Appreciate the input, Ed. I'm renovating a Victorian house, and framing inside of the rock basement wall with 2x4's, leaving enough space behind the wall for the 4" pipe. PVC seems to work good because of its rigidity and smooth interior, and it just looks good framed into the wall....I just read a posting stating that gas dryers (which mine is) should have no plastic vent pipe, but that may have been referring to the plastic flex tubing. How about fiberglass insulation next to the pvc - fire hazard? Most gas venting pipes need to be double wall or have a air space of 1", but I can't seem to find any info on gas dryers.