I am running a new dryer vent, 4" thinwall pvc, 5' up an exterior wood-frame wall, through two elbows and out a self-closing vent (all rigid PVC). For the connection to the dryer I have framed in a little "box" at the bottom of the wall for a metal 4" 90deg. duct, and I'll use a short length of straight duct to connect to the dryer. My question is if I can insulate with typical fiberglas batts against the PVC in the wall cavity. My theory is if the clothes are not catching on fire, the insulation will not either, but there may be more to this than I think. There will be no insulation on the first 1' of vent pipe located in the framed "box" by the dryer outlet. Any thoughts? Thanks.