to further geek out: * If air temp will never cross the dew point, you don't need a vapor retarder. * Though the normal is "vapor retarder on the hot side", t

to further geek out: * If air temp will never cross the dew point, you don't need a vapor retarder. * Though the normal is "vapor retarder on the hot side", they can be on the outside of the wall assembly if condensed water can drain away. This is important for climates that have heavy heating and heavy cooling seasons, or for it's easier to put it on the outside.