Do you need a vapor barrier under a concrete basement floor?

Do you need a vapor barrier under a concrete basement floor?

For most new heated spaces, the model code says yes. The International Residential Code (IRC) section R506.2.3 puts a vapor retarder “between the concrete floor slab and the base course or the prepared subgrade.” It sits under the slab, in the gravel or soil layer, and goes in before the pour.

Two things change the answer: which IRC edition your local building department enforces, and whether your slab falls under an exception. Ask your city or county permit office. Do not assume.

What do St. Louis City and County require?

Both follow an older edition than 2021, and both name 6-mil polyethylene. The City of St. Louis lists the “2018 International Residential Code Ordinance 70795” among its adopted codes. St. Louis County lists a “Residential Building Code (CH. 1116 - 2015 IRC),” and its building code history says the ordinance “came into effect on April 1, 2020.”

The County says that if your property is in a municipality, you should check its Municipal Matrix, because many towns set their own codes. Both the City and the County adopt the model code with amendments, so confirm the slab rule with the building office for your address.

What do the 2018 and 2021 IRC say?

Both require a retarder with joints lapped at least 6 inches. The thickness changed.

Stego Industries, a vapor barrier maker, writes that under the newer code retarders “must have a minimum thickness of 10-Mil.” Stego sells these products, so it has a stake. The code text itself matches what Stego says.

The 2015 IRC text matches the 2018 text on this point: 6-mil polyethylene or an approved vapor retarder, lapped at least 6 inches. The model code only becomes law when a jurisdiction adopts it. Ask which edition your building department enforces, and whether it has local amendments.

Is a vapor barrier the same as a vapor retarder?

Close, but not identical. Americover, which sells plastic sheeting, writes: “A vapor barrier is a type of vapor retarder with a vapor permeance rating of 0.1 perms or less.” The code says “retarder.” Many people say “barrier.” Ask your contractor for the product name and its perm rating, not only the thickness.

Which slabs do not need a vapor retarder?

Both editions list exceptions. In summary, the retarder is not required for:

• Garages, utility buildings and other unheated accessory structures.
• Unheated storage rooms under 70 square feet, and carports.
• Driveways, walks, patios and other flatwork not likely to be enclosed and heated later.
• Cases the building official approves based on local site conditions.

Read the storage-room item closely. It covers only unheated rooms under 70 square feet. A basement you plan to finish and heat is not that. If you want a finished basement, ask the building department before you assume an exception.

Why does the barrier matter for the floor on top?

Water vapor moves up through concrete. If flooring or a coating seals the top, that vapor has nowhere to go. AES says its liquid moisture barrier “will prevent vapor from penetrating the flooring reducing the risk for future problems and mold growth.” Mold is the long-term concern. Read about mold in a basement and the mold after water damage guide for what that looks like.

Wagner Meters, a moisture testing company, warns that sealers can end up “trapping water vapor inside the slab.” That applies to coatings and polished finishes as well. See how long concrete should cure before polishing and how long an epoxy floor takes to cure. Our concrete polishing page covers that side of the work, and the polished concrete and epoxy floor coating entries explain the terms. If moisture has already caused mold, see the mold remediation entry.

What if your existing slab has no barrier?

You cannot slide a sheet under a poured slab. You have three practical choices:

1. Test first. Find out whether moisture is actually a problem before you spend money.
2. Use a liquid moisture barrier on top of the slab, before the flooring.
3. Pick a floor that tolerates moisture, and accept the risk.

AES installs “2-component, liquid epoxy coating moisture barriers developed to be applied over concrete floors.” The AES page lists these features: it can treat 25 lb. MVER concrete at 100% humidity, reduces vapor transmission “to near zero,” has zero VOC, and is compatible with most floor coverings. A manufacturer’s 10 year guarantee is available. AES says the barrier can be applied under sheet vinyl, tile, carpet and hardwood.

AES also says the product can go on dry or damp surfaces, “even 5-day old concrete.” That helps on fast-track jobs where you cannot wait for the slab to cure.

Why is a retarder under the slab different from a barrier on top?

They solve the same problem from opposite sides. A retarder under the slab slows ground moisture before it enters the concrete. A liquid barrier on top slows vapor already in the slab before it reaches the flooring. A new slab can have the first. An existing slab can only get the second. Neither one replaces a moisture test, and neither makes a leaking wall or a bad gutter go away. Fix water sources first. Our post on whether all basements have mold explains why damp basements grow mold.

How do you test a basement slab for moisture?

AES tests the slab before it installs a barrier. It offers “calcium chloride testing and relative humidity testing” to measure the vapor transmission rate and decide whether a barrier is needed. After testing, AES says the surface should be clean, sound, solid and absorptive. Cleaning can be done with a pressure washer or damp mop, then a thorough vacuum.

Wagner explains that calcium chloride tests measure only moisture leaving the top of the slab. In-slab relative humidity sensors give a fuller picture. Ask which test your installer uses and what number they need to see.

What should you do in the St. Louis area before you pour or finish a basement?

Follow these steps:

1. Call the building department for your address. In the City of St. Louis that is the Building Division. In the County, check whether your town contracts with the County or has its own office. Ask which IRC edition applies and whether a vapor retarder inspection is part of the permit.
2. For a new slab, ask the contractor to show the retarder, its thickness and the lapped seams before the pour.
3. For an existing slab, get a moisture test before you choose flooring.
4. If the test is high, ask for a liquid barrier quote and read the guarantee.
5. If you find mold or a musty smell, deal with that before covering the floor. AES offers mold remediation and mold inspection.

AES quotes each job and does not publish a price for moisture barriers. Call (314) 865-2662 or use the quote form.

Next step

Advanced Environmental Services installs moisture barriers in St. Louis, St. Charles and Columbia, Missouri, and takes projects within 300 miles. Call (314) 865-2662 or use the quote form to have your slab tested. Read the moisture barrier entry for the term, and see our epoxy floor coatings page if you plan to coat the floor.