The International Residential Code does not require or prohibit the use of vapor retarders in climate zones 1,2, 3 and 4. NAIMA recommends using either a Class II or III vapor retarder in these warmer climate zones and not using Class I (very low perm) vapor retarders. Kraftfaced batts can be installed in all climate zones.

In the warmer climate zones installing vapor retarder with a very low perm rating on the interior of a wall assembly can lead to moisture problems. Even vinyl wall paper,which has a low perm rating,can induce moisture problems in warm humid climates where hot moist conditions tend to drive moisture into the wall from the outside of the building.

In very warm humid climates if a vapor retarder is used,NAIMA recommends installing it to the exterior side of the wall.