Commercial building owners are familiar with the most basic principle of accounting – revenue minus cost equals profits. When costs rise, profits start shrinking. While “R-value” may not be as familiar a term to commercial building owners as “revenue,” it can have an equally critical impact on a company’s bottom line.
Energy is one of the most significant expenses for commercial facilities. Typically, heating and cooling costs represent about 32 percent of a building’s operating budget. As this cost is multiplied over the average 10-to 20-year ownership period, energy expenses rank as a high-priority lifecycle issue – becoming even more significant to owners with multiple retail facilities.
This article will look at important principles of thermal conductance for commercial buildings, examine the structural integrity and efficiency of different construction materials and methods, and discuss how owners can reduce their total cost of ownership by making energy efficiency an important part of the planning process.
Thermal Conductance: How to Measure Effectively
The most important factor in maximizing the energy efficiency of commercial buildings is in the design, materials, and construction of exterior wall systems. But before deciding which materials and wall construction methods will enhance energy savings, owners must first be familiar with thermal calculation techniques.
“R-value” is a unit of measurement that describes the resistance of construction materials to the flow of heat. Unfortunately, some of the most common methods of calculating a building’s energy efficiency using R-values are the least accurate. Here’s why.
Heat is transferred in three ways: by conduction, radiation, and convection; however, in all methods, heat flows from warm to cold. In fact, heat transfer is similar to fluid movement in many ways. Because fluids take the path of least resistance, they will usually find a way through almost any material. If a plastic sheet – which is essentially impermeable – has a pinprick in it, water will flow through the pinprick as fast as the hole will allow. The permeability of that sheet, in essence, becomes the same as the permeability of the pinprick, rendering the permeability of the plastic unimportant. In the same way, heat will find the path of least resistance and the R-value of the wall approaches that of the least insulating portion of the wall. Failure to recognize this can lead to inaccuracies in a building’s R-value.
The most common method of calculating R-values for wall systems is to add the R-values of all the materials that make up the panel. For example, with a wall panel that has a three-inch layer of concrete, a two-inch layer of foam, and another three-inch layer of concrete, the total R-value would simply be the combined R-value of all the materials. This method assumes a steady-state heat flow, in which the difference in temperature across all material layers is steady at all times.
Owners should be aware, however, of the potential for thermal breaks. Thermal breaks are areas that violate the insulation area and greatly reduce energy efficiency. With wall panels, thermal breaks can range from the obvious, such as doors and windows, to those that are invisible to the eye, such as highly conductive structural ribs embedded in the wall. In the previous example of two concrete panels sandwiching a layer of foam, a wall panel like this often uses metal ties and concrete to hold the different layers together. These metal ties and concrete are thermal breaks that allow heat to pass through the concrete layer, reducing energy efficiency.
Besides thermal breaks, owners should also be aware of the benefits of thermal mass. In real-life situations, inside and outside temperatures are not always constant. The force behind conductive heat flow between the exterior and interior of a wall can change significantly and even reverse during the day. For example, if a building’s exterior wall is dark-colored and in the sun, it will be significantly hotter than the outside temperature. If the temperature on the inside of the building is cooler than on the outside, heat will conduct from the outside surface of the wall inward. But as the exterior temperature falls at night, the driving force for heat flow reverses. Energy, as heat, is drawn from the inside to the outside of the building through conductive heat transfer.
With high mass materials, such as concrete, this heat transfer is delayed and even blocked by the high heat retention capacity of the wall mass. This “mass effect” has a quantifiable effect on the energy efficiency of a wall that cannot be determined by R-value alone. When choosing a building material, considering how “mass effect” can enhance energy efficiency is as important as determining a material’s R-value.
Before beginning new commercial construction, owners should examine the R-value of different materials, take into account the mass effect produced by certain high-capacity materials, and ask contractors about the presence of thermal breaks. Otherwise, an owner should be prepared for energy cost surprises down the road.
Examining Construction Options: Maintenance, Structural Integrity, and Energy Efficiency
Masonry, metal, and concrete are the most common materials used to build an exterior wall. When deciding between building materials and construction methods, owners need to examine the energy efficiency, maintenance, and structural integrity of each option.
Masonry’s widespread use makes it the most popular wall construction method available. Brick walls offer security, quality and long-term durability. With masonry, because the finished face is brick, you get high aesthetic values, choice of colors, and patterns.
But because of the sheer number of components bound by mortar, masonry is the most susceptible to cracking of any exterior wall option. When cracks appear, it’s necessary to tuck-point, a time-consuming and costly repair.
