The Secret Below-Slab Ingredient for Building an Energy-Efficient Home

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 (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.

My job is to talk to contractors about the materials and products they use on a work site. I ask lots of questions about their processes so I can understand how to design materials that make their labor more efficient. I often hear that timeliness and a long lasting, quality outcome are two things these contractors strive for. Believe it or not, a lot of these things can be boiled down to the kind of seaming tape they use when putting up vapor barriers in crawl spaces.

If you’re considering renovations in your crawl space or basement, it’s important to know some commonly used terms. That way, whether you’re doing the work yourself and need to pick up some supplies at the hardware store or you want to be able to discuss the installation process with your contractor, you’ll be on solid ground (pun intended).