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Thermal resistance and polyiso insulation

There's a new method for determining the long-term thermal performance of polyisocyanurate insulation

by John Clinton

Almost since the introduction of polyurethane and polyisocyanurate foam insulation, end users and design communities have demanded R-values that more accurately describe long-term thermal resistance (LTTR). During the past several decades, polyisocyanurate rigid foam insulation manufacturers have sponsored research projects resulting in consensus laboratory methods that can be used to determine the design LTTRs of permeably faced plastic insulating foams typically used as roof insulation.

Determination of Long-Term Thermal Resistance of Closed-Cell Insulating Foams," polyisocyanurate foam insulation producers will be able to provide long-term design R-values for those who require them. In addition, formulation improvements can be evaluated in a reasonable time period, providing manufacturers a new tool for product development.

What is LTTR?

LTTR forecasts the future thermal insulation performance of closed-cell plastic foams. There are at least two methods for calculating LTTR that rely on a technique called "slicing and scaling." Thin (between 1/4- and 1/2-inch-[6- and 13-mm-] thick) slices of foam are taken from standard production boards, and the slices' R-values are measured after a set time period. Because the slices are thin, the foam experiences accelerated aging and a design LTTR value can be determined.

The use of an LTTR value provides the following advantages compared with using values determined after an arbitrary fixed conditioning period:
  • The thin slices are taken from current production foam samples. Prior methods used samples that were at least three months old as a requirement; some were six months old.
  • Determining an LTTR value is fairly rapid and, depending on a slice's thickness, can produce an LTTR design value for 2-inch- (51-mm-) thick polyisocyanurate foam in about 90 days.
  • A formula is used to determine the aging time period for a particular thickness. Previously, the same conditioning period was used for products of all thicknesses.
The methods

The first method that can be used to determine a long-term design value is ASTM C1303-00,"Standard Test Method for Estimating the Long-Term Change in the Thermal Resistance of Unfaced Closed Cell Plastic Foams by Slicing and Scaling Under Controlled Laboratory Conditions."

ASTM C1303 was, in part, a result of a research project at the Oak Ridge National Laboratory (ORNL), Oak Ridge, Tenn., that was co-funded by the U.S. Environmental Protection Agency, Department of Energy, Polyisocyanurate Insulation Manufacturers Association (PIMA), NRCA and Society of the Plastics Industry. The project began in 1988 and ended in 1993.

Although the project's primary intent was to assess the performance of polyisocyanurate foam produced with chlorofluorocarbon substitutes that have reduced ozone depletion potential, the test roof structure constructed for the research program at ORNL was fitted with thermocouples to measure in-place thermal performance. These values were compared with retained foam samples held under laboratory conditions. An accelerated aging method was developed by ORNL investigators and validated with the same polyisocyanurate used in the project. This method became an ASTM standard in 1995. The method is accurate but quite complicated to perform and limited to unfaced, homogeneous materials. Because of these factors, it rarely has been used.

Further work in Canada led to the development of CAN/ULC S770. Although based on the same thin-slicing and accelerated-aging concept as ASTM C1303, this method accounts for the effect of permeable facings, or skins, on the long-term thermal resistance of foam insulation in addition to a number of other factors. Considered to be a prescriptive way to perform ASTM C1303 (a more narrowly defined procedure within the bounds described in the ASTM standard), CAN/ULC S770 predicts what the foam's R-value will be after a five-year aging period.

The Canadian method further established that foam's R-value at the five-year point is equivalent to the average performance of a closed-cell foam insulation product during a service life of 15 years. In Canada, this method is used to establish the design R-value.

The evolution of test methods

Reaching the current test methods has been a long process. In the early days of the rigid polyurethane foam industry (1950s and early 1960s), manufacturers measured foam's thermal conductivity (k factor or k value) soon after foam was manufactured. At that time, most foam was molded into cavities surfaced with thick metal or plastic and used in end products such as refrigerators. In these cases, any aging was reduced or eliminated.

The pace of change in the rigid polyurethane foam industry was rapid, and if materials were retained to measure long-term conditioned k values, it was likely the foam recipes had changed. In many cases, foam's insulating materials were used in such a way that it was difficult to cut and retain samples that would be representative of "aged" materials in the actual use condition.

In 1981, The Roof Insulation Committee of the Thermal Insulation Manufacturers Association published a standard practice (RIC/TIMA 281-1) for manufacturers to follow when measuring k values or quoting R-values of finished products. Before measuring the thermal performance of a closed-cell foam product, a manufacturer was directed to condition the product for 180 days at 75 F (24 C). The value measured then could be advertised as the R-value after conditioning. PIMA members, those companies that manufacture polyisocyanurate rigid foam roof insulation board, currently use this same method and call it PIMA 101.

LTTR and polyiso

Because of its close involvement with these testing developments, the polyisocyanurate industry has taken a step in beginning the transition to LTTR values. At the most recent meeting of ASTM C16 Committee on Thermal Insulation, an LTTR method was proposed as a revision to ASTM C1289, "Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board."

This method, more practical and representative of actual products, is based on CAN/ULC S770 and is a prescriptive version of ASTM C1303. Once the balloting is completed on this proposed revision, PIMA will begin an educational initiative geared toward the proper use of LTTR values for users, designers and regulators.

Additional outreach is planned with several government agencies and recognized organizations (The American Institute of Architects, Construction Specification Institute and Roof Consultants Institute) that currently rely on PIMA 101.

The bottom line

For at least 35 years, the long-term thermal resistance of polyisocyanurate rigid foam insulation has been studied and debated. Although much research has been conducted and RIC/TIMA 281 and PIMA 101 have been accepted as test methods, the question remains: What is the best way to measure the long-term thermal performance of polyisocyanurate? For permeably faced polyisocyanurate, the industry now has a definitive answer with LTTR.

John Clinton is vice president of Intech Consulting Inc., Kennebunkport, Maine.

Copyright 2004 National Roofing Contractors Association