Rigid gas-filled closed-cell foam insulations include all cellular plastic insulations which rely on a blowing agent (or gas), other than air, for 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. As time passes, the relative concentrations of these gases change due primarily to diffusion. This results in a general reduction of the thermal resistance of the foam due to an increase in the thermal conductivity of the resultant cell gas mixture. These phenomena are typically referred to as foam aging.

For some rigid gas-filled closed-cell foam insulation products produced using blowing agent gases that diffuse very rapidly out of the full-thickness foam product, such as expanded polystyrene, there is no need to accelerate the aging process.

5.1.2 Physical gas diffusion phenomena occur in three dimensions. The one-dimensional form of the diffusion equations used in the development of this practice are valid only for planar geometries, that is, for specimens that have parallel faces and where the thickness is much smaller than the width and much smaller than the length.

Note 2-Please see Appendix X3 for a discussion of the theory of accelerated aging via thin slicing.

Note 3-Theoretical and experimental evaluations of the aging of insulation in radial forms, such as pipe insulation, have been made. (6) However, these practices have not evolved to the point of inclusion in the test standard.

The change in thermal resistance due to the phenomena described in 5.1 usually occurs over an extended period of time. Information regarding changes in the thermal resistance of these materials as a function of time is required in a shorter period of time so that decisions regarding formulations, production, and comparisons with other materials can be made.

Specifications C578, C591, C1029, C1126 and C1289 on rigid closed-cell foams measure thermal resistance after conditioning at 23 ± 1°C (73 ± 2°F) for 180 ± 5 days from the time of manufacture or at 60 ± 1°C (140 ± 2°F) for 90 days. This conditioning can be used for comparative purposes, but is not sufficient to describe long-term thermal resistance. This requirement demonstrates the importance of the aging phenomena within this class of products.

The Prescriptive Method in Part A provides long-term thermal resistance values on a consistent basis for a variety of purposes, including product evaluation, specifications, or product comparisons. The consistent basis for these purposes is provided by a series of specific procedural constraints, which are not required in the Research Method described in Part B. The values produced by the Prescriptive Method correspond to the thermal resistance at an age of five years, which corresponds closely to the average thermal resistance over a 15-year service life (7, 8).

It is recommended that any material standard that refers to C1303 to provide a product rating for long-term thermal resistance specify the Part A Test Method of C1303.

The Research Method in Part B provides a relationship between thermal conductivity, age, and product thickness. The calculation methods given in Part B can be used to predict the resistance at any specific point in time as well as the average resistance over a specific time period.

Note 4-The 5-year aged values produced in Part A can be derived from the Part B data only if all other Part A requirements are met.

This test method addresses three separate elements relating to the aging of rigid closed-cell plastic foams.

Specimen Preparation-Techniques for the preparation of thin flat specimens, including their extraction from the "as manufactured" product, and the measurement of specimen thickness are discussed.

Measurement of the Thermal Resistance-Thermal resistance measurements, taken at scheduled times, are an integral part of the test method.

Interpretation of Data-Procedures are included to properly apply the theory and techniques to achieve the desired goals.