Bruce Harley's book "Insulate and Weatherize" is filled with lots of great information. On the issue of vapor barriers and climate differences, however, there are things that are confusing to me. I am hoping someone here can help me make sense of all this. Early in the book he talks about how kraft paper is the best interior vapor barrier to use in mixed humid climates and in cold climates in homes with central A/C. Mixed humid climates include Kentucky, which is where I am. He adds that polyethylene sheeting should only be used in the cold climates. This all makes sense to me, since in a mixed climate like Kentucky it is important for a wall to be able to dry to the interior OR the exterior.
Now, here is where I get confused. Later in the book he discusses the use of foam board in remodeling situations. He states that, "Extruded polystyrene and foil-faced polyisocyanurate board are both good choices for interior or exterior applications, regardless of the climate." Now wouldn't foil-faced polyisocyanurate (PIC) be equivalent to polyethylene sheeting, so why would it be acceptable to use in ANY climate??? This seems to contradict his remark about kraft paper being the preferred vapor barrier in mixed humid climates, and he does not appear to offer an explanation as to why foil-faced PIC would be an exception to this rule. I can see why extruded polystyrene (XPS) would be alright since it is somewhat permeable, but I can't figure out why something impermeable like PIC would be OK?? If polyethylene sheeting is not a good idea in a mixed humid climate, then why would something similarly impermeable like foil-faced PIC be OK?
Can someone shed some light on this?
Now, here is where I get confused. Later in the book he discusses the use of foam board in remodeling situations. He states that, "Extruded polystyrene and foil-faced polyisocyanurate board are both good choices for interior or exterior applications, regardless of the climate." Now wouldn't foil-faced polyisocyanurate (PIC) be equivalent to polyethylene sheeting, so why would it be acceptable to use in ANY climate??? This seems to contradict his remark about kraft paper being the preferred vapor barrier in mixed humid climates, and he does not appear to offer an explanation as to why foil-faced PIC would be an exception to this rule. I can see why extruded polystyrene (XPS) would be alright since it is somewhat permeable, but I can't figure out why something impermeable like PIC would be OK?? If polyethylene sheeting is not a good idea in a mixed humid climate, then why would something similarly impermeable like foil-faced PIC be OK?
Can someone shed some light on this?