Masonry is also porous and needs a sealer (most often paint) to be applied at installation and at regular intervals ranging from two to 15 years (depending on the quality of sealer applied and the skills of those doing the sealing). This is an added cost.
Finally, the porous nature of masonry also requires an added insulation layer to simply meet code, further driving up installation costs. And if the building owner wants a smooth finish on the interior walls, the masonry must be furred out and sheetrocked. All of these added requirements increase the cost of the building initially and throughout its life-cycle.
Metal buildings – the least expensive of wall construction options—can be a cost-effective, viable option. While initially inexpensive, some metal-based walls are not as inherently strong or as durable as concrete or masonry. A hailstorm, strong wind, or misguided delivery truck can easily disfigure a metal exterior.
The extensive studwork associated with a metal wall creates a natural passageway for air, negatively affecting the energy efficiency of a building. And when an exterior skin of thin metal panels covers a metal frame, each screw and cut edge acts as a possible miniature point of air penetration or rust.
The design limitations of metal walls also can be a concern. Large steel frames of beams and columns need to bear the load, which can be especially inhibiting for retail and distribution outlets where storage space is at a premium. Columns, which often extend two to three feet from the wall into interior space, impede the placement of racks, pallets, and other storage devices, making the space less flexible.
Because of its high structural integrity, durability, and low maintenance requirements, concrete is often the material chosen for exterior walls on commercial buildings. Unlike other building materials, concrete actually gains strength over the life of a building since hydration causes the compounds in cement to elongate, lengthen, intertwine, and create an impermeable surface. As a result, concrete walls require minimal maintenance. A sporadic, high-pressure washdown is all that is needed to maintain its finish, and re-caulking about every 15 years helps eliminate fissions that may appear over time. Because concrete is a high-capacity material with strong heat retention capabilities, it also can take advantage of the mass effect phenomenon.
Yet, not all concrete-based wall systems are created equal. There are three primary types, each having unique insulating characteristics: composite precast concrete wall panels, tilt-up concrete wall panels, and non-composite precast concrete wall panels. Knowing the difference between each can help owners pinpoint the construction method that will best minimize their life-cycle costs.
Composite wall panels are those that combine two separate layers of concrete with steel ties and solid areas of concrete to achieve one “composite” panel. Typical composite wall panels are constructed with a three- to seven-inch exterior layer of concrete, a two-inch layer of foam, and a three-inch interior layer of concrete. The Department of Energy (DOE) examined the thermal efficiency of “common, insulated, concrete sandwich wall systems,” the composite method of wall construction. The DOE interim report indicated that 5 percent solid concrete and steel ties resulted in a 60.59 percent loss of the claimed heat-flow resistance (or R-value) of 10.14, reducing it to only 4.13. This is mainly due to the presence of thermal breaks. Another drawback is that because the inside and outside of the panel are forced to work together, thermal difference from the interior to the exterior may force the panels to bow. This further strains the panels and the joints between them.
Tilt-up wall systems are similar to composite type precast panels, but are not manufactured in a controlled environment. The concrete for tilt-up panels is generally poured outside, and its exposure to environmental conditions, such as rain and freezing temperatures, can alter its structural integrity and durability. Most tilt-up panels are also created without integral insulation, requiring the interior surface to be furred out and insulated.
Non-composite panels combine a unique structural core, a rigid insulation layer, and a nonstructural façade for exceptional strength without the need for a thermal bridge. Eliminating highly conductive thermal breaks increases energy efficiency. For example, Fabcon Inc. produces non-composite precast sandwich wall panels composed of an eight-inch hollow core layer of concrete, a 2.5-inch layer of foam, and a 1.5-inch layer of concrete. Due to its high density, a Fabcon wall panel has an R-value of 15.99.
Besides the calculated energy efficiency, the thermal mass effect on Fabcon’s non-composite wall panels can increase the R-value to 23.5. This improves livability for building occupants by eliminating cold spots in the building’s walls. With a wide variety of exterior and interior finishes, non-composite panels also increase the aesthetic options available for commercial buildings. Fabcon’s non-composite panels are completely manufactured indoors, ensuring consistent quality.
Commercial building owners recognize that improving energy efficiency can lower operating expense and improve tenant comfort. The economic and environmental benefits also make a property more competitive, resulting in better tenant retention and attraction. Lower expenses, in turn, produce higher net-operating income, resulting in increased property value.
Tom Kuckhahn is director of Engineering at Fabcon Inc. (www.fabcon-usa.com), Savage, MN.