It started with a soft spot.
Mark and Jessica had just moved into their charming fixer-upper—a cozy ranch built in the early ‘80s with a concrete slab foundation. Wanting to give it a modern touch, they ripped up the stained carpet and installed luxury vinyl plank flooring across the entire main level. The floors looked gorgeous—warm, wood-like, and easy to clean.
But they skipped one step: the vapor barrier.
“I read on a forum that LVP is waterproof,” Mark said. “I figured that meant we didn’t need the plastic sheet underneath. Plus, it was one less expense.”
For the first few months, everything seemed fine. Then came the rainy season.
The living room floor started to lift. At first, it looked like air bubbles. Then the planks began to curl. Some clicked apart. The worst spot? The center of the room—right over a cold, damp patch of concrete.
A local flooring expert came to inspect. He peeled up a plank and immediately smelled mildew. A moisture meter told the rest of the story: the concrete was sweating from hydrostatic pressure. Without a vapor barrier, the moisture had nowhere to go but up—into the LVP and into the air.
Mold spores were found underneath. The entire floor had to be ripped up, cleaned, and redone—with a vapor barrier this time. The cost? Over $7,000 in repairs.
The worst part? The LVP warranty was voided because the product had been installed directly over concrete without a moisture barrier for flooring —clearly written in the fine print.
Why Moisture Barriers Matter
Concrete, even when dry to the touch, emits vapor. Over time, this invisible moisture can wreak havoc on your flooring, especially in:
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Basements
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Ground-level slabs
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Regions with high humidity or rainfall
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Homes without a proper vapor retarder beneath the slab
“Even a newly poured slab can emit 8 to 10 pounds of moisture per 1,000 square feet per day,” says Timothy Grant, a certified flooring inspector with 20+ years in the industry. “And in older homes, slabs are often unsealed. If you don’t put a barrier down, you’re just inviting problems.”
Moisture vapor emissions are measured in lbs/1,000 sq. ft./24 hrs, and most LVP manufacturers require the rate to be below 3 lbs. According to the ASTM F1869 standard, anything above that can lead to adhesive breakdown, mold growth, and plank deformation.
When You Might Not Need a Moisture Barrier
There are a few scenarios where a vapor barrier underlayment may not be necessary:
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Wood Subfloors: If you're installing LVP over plywood or OSB above grade (e.g., on the second floor), you typically do not need a vapor barrier. In fact, one can trap moisture and cause mold in the wood itself.
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Concrete with Proven Moisture Control:
“If a builder used a poly vapor retarder beneath the slab—and you verify moisture levels with an RH or calcium chloride test—you might be safe,” says Emily Roth, a technical rep from a national LVP brand.
Most manufacturers require:-
RH ≤ 75% (ASTM F2170)
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MVER ≤ 3 lbs (ASTM F1869)
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LVP with Built-In Underlayment & Barrier Layer:
Some high-end click-lock LVPs feature an attached pad with a built-in vapor barrier.“Even then, double-check the specs,” Emily adds. “Not all ‘underlayments’ include a vapor barrier layer, and some only work on wood subfloors.”
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Dry Interior Conditions in Arid Climates:
If you live in a desert climate and have a well-tested, sealed slab, some installers will skip the barrier. But this is a calculated risk.
The Bottom Line
“Skipping the vapor barrier is like going without insurance,” says Grant. “You might get away with it—until you don’t. And then you’re paying thousands.”
The vapor barrier is one of the cheapest parts of your floor—costing as little as $0.05–$0.15 per square foot—and potentially the most critical. Don’t risk your investment, your indoor air quality, or your warranty by skipping it.
If you’re unsure, test your subfloor, read the manufacturer's guidelines, and when in doubt—install the barrier.
Because the only thing worse than a ruined floor… is paying to do it all over again.