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The accurate estimation of the thermal performance of insulation products used in buildings over their expected lifetime  has been a recognized challenge for over 25 years. This is because the lifetime of such products is long, is caused by the diffusion of a multitude of gases, and the insulation product is not homogeneous. The task of developing a
standard test method for predicting long term thermal performance which applies to a variety of closed-cell foam products is even more complicated as diffusion processes occur at rates that depend on the type of polymer, the cellular structure, the  temperature, the gas type, and its pressure. Both classical approaches to accelerating diffusion controlled phenomena, namely aging at higher temperature and aging a thin slice, present challenges especially if a single method is desired for a variety of cellular foam products such as polyisocyanurate （polyiso） and extruded polystyrene （XPS）.

Though Europe has favored standard test methods based on aging at elevated temperature, slicing and scaling techniques have been the leading approach in North America. Lay, two prescriptive test methods, ASTM C 1303 in the USA and CAN/ULC-S770 in Canada have emerged in North America. Both are based on accelerating the foam aging process by slicing the foam into thin specimens. Both methods use the projected thermal conductivity at five years of age to represent the insulation s long term thermal resistance value （LTTR）. The two methods have many other similarities, such as use of Fickian law for one-dimensional diffusion to calculate aging period, and use of a thin slice from the core and surface areas of the foam. However, they do vary in precisely how the long term thermal resistance is calculated. The C
1303 test method prescribes that the thermal resistance value of a stack of thin slices after aging for a calculated time is the long term thermal resistance value. The S770 test method calls for multiplication of an aging factor to an initial thermal resistance value to determine the five-year value. Though the basic frameworks of the two methods are in place, the precise parameters are still being debated and balloted.

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