With the ongoing impetus to reduce energy use, outfitting a building envelope with materials effective in reducing heating and cooling costs is more important than ever. The insulation specified as part of the overall building envelope system has a significant impact on how a system will perform in terms of energy efficiency. As such, it is important to consider multiple factors before deciding which insulating product is best for a given facility.
One factor touted as centrally important to this goal is the R-value of the insulation. The generally agreed upon definition of this term is “a measure of the material’s resistance to heat flow.” This means—the higher the R-value, the greater its thermal resistance—whether it’s keeping the heat from exiting or entering a structure as the heat flows naturally from warmer to cooler spaces.
Simply stated, the R-value of a material depends upon its thickness and density. However, once it is installed in a building, insulation does not exist independently. Rather, the material becomes part of the entire building envelope, and, consequently, its level of effectiveness is influenced by other materials used in the structure, for better or worse.
So how important is the R-value of an insulation? And what other factors should be taken into account when specifying one?
The space where the insulation will be installed is one consideration. For instance, if a wall cavity is relatively spacious, the thickness of the insulation is not a crucial issue. However, when spaces are tight, insulation materials that are designed to fit in smaller areas may be a better choice.
Experts point out that R-values decrease when certain insulation materials are compressed unnaturally. Because the R-value is a function of both the density and thickness of the material, that thickness needs to be maintained. Consider fiberglass as an example. Therese Stovall, senior research engineer at Oak Ridge National Laboratory in Oak Ridge, TN, cites this scenario: “In buildings with metal roofs, it is common to drape insulation batts above the rafters. This compresses the batts where the rafters meet the roof, resulting in a reduced R-value.”
Robert Dehne, architectural services manager at Owens Corning in Toledo, OH, notes another potential concern. “R-value can be defeated if there are thermal shorts going through the wall. In commercial construction, for example, steel studs are frequently the main support for wall systems. Those studs conduct heat very efficiently, so if there is no insulation on the exterior of the stud, thermal shorting, or transference heat flow, will occur across the wall. This effectively reduces the R-value of the wall construction, sometimes by as much as 50% depending upon climatic and interior conditions. Insulated sheathing can be used at the outside face of the steel stud; an extruded foam product is ideal, because it does not absorb water.”
It’s clear that insulation and R-values are just one part of the building envelope equation. Vapor barriers, air sealants, and other building envelope products all have an effect on R-value, and therefore impact the overall assembly. It should be noted that when there are multiple materials present, each with their own R-value, those values are often added together. However, system effectiveness is often reduced due to gaps and voids that may exist.
Says Dehne, “A common misconception pertaining to R-values is that they are additive—that you can simply add the R-values of all the assembly components. However, when you factor in any thermal shorting or air leakage that may be present, this is not an accurate method. While the components’ R-values may total R-16, for instance, the value might actually be around R-11.”
The impact of air leakage on an effective insulation system has been addressed by the U.S. Department of Energy, which estimates up to 40% of the cost of heating and cooling a building is lost to such leakage. Tom Harris, product manager for BASF Polyurethane Foam Enterprises LLC in Minneapolis, MN, says, “What is needed is superior insulating performance, not necessarily R-value, coupled with a continuous, effective air barrier system. This recognition is reflected in the fact that Massachusetts, Michigan, and Wisconsin now include air barriers in their commercial energy codes.”
Additionally, ASHRAE is considering including continuous air barrier recommendations under Addendum z to its Standard 90.1-2004. The issue is currently under discussion.
As would be expected, geography is an important factor when looking at R-values. A useful starting point is to find out what is required by local building codes. Since these types of codes normally reflect minimum requirements, facility managers aiming for a significant boost in energy efficiency can go beyond the R-value recommendations.
Buildings in more extreme climates can benefit from R-values beyond what is recommended in building codes. “When there is a big differential, it’s desirable to have a higher R-value,” explains Gale Tedhams, product/programs manager, residential insulation for Owens Corning. “A facility manager of a building located in a really cold or really hot climate may want the higher insulating power in order to maintain the inside temperature with the least amount of energy required for either heating or cooling. In a very temperate climate, you can be effective with a lower R-value.”
While R-value is only one part of the equation, it can serve as a point of reference when assessing the needs of a building. Through careful consideration of building structure, geographic location, and even the conditions which the product was tested under, facility managers can curb wasted energy, which is becoming more valuable each day.