This article is a very good source of information for all.
Also available for Continued Education.
http://archrecord.construction.com/resources/conteduc/archives/0601duponttyvek-1.asp
I listed a few of the building wraps available and their performance specification for comparison.
Perm ratings differ by quite a bit.
Permeability, rated in perms, is a measure of the transference of grains of water through a material at a standard vapor pressure and temperature. Vapor retarders have permeability ratings of 1.0 or lower. Asphalt felt paper (tar paper](http://www.answers.com/topic/tar-paper-1)) has a permeability rating of 5, so it is inappropriate as a vapor retarder.
I have provided these different products and their specifications for comparisons.
Product Name: Tyvec
Water Vapor Trans. Rate
58 perms
Air Leakage Rate (Air-Ins)
0.007 cfm/sq ft @ 75 pascals
Typar
Water Vapor Trans. Rate
13.7 perms
Air Leakage Rate (Air-Ins)
0.0023 L/S-sq m @ 75 pascals
ProWrap
Water Vapor Trans. Rate
ASTM E 96, Procedure A: 125 g/24 h•m2
Air Leakage Rate (Air-Ins)
ASTM E283: 0.017 CFM
GreenGuard **Water Vapor Permeance**
ASTM E 96, Procedure A: 105 g/m2/24 hrs (@75ºF/50% RH)
Air Leakage Rate (Air-Ins)
ASTM E 283: 0.03 cfm/sq ft (@ 75 pascals/25 mph)
Prime Wrap
Water Vapor Permeance
ASTM E-96-A: 5-15
Air Leakage Rate (Air-Ins)
ASTM E283: 0.017 CFM
Valeron Vortec **Water Vapor Trans. Rate**
-
ASTM E-96A: 50 gms/m2/24 hrs @ 50% RH
-
ASTM E-96A: 6.5 perms
Air Porosity (Gurley Hill)
TAPPI T-460: 8.1 sec/100 ccGreen Guard RainDrop **Water Vapor Trans. Rate**
ASTM E 96, Procedure A: 10 perms (@75ºF/50% RH)
[size=3][FONT=Times New Roman]Water Vapor Permeance[/size]
ASTM E 96, Procedure A: 73 g/m2/24 hrs (@75ºF/50% RH)
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Look at the difference in the Product Performance and make your own decision as to which is better.
Hope some enjoy.
Marcel