We can use life-cycle inventories to compare the environmental performance of two systems performing identical functions, but comparing two products or materials is risky business. Here is more on this subject from a report by Boustead for Plastics Europe:
All life cycle inventories are concerned with describing the behaviour of industrial systems, not products; a product is simply one materials flow within the system. The system itself is identified in terms of its function. Consequently if any comparisons are to be attempted, then they must be confined to comparing systems that perform identical functions. This is the basis of all life cycle assessments. Despite this requirement, which has been stated many times, there are still some users of life cycle inventories who continue to compare materials on the basis of 1 kg of polyurethane versus 1 kg of some other material. Such comparisons are meaningless.
Also, when comparing systems, which perform equivalent functions, it is important to compare the data for the whole life cycle and not just data for those elements of the life cycle that differ. Comparison of parts of systems can lead to misleading conclusions. For example, if polyurethane foam is used to insulate a house, the value of the exercise is demonstrated by a comparison of the system describing the un-insulated house with that describing the insulated house. Essentially the investment of a small amount of energy and feedstock in the production of the foam insulant leads to a saving of at least 100 times this initial investment in heating fuels over the lifetime of the house. The investment of 1 unit of energy results in a saving of 99 units. If a different type of insulant is used with a production energy that is 25% greater than the polyurethane, the net energy saving will be 98.75 units for every 1.25 units of initial investment.
Such a comparison indicates that the important feature is to install insulation materials of any type to achieve the saving of energy. Comparing the insulation materials on the basis of their production energies is a marginal exercise of little value since the difference in production energies of different materials is trivial compared with the ultimate saving.