When meeting with homeowners to discuss garage and basement floor coatings, we often find the customer is concerned that the homebuilder did not put vapor retarding material under the concrete slab. Specifically with garages, they often think the builder skipped a step because they saw the concrete poured directly onto the prepared base.
Is a vapor retarder or barrier necessary for under the concrete slab of a garage or basement?
Concrete Garage Floors & Vapor Transmission
We find ourselves having to explain that a vapor retarder isn't required or necessary for a concrete garage floor poured at or above grade.
The 2012 International Residential Code states:
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R506.2.3 Vapor Retarder. A 6-mil (0.006 inch; 152 µm) polyethylene or approved vapor retarder with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists. Exception: The vapor retarder may be omitted:
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Since the typical concrete garage floor sits upon a base of 4-inch-thick (or deeper) base course of gravel or stone, the moisture transmission through the slab is minimal. That slab is not in contact with wet soil, so the amount of vapor transmission is limited.
Concrete Basement Floors & Vapor Retarders
The basement’s concrete slab in newer construction is different. The foundation typically has a drainage system to move water away from the walls and not allow it to build up below the slab. The crushed stone or gravel base serves as a capillary break (a space between two surfaces which prevents the movement of moisture through the space by capillary action) and the sheet polyethylene acts as the vapor barrier.
The problem with the minimum basement vapor barrier (6 mils) is that it may be a material that allows too much permeance (the amount of vapor passing through) and the plastic film may get damaged/compromised during the placement of reinforcement (rebar and wire mesh) and concrete.
Newer materials from a variety of manufacturers are 10 or 15 mils thick. These meet the ACI 302.1 Guide for Concrete Floor and Slab Construction and are much less susceptible to punctures or tears. These newer materials, such as W.R. Meadows’ PERMINATOR under-slab vapor barriers also have a lower permeance.
What About Older Homes?
Many older homes have slabs that were poured directly onto the compacted earth. No stone base is there to act as a capillary break so vapor transmission is much higher. Older homes often have basements where no vapor barrier is present. In most situations where we suspect a problem, we either have to pass on quoting the job or our quote requires us to install an epoxy moisture vapor barrier (a membrane forming moisture mitigating coating which reduces the moisture vapor emission rate through the slab).
In garages where we suspect there is no stone base, we require inspection under the slab to determine if the concrete slab is, in fact, directly in contact with the earth below. This can be accomplished by drilling through the slab.