VAPOR BARRIER vs. VAPOR RETARDER
Let’s first look at the terms used in the industry.
Is it a vapor retarder or is it a vapor barrier? This is a difficult question to answer as these terms are used interchangeably in the construction industry.
For example, within ASTM E1745: Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs, a vapor barrier is defined as
A material or construction that impedes the transmission of water vapor under specified conditions.”
It does not define the term vapor barrier.
ACI approaches this a little differently:
ACI 302.1R: Guide for Concrete Floor and Slab Construction states
A number of vapor retarder materials have been incorrectly referred to and used by designers as vapor barriers. True vapor barriers are products that have a permeance (water-vapor transmission rating) of 0.00 perms when tested in accordance with ASTM E 96.”
ACI 302.2R: Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials states
Historically, the construction industry used the term vapor barrier to describe a polyethylene-based material below a concrete slab. Polyethylene, however, does not completely stop the transmission of water vapor. These products only reduce or retard water vapor transmission. It was therefore considered more appropriate to call these products vapor retarders instead of vapor barriers.”
The same ACI committee has made a recommendation that:
If it is determined that a vapor retarder meeting the ASTM E1745 for vapor permeance requirements is not sufficient to protect the flooring material to be installed, then a vapor barrier with a perm rating of .01 or less should be specified.”
While this statement implies that a material with a 0.01 or less perm rating may be considered a vapor barrier, it does not define a vapor barrier to be a product that has a permeance of less than 0.01 perms.
Confusing so far?
In addition when we start to look at categorizing vapor retarders, materials can be separated into three general classes based on their permeance and are defined in the International Building Code (IBC):
- Class I vapor retarder: 0.1 perm or less
- Class II vapor retarder: 1.0 perm or less and greater than 0.1 perm Class III vapor retarder 10 perms or less and greater than 1.0 perm
- Anything above 10 perms is defined as a vapor permeable membrane.
So what is a vapor barrier? Many people in the industry typically defines it as being a Class I vapor retarder. However, when looking at the current International Building Code (and its derivative codes), there are no specific performance requirements when it comes to underslab vapor protection. The term vapor retarder is still used. Section 1907.1 of the 2012 IBC calls for 6-mil polyethylene or “other approved equivalent methods or materials shall be used to retard vapor transmission through the floor slab”.
So, what should be done?
We need to get away from using the term vapor barrier and vapor retarder and actually define what performance characteristics the material is required to meet based on the actual project requirements. This is worded nicely within ACI 302.1R,
The committee recommends that each proposed installation be independently evaluated as to the moisture sensitivity of subsequent floor finishes, anticipated project conditions, and the potential effects of slab curling, crusting, and cracking. The anticipated benefits and risks associated with the specified location of the vapor retarder should be reviewed with all appropriate parties before construction.”
Irregardless of the term, it is the function of the material that needs to be evaluated – to control the entry of water vapor into the building by vapor diffusion.