Vapor barrier

What the test measures

A vapor barrier, or vapor retarder, is a layer that slows moisture moving up from a slab or a crawl space into the floor. It is not a reading but a material, chosen by how much vapor it lets through. NWFA’s installation guidelines class it by permeance:

Class I

“Class I vapor retarder ≤0.1 perm.”¹

Class II

“Class II vapor retarder >0.1 perm and ≤1.0 perm.”¹

Class III

“Class III vapor retarder >1.0 perm.”¹

Over concrete, whether one is needed often turns on a moisture test; see Concrete moisture test: the RH probe (ASTM F2170) and Calcium chloride test (ASTM F1869). What the floor’s own document asks for is in the table under Red flags.

What a valid report looks like

A complete record names, at minimum:

• the moisture test result that the choice rests on, where the floor goes over concrete;
• the material used, by product name, with its class, permeance or thickness as its maker states it;
• where it went: over the slab, under a wood subfloor, or over the crawl space ground;
• how the seams were overlapped and sealed;
• the product document it was chosen against.

NWFA’s installation guidelines also say when each class is called for. Where a record says what was used, these are the points to hold it against, quoted from NWFA:

Concrete, by test result

“Install a Class I impermeable vapor retarder when calcium chloride readings are greater than 3 pounds, relative humidity readings are greater than 80%, or calcium carbide readings are greater than 2.5%.”¹

Concrete on and below grade

“In on- and below-grade applications, due to the ever-changing moisture variability with a concrete slab, and the likelihood of sub-slab moisture barrier degradation over time, a Class I impermeable vapor retarder is always recommended.”¹

Sheet materials

“Class I underlayment sheet-good vapor retarders may include 6-mil polyethylene film or other premium polymer sheet goods, closed-cell foam pads, or peel-and-stick membranes.”¹

Liquid-applied materials

“Class I liquid-applied vapor retarders may include rolled- troweled- or sprayed-on sealers, epoxies, or urethanes.”¹

Wood subfloor over an unconditioned space

“A Class II vapor retarder (sheet-good or liquid-applied) may be used on wood subfloors over unconditioned spaces to slow the rate at which potential moisture-laden air moves through the assembly and into the wood flooring, unless otherwise directed by the flooring manufacturer.”¹

This is NWFA’s guidance, not the floor document’s; where your product’s document names a barrier, that document applies.

Red flags

1. Concrete: a moisture result above NWFA’s figures, and no Class I retarder named. NWFA’s sentence on when Class I is called for is the first point under What a valid report looks like.

2. Any floor type: the record names a barrier but not the product document it was chosen against. The documents read ask for different materials, placed differently; the table lists them. The limit that binds you is in your product's installation document.

3. Laminate: the seams were not overlapped as the product’s document asks. One laminate guide, Pergo’s, gives the overlap with the material:

   On concrete subfloors, use a non-recycled 6-mil. (.15mm) non-recycled (100% virgin) resin polyethylene film as a vapor barrier, overlapping edges 8", or follow the directions for installation of an equivalent vapor barrier.

   Pergo Installation Essentials Guide (Mohawk), PDF p. 2¹⁴

4. Laminate: the barrier on the record does not match what the product’s document prints. A record should follow the document’s own words. Swiss Krono’s installation FAQ prints its requirement this way, quoted as printed:

   Swiss Krono specifies that the vapor barrier for its laminate floors must be 6mm thick and made of virgin polyester.

   Swiss Krono USA installation FAQ, web page¹⁷

   Where the record and the document’s words differ, ask the installer what was used and which page of the document it rests on.

Questions to ask the installer

1. Which product’s installation document did you follow for the vapor barrier, and what does it ask for?
2. What material did you use, and what is its class, permeance or thickness?
3. Over concrete: which moisture test result did you base the choice on?
4. Where exactly did it go, and how were the seams overlapped and sealed?
5. Over a crawl space: is the ground covered, and with what?