In January's article on radiant heating, Paul Hanke discussed the use of polybutylene (PB) plastic, cross-linked polyethelene (PEX) plastic and EPDM rubber tubing as alternatives to copper pipe in hydronic radiant slabs. As he mentioned, each of these products is susceptible to the phenomenon of "oxygen diffusion,'' which can cause corrosion elsewhere in the system. This month, Paul expands on the issue with this special, in-depth report. He explains how the problem arises, its extent, and what can be done about it during the design and installation of radiant heating systems. Most readers probably are aware that water in the form of gas (i.e., vapor) can pass through the solid walls of a house. It