Energy codes and other motivators are pushing all of us to not only design to higher levels of performance in buildings, but to ensure we achieve them. This requires an understanding of not only the ideal laboratory test conditions of building products, but also the actual in place field conditions that influence that performance. When it comes to selecting or specifying building insulation, it has been demonstrated repeatedly that there are a variety of factors that are important to consider. It is not just about lab-tested R-values, but about understanding how insulation plays into the total building performance in specific locations with a specific climate. It also means understanding the differences in characteristics between different types of insulation products. This is true in all parts of a building, but particularly so in roofing assemblies since the roof area of many buildings is a significant portion of the total building envelope and its resulting thermal performance.

Determination of Roof Insulation R-Values

The U.S. Federal Trade Commission (FTC) has authority in this country over manufacturers' products and any claims of performance that are presented to consumers of those products. In 2005, they updated the Trade Regulation Rule titled: Labeling and Advertising of Home Insulation (part of 16 CFR Part 460) in which they identify the “R-value Rule.” This rule specifies requirements (applicable to insulation manufacturers, professional installers, new home sellers, and retailers) to substantiate and disclose information related to thermal insulation products used in the residential market. It also prohibits certain claims unless they can be proven.

The primary disclosure required is the insulation product's R-value which the FTC simply defines as “the numerical measure of the ability of an insulation product to restrict the flow of heat and, therefore, to reduce energy costs—the higher the R-value, the better the product's insulating ability.” It does go on to state that the disclosure of the R-value shall be based on uniform, industry-adopted standards which in most cases means tests performed under ASTM International standards (formerly known as the American Society of Testing and Materials). These standards are performed in laboratory conditions with specified requirements regarding test procedures, apparatus, and reporting. The ASTM tests are applicable to both commercial and residential products and are referenced widely in architectural specifications. The R-value Rule technically only applies to residential insulation products although the FTC has left the door open to revisit its applicability to commercial products. Either way the tests and standards are the same and all manufacturers follow all the requirements since they don't dictate whether their insulation is used in residential or commercial buildings.

The R-value Rule goes on to list some of the various ASTM tests that are recognized depending on the type of insulation being addressed. In addition, the rule requires that R-value tests be conducted at a mean temperature of 75°F and a temperature differential of 50°F (all plus or minus 10°F). Hence, insulation is usually tested with the cold side at 50°F and the warm side at 100°F to create the mean or average of 75°F. It also requires that the tests must be done on the insulation material alone (excluding any airspace). While this creates a very uniform and repeatable method of comparing insulation materials and eliminates any interference from other materials or factors, it may not necessarily replicate real-world field conditions across different locations, different times of year, or different construction systems. In other words, while the tested R-value is a useful comparative factor, actual real-world results may vary to a large degree.

Buildings perform better when insulation material is used that is responsive to their climate and location. Image courtesy of ROXUL, Inc.

When it comes to certain closed cell foam insulation products such as polyurethane, polyisocyanurate, and extruded polystyrene, there is another variable that will affect performance. These closed-cell foam insulations rely on a blowing agent or gas, other than air, to achieve their thermal resistance values. At the time of manufacture, the cells of the foam usually contain their highest percentage of blowing agent and the lowest percentage of atmospheric gases (air). As time passes, the relative concentrations of these gases change due primarily to diffusion. This results in a general reduction in the thermal resistance of the foam due to an increase in the thermal conductivity of the eventual mixture of gases in the foam cells. These phenomena are typically referred to as foam aging and the R-value Rule requires that tests must be done on samples that fully reflect the effect of aging on these products.

The R-value Rule identifies either the very specific procedure in paragraph 4.6.4 of GSA Specification HH–I–530A, or another reliable procedure to determine aging. However, since HH-I-530A is no longer an active specification, the generally accepted procedure is to use ASTM C-1303 Standard Test Method for Predicting Long-Term Thermal Resistance (LTTR) of Closed-Cell Foam Insulation. This test seeks to estimate the 5-year aged value of the tested products which are considered to predict the 15-year time-weighted average R-value.

While all of this testing is useful, there are two other factors that are not tested, but can certainly influence the real-world performance of the installed insulation. The first is the different range of temperatures that the installed insulation will be exposed to. A building will likely be exposed to much more than a 50-degree temperature differential with the exterior surface being exposed to either very hot or very cold conditions depending on the location and climate. Second, the presence of moisture in a construction assembly has been shown to sometimes affect performance, but generally that is minimal. If there is a breach in the assembly and rain water penetrates, causing the insulation to be wet, then in some cases it can lose its thermal effectiveness. However, for the purposes of this article, we will consider that an anomaly not relevant to the rest of our discussion.