Exception 3 occurs in locations where the local environmental conditions do not support a recommendation for vapor retarders. These locations, in general, have high outside temperature and humidity levels during a significant portion of the year. These locations may be found along the Southern Coast (SC, GA, FL, AL, MS, LA, TX), in parts of Arkansas and Oklahoma, and in Puerto Rico and Hawaii. See the attached map for the locations specified for Exception 3.

One practice for humid and slightly cooler "fringe" areas is to use either an interior or exterior side vapor retarder with moderate permeance such as stapled or unstapled kraft facing with a permeance of about 1 perm.

Check local practices and code regulations for the specific area.

V. MATERIALS THAT ARE VAPOR RETARDERS
Many insulation products are faced with an asphalt-impregnated kraft paper or a foil laminate. Each of these facings is a vapor retarder. Other materials such as polyethylene sheet or foil backed gypsum board are also vapor retarders that are typically used with unfaced insulation.

Important: Many standard insulation facings will burn and must not be left exposed in an occupied building. Standard facings must be covered with gypsum board or another code approved interior finish. Use only flame resistant facings for exposed applications. See ICAA Technical Bulletin No. 27 and No. 28 for details.

Any material that has a perm rating of 1 or less is considered to be a vapor retarder. The following table shows the perm rating of some common building materials whose values are consistent with ASHRAE Handbook of Fundamentals and other industry sources.

Vapor Retarders	Perm Rating
Insulation Facing, Kraft	1.0
1/4" Plywood (douglas fir, exterior glue)	0.7
Insulation Facing, Foil Kraft Laminate	.5
Vapor Retarder Latex Paint, 0.0031" thick	0.45
0.002" Polyethylene Sheet	.16
0.004" Polyethylene Sheet	0.08
0.006" Polyethylene Sheet	.06
Aluminum Foil 0.00035" thick	0.05
Aluminum Foil 0.001" thick	0.0