7 Factors to Consider When it Comes to Under-Slab Vapor Barrier Placement

Vapor barrier thickness is a key attribute of a below-slab vapor barrier but it does not tell the whole story. Tune into this Stego IQ episode where Stego Technical Director Dan Marks provides some insight on all the key attributes that make up a high-performing under-slab vapor barrier which is puncture resistance, tensile strength, and most importantly permeance and longevity.

Stego IQ returns with the latest episode dispelling the myth about vapor barriers and slab curl. Dan Marks, Stego's Technical Director, returns to the screen to explain the short term slab curl effects with a vapor barrier as well as the long term effects of slab curl if you do not install a below slab vapor barrier.

For starters, is it Class I or Class 1?  Fun fact:  it’s actually Class I.  I get asked by both homeowners and contractors alike what a Class I vapor retarder is.  If you’ve been researching a DIY crawl space encapsulation project, it is possible you’ve stumbled across this reference as well.  In this blog post I’ll answer what a Class I vapor retarder is, but I’m also going to use this space to explain why a Class I vapor retarder, although referenced in code as a threshold, may not be satisfactory for your project.  To get you started on the right foot, we’ve made it easy for you to look-up the current version of code governing your jurisdiction.    

“Vapor retarder” is the term used to describe materials of varying types and classifications which impede the infiltration of water vapor into a structure. Often the term “vapor barrier” will be used interchangeably with the term “vapor retarder,” with no repercussions. But for under-slab applications, it is important to understand the distinctions between the two terms to avoid negative impacts on your project.

ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized leader in the development and delivery of international voluntary consensus standards. Today, some 12,000 ASTM standards are used around the world to improve product quality, enhance safety, facilitate market access and trade, and build consumer confidence.

ASTM (American Society for Testing and Materials) E1745 is the Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs. This standard requires products undergo conditioning testing designed to simulate in-service (under-slab and installation) conditions, and it classifies vapor retarders based on their tensile strength, puncture resistance, and water vapor permeance ratings.