What is Molded Expanded Polystyrene (EPS)? Continued

No, extruded polystyrene (often in colors such as pink, blue or green) is produced using a different manufacturing process and different blowing agents. White EPS insulation is molded into blocks using a hydrocarbon blowing agent and, unlike extruded, is available in many different densities and thickness. EPS manufacturers, in compliance with the Federal EPA Clean Air Act, control blowing agents during processing. In addition, EPS foam used as thermal insulation reduces harmful air emissions by saving resources and energy which would be consumed due to greater heating and air conditioning requirements.

What is R-value and how is it measured? How does R-value relate to K-value?

K-value is thermal conductivity, expressed as the quantity of heat (BTUs), which will flow through a one square foot section of a 1-inch thickness of a homogeneous material, during one hour when there is a 1 degree F difference in the hot to cold side temperature.

R-value is a measure of the thermal resistance of a material. Thermal resistance is an index of a materials' resistance to the flow of heat. It is the reciprocal of the K-value. K-values are determined by either of two tests: ASTM C 177 or ASTM C 518.

The higher the R-value, the better the resistance to the flow of heat (expressed in BTU's) and the better the insulation.

R-values are reported for 1-inch of thickness and are not necessarily per inch of thickness (for residential construction only). R-values are usually reported at mean temperatures of 75 degrees F. per FTC regulations.

In an article in the 1987 Autumn issue of Exteriors magazine, Andre Desjarlais, then with Dynatech Scientific Inc., of Cambridge, Massachusetts, discussed the need for testing real-world roof environments when evaluating roof insulation. He said, "In addition, some isocyanurates may change chemically when held at 140 degrees F. for 90 days. Lab results can be used as valid representation of the product but the R-values one gets are not absolute." This further substantiates the issue that R-values must be explored not only from a laboratory perspective, but from their "real world" applications as well.

How does the R-value of EPS compare with other insulations?

The R-value of EPS is stable and does not change over time.

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

The following table compares two examples of published R-values to samples taken from actual roof decks:

 

AGE DENSITY R-VALUE Published Initial Values at time of manufacture 1.00 pcf 1.25 pcf 3.85 3.92 EPS Insulation Samples 13 years 15 years 1.28 pcf 1.09 pcf 3.94 4.07

Those who specify or purchase insulation pay for a certain amount of R-value per inch according to their needs and budget. To invest in a product where the dollar value drifts as the thermal values shrink can be a costly mistake.

In November 1987, the National Roofing Contractors and the Midwest Roofing Contractors Association issued a joint technical bulletin on in-service R-values for polyisocyanurate and polyurethane foam roofing insulation boards. In that bulletin the associations recommended that designers and users use an R-value of 5.6 per inch for polyiso and urethane products, not the higher value often referenced by manufacturers' literature.

In November 1992, NRCA issued another bulletin in which it reaffirmed recommending an in-service R-value of 5.6 per inch of foam thickness for HCFC-blown polyisocyanurate board products.

In the manufacturing process for polyisocyanurate, blowing agents defuse from the cellular structure of the foam for many years after manufacture. The initial loss of blowing agent has been associated with a loss of R-value. This may be referred to as short-term thermal aging. This loss of blowing agent continues for many years after manufacture. These gases are replaced by air which has a lower thermal resistance than the initial blowing agent. This phenomenon has a significant influence on the long term thermal performance of the insulation material.

The paramount issue to consider is the long-term thermal performance (10-15 years), not the short-term. Often manufacturers of polyisocyanurate insulations report their R-values after 180 days, a short-term loss period only. Although polyisocyanurate producers may be reporting 6-month aged R-values in accordance with federal regulations, the R-value of these products continues to decrease for years. This is not the case with EPS thermal insulation.

Ask your EPS supplier about R-value warranties available with their products.

What is cost per "R"?

An effective approach to a specifier is a comparison of dollar cost per unit of resistance (R).

For example, Product "A" may cost $.10 per board foot and provide an "R" of 4. Its cost per unit of resistance is then $.10/4 or $.025.

Product "B" may cost $.24 per board foot but provide a higher "R", say 6 units. Its cost per unit of resistance is then $.24/6 or $.04.

Conversely, Product "A" delivers 40 units of "R" per dollar of cost; whereas Product "B" only provides 25 units of "R" per dollar.

Make sure you get what you pay for over the entire life of the project.