We as trades professionals (yes, we are) are put upon by a multitude of products, names, configurations, and trademarked items. Specifically, for home inspectors, our goal is to identify and/or explain many of these to our clients. So, being as consumer oriented as I tend to be (read the back issues), we are failing our clients because we often misuse terminology.
Let’s look at two common terms: vapor retarder and vapor barrier—I could make every column about misused words and concepts, there are so many.
It drives me bonkers when I hear other inspectors (or read their reports) talk about vapor barriers in a crawlspace. If you live in a region of our country that doesn’t have crawlspaces, well…good for you. I dislike you, but please feel free to continue reading for fun.
Technically, inspectors are correct in calling the plastic on the ground in a raised foundation environment a “vapor barrier.” By definition of materials, set by ASTM E96, the 6mil polyethylene in most crawlspaces is a Class I vapor barrier as its rating is around 0.03 perms on average. Now, this column isn’t intended to get into the weeds on permeance and permeability—both of which are very important concepts to understand related to our building materials and how our homes perform related to water management—but, the lower the rating, the lesser amount of water vapor (soil gases in general) diffusion occurs.
A Class II vapor retarder has a permeance between 0.1 and 1.0 perm, per definition. So, again, that plastic in most crawlspaces is a vapor barrier. However, this is highly misleading and inaccurate when it comes to crawlspace environmental context.
When inspectors write, or identify, the presence/absence/condition of a vapor barrier in a crawlspace, they are almost never intending to comment on the perm rating of the material itself. How do I know this, you ask? Because most inspectors are never taught about perm ratings, permeability, or permeance. What they are doing is identifying presence/absence of said material in the crawlspace because not commenting on such a thing could be costly in the event of a fungal growth/moisture issue in the future.
To that end, within the context of an average, passive venting crawlspace, a vapor barrier is actually a vapor retarderbecause it is unsealed and not always fully covering the ground. In order for a vapor barrier to be a vapor barrier within the context of the same crawlspace, the sheeting must be overlapped and sealed at seams and edges (the IRC clearly lays out the minimum parameters for such configurations). When properly and diligently performed, this creates an actual vapor barrier that does not retard (read, slow the entry of), but fully stops vapor intrusion within the crawlspace. This, of course, is a key component for an encapsulated crawlspace (and, for sub-slab applications where the slab is to be within conditioned space). Keeping water, in all phases, out of any crawlspace is pivotal and ideal.
Our industry should refer to the plastic on the ground in non-encapsulated crawlspaces as a vapor retarder or vapor retarding system. This way, the notion can be more clearly conveyed to the client that the presence of said plastic will not prevent moisture accrual in vapor form (there are a few other critical entry ways, as well), but will help to slow the rate of said accrual.
Our industry should refer to a vapor barrier if, and only if, the plastic on the ground has been fully sealed. Even then, commenting only on the configuration of the material isn’t enough for clients to understand the long-term ramifications of one of the most confused air spaces in our residences.
Please, stop saying vapor barrier unless it’s an actual vapor barrier by configuration, or if you are specifically addressing the characteristics of the material itself. If you are doing the latter, God’s speed to you—you will need to do that for all of the other materials in that given structure…or find yourself trying to educate a jury about why you identified the physical properties of one component, but not another one. http://cookeville.wini.com