As new building and insulation materials have entered the market, confusion over the nature and meaning of R-Value has raised questions among building professionals and consumers alike. What exactly is R-Value? Is it an effective and objective measure of the thermal performance of an insulation product or construction system? And how should the marketplace use it to compare the benefits of one insulation material over another?

Defining R-Value

During the 1970's demand for quality building insulation soared, when an oil crisis sent heating and cooling costs skyward, With many new products on the market -- and with so many conflicting claims pertaining to the insulating abilities of those products -- the Federal Trade Commission, with the participation and support of the insulation industry, created an objective method for reporting the performance of residential insulation materials. This method is called the R-Value Rule.

The rule provides requirements for product labeling (R-Value) and advertising, and mandates specific ASTM methods for thermal testing. The R-Value rule attempted to create a level-playing field for competing insulation materials. "The R-Value Rule has been helpful in comparing different brands of the same type of insulating material," said Betsy de Campos, executive director of EPSMA, "but as more sophisticated materials and higher technology construction systems are introduced into the building industry we find that the R-Value of a material does not tell the whole story."

R-Value is based on a mathematical term known as R-Factor. The term R-Value was developed to represent the ability of an insulation material to restrict heat flow. It is determined by placing test specimens between two plates in a laboratory apparatus and measuring heat-flow through the insulation. The test specimen usually consists of a square foot of material exactly one inch thick whose surfaces have a temperature differential of 1degree Fahrenheit. The thermal conductivity (k) of a material is expressed as the rate of heat flow in BTUs per hour.

R-Value then, is the R-Factor of an insulation material multiplied by the amount of material used. For instance, if the specified insulation has an R-Factor of 3.8 and you are using 3.5 inches of insulation, the R-Value is 13.3. Thermal resistance (R) of a material is its resistance to heat flow, and R-Value is expressed as the reciprocal of the materials thermal conductivity.

Pretty technical stuff, but the idea that a consumer should be able to compare insulation is essential to ensuring homeowners and building professionals are capable of making informed product decisions. Simply put, the greater the R-Value the better the insulation.

But, there is more to consider when making those decisions today.

Clear Wall verses Whole Wall R-Value

When the R-Value Rule was instituted, most homes and buildings were constructed and insulated using dimensional lumber, two by fours, and fiberglass insulation. To increase a home's insulation, generally, a builder would have selected a fiberglass insulation product that had a higher R-Value, but today we have learned that it isn't necessarily the R-Value of the insulation that makes the wall more efficient. It is a combination of insulation materials, construction details, and installation care that provide ultimate thermal performance.

Today we understand that to insulate the space between the studs with fiberglass is not the same as insulating an entire wall. We must consider the wall as a system. The lumber creates thermal bridging and air infiltration within the wall, around windows and doors and at connections between the wall, the ceiling and the floor. Now we know that it is important to insulate the wall and not just the area between the framing members.

Consider though, fiberglass insulation performs well in controlled laboratory tests, how does it perform when actually installed in a home? Fiberglass is susceptible to air exfiltration; when air enters a wall through a crack in the siding or near a window opening; it flows right through the fiberglass, significantly reducing its ability to resist thermal flow. Moisture, also, causes fiberglass to significantly lose its insulating ability. R-Value doesn't account for these problems.

In an effort to create more comprehensive standards, researchers at the Building Technology Center at the Department of Energy's Oak Ridge National Laboratory in Tennessee are proposing a system for rating "Whole Wall R-Value," which is the insulating value of a whole wall system.

According to Builder Magazine, most R-Value calculations are based on conventional wood frame construction using "clear wall" or "center of cavity" criteria. R-Value only takes into account the insulation and necessary framing members that make up the "clear" section of the wall, not the corners and intersections with the roof or floor. Further, the center-of-cavity method rates R-Value only at the point where the insulation is the thickest -- right between the studs -- and the rating is based on laboratory tests, not actual conditions. For these reasons, R-Values tend to be overstated.

Whole R-Value, however, takes into account the interface details of an exterior wall, which are the intersections of the wall with other walls, the roof, deck, doors and windows. The framing and connections create what are called thermal shorts -- points that can detract from the wall's overall R-Value.

"EPS insulation used in structural insulated panels or insulating concrete form homes offer tighter interface details than fiberglass batts by eliminating air infiltration," said de Campos. "For this reason, the use of whole wall R-Value is more accurate in describing thermal performance of the system than the respective R-Values of the components. This is just one other consideration that consumers need to make when determining the best insulation system."

Thermal Drift

There are other factors that affect the performance of insulation products after they are installed in a building, including thermal drift.

"Depending on the insulation material used, the R-Value can slowly be reduced over time as the material ages," said de Campos. "This should be considered when the designer calculates the expected performance of the insulation materials that he is recommending."

Some foam plastic insulation materials use blowing agents that have a high resistance to heat flow causing the insulation to have an abnormally high R-Value at the time of manufacture. It is now known that these blowing agents diffuse from the cellular structure of the foam until a level of equilibrium is reach many years after it is manufactured. As the high R-Value gases diffuse out of the cellular structure, the ability of the insulation to prevent thermal flow is reduced, losing up to 30 percent of its original insulating ability. EPS foam does not use these types of blowing agents, therefore, its insulation performance remains stable over its entire life.

"If you compare EPS side-by-side with some foam plastic insulation materials right after they are manufactured, the other materials may have a higher R-Value," said de Campos. "But EPS is stable and does not experience any thermal drift and does not lose R-Value over its life. In the long run, the thermal performance of EPS insulation is constant, and when all cost and performance factors are considered, it typically provides the greatest insulating value available."

EPS Maintains R-Value

In the "Report on Expanded Polystyrene Insulation for Use in Built-Up and Single Ply Roofing Systems," dated August, 1984, researchers Rene M. Dupuis and Jerome G. Dees show that samples of EPS insulation had no deterioration in R-Value. The test results at 70°F for thermal resistance of EPS insulation samples taken from roof systems of various ages indicated no deterioration in the R-Value over time.

Beyond R-Value

Building codes require minimum levels of insulation in order to conserve energy and make the building more comfortable. Energy and resource conservation continue to be important issues in deciding what materials are used in today's construction. As technology improves and our understanding of thermal performance increases we now know that there are many more factors to consider than just R-Value. Thermal bridging, thermal shorts, air infiltration, poor construction details, and bad workmanship all are factors that contribute to the thermal performance of a material or insulation system beyond R-Value.

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