, and gymnasiums—locating the dual-purpose panels in the upper third or half of the wall (above the “impact zone”), plus some increased insulation at the roof. Lastly, for more northern climates, the lower portion of the wall may require cavity wall insulation, which can be protected by an interior brick veneer wythe; the wall assembly becomes structural reinforced masonry, insulation, interior brick veneer—from outside to inside, respectively. The upper portions of the walls can be as previously discussed, insulated with dual-purpose insulating and sound-attenuating materials.

Conclusion
As noted above, accounting for the enhanced energy performance of brick veneer used in a traditional cavity wall system can reduce the amount of required insulation by about half or more, and even more reduction to the insulation required to meet code when modeling the heat capacity of the entire wall. This approach can produce significant savings in construction by reducing material and labor, with the collateral benefit of improving the wall performance regarding moisture control—forcing the dew point to the exterior side of the WRB. Additionally, it has been shown that single wythe reinforced structural clay masonry has the same mass wall thermal benefit as brick veneer, which can be augmented by various means to meet the performance dictated by the energy codes. Brick masonry is much more than a durable, aesthetically pleasing solution to the exterior walls of a building; it can have a significant positive impact on the long-term energy performance and energy savings of a facility.



Footnotes:
N. Huygen and J Sanders; Air Flow Within a Brick Veneer Cavity Wall, and, N. Huygen and J Sanders; Dynamic Thermal Performance Measurements of Residential Wall Systems Part II, with Numerical Validation of Steady-State Performance; (both papers published in the 2019 13th NAMC proceedings in Salt Lake City, Utah).


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