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Understanding Perm Rating

So many people stumble across this term “Perm Rating” and struggle with understanding what it really means. Some companies are completely dishonest with their vapor barrier’s performance ratings, but we will get into that in a little while. Perm rating is a number assigned to a product that denotes the amount of moisture that will pass through the product in a set amount of time. Perm ratings are shown in both US and metric units. It becomes clear that a lower per rating is better. The metric unit in perm ratings is smaller than the US unit for per ratings. Pure virgin polyethylene resin has nearly the same perm rating for the same thickness of plastic film from one product to another. The one exception to this statement is the labor-intensive process of laminating multiple layers (more than 6) in a plywood style stacking to get a lower perm rating. This is a product that has a very specific use. It is not a general-purpose plastic and it is not priced to compete with that kind of plastic. The truth is, there is no super vapor barrier with perm ratings that start with a period and is followed by five zeros before you see an actual number greater than zero.

There is no polyethylene vapor barrier on the market today that has a zero perm rating without a layer of metal. The metal is actually what would give that barrier a zero perm rating. There are products on the market made this way. The cost, as you can imagine is not competitive with vapor barriers made without a layer of metal.

Chemical additives to a polyethylene vapor barrier such as fire retardant, antimicrobial and antistatic additives raise the products perm rating which decreases its performance as a moisture barrier and its ability to last over time. Vapor barriers with these chemicals added to it also have a very specific use. For example, all plastic film used in a commercial setting such as a renovation must have a fire retardant in it according to safety regulations. The fire retardant does not make the plastic fireproof. It only suppresses the flame, slowing the excitement of the fire. There is a trade-off there, the more fire 

retardant, the more smoke. The fire will burn you, but the smoke will suffocate you. So the next time you’re in the market to purchase a vapor barrier, consider where you are going to install it and if it is truly necessary to have a fire retardant chemical additive in the vapor barrier.

Did you know that the chemicals that make plastic antimicrobial are highly hazardous in its raw form? In fact, the EPA requires any facility using antimicrobial in a plastic to not only test the air regularly, but test the health of all the employees regularly in order to keep their certification. Did you also know that pure virgin polyethylene naturally resists mold and mildew growth. An antimicrobial additive is absolutely not necessary. The slick marketing trick is to let you believe that since there is an antimicrobial in the plastic that you will never have mold growth in your home.

Perm ratings do not override the quality of workmanship used to install the vapor barrier. In other words, if a contractor installs a “Zero Perm” crawl space vapor barrier or any other vapor barrier for that matter and does not tape the seams it will hardly control 100% of the moisture. With that said, even if they do use a tape to seal the seams, that tape must be “Zero Perm” as well and the workmanship must be flawless. Then, of course the area that the vapor barrier is terminated must also be sealed with a “Zero Perm” product. The truth is, a great installation of a poor quality product will give far better results than a poor installation of a superior product. Just understand that using a poor quality product will give a short service life.

Perm ratings (for US and Metric) are measured for results in a 24 hour period. One marketing trick used by a very will known yellow vapor barrier is to divide the perm rating by 24 (hours in the day) and present a perm rating by the hour. So it looks like this; actual perm rating (24 hours) .053 perms, but they present their perm rating as 0.0022 perms. That is very impressive when the only thing the shopper knows about perm ratings is a lower number is better.