Short answer
Identify moisture under vinyl by looking for bubbles, cupping, lifting seams, or musty odors, then confirm with a moisture meter and by checking nearby plumbing and the subfloor. Fix it by stopping the water source, removing affected flooring, drying and disinfecting the subfloor, testing moisture to manufacturer specs, and reinstalling with proper vapor control. If mold covers more than ~10 sq ft or the slab won’t meet moisture limits, call a pro.
Why moisture builds up under vinyl
Vinyl (LVP/LVT or sheet) is relatively watertight on top, so spills, plumbing leaks, or concrete vapor can get trapped beneath. Common sources:
- Leaky toilet wax ring, dishwasher, fridge ice-maker line, or sink supply/drain
- Foundation or slab moisture vapor emission (especially basements)
- Condensation in high-humidity rooms
- Roof or wall leaks running down into base areas
How to diagnose (step-by-step)
- Visual and smell check
- Look for bubbles, tenting, soft spots, dark edges, or white haze under clear wear layers.
- Musty smell along baseboards or transitions is a red flag.
- Check plumbing and appliances
- Inspect around toilets, tubs, sinks, dishwashers, fridges, and laundry. Look for drips, corrosion, or loose connections.
- Measure moisture
- Use a pin or pinless moisture meter on the subfloor (wood) or concrete. A hygrometer helps monitor room RH.
- For concrete, consider a slab test:
- Calcium chloride (ASTM F1869) or in-situ RH probes (ASTM F2170).
Typical acceptance (verify your flooring/adhesive):
- Wood subfloor MC: ≤ 12–14%
- Concrete RH (F2170): ≤ 75–85%
- MVER (F1869): ≤ 3–5 lb/1000 sq ft/24 hr
- Access a hidden spot
- Pop a threshold/transition or remove a baseboard and lift a piece to see the underside. For click LVP, unlock a few planks from a doorway. For sheet vinyl/glue-down, carefully cut a small test flap in a closet area.
- Map the wet area
- Mark boundaries with painter’s tape. This guides how much to remove.
Time: 1–3 hours. Cost: moisture meter $30–$200; slab test kits $15–$25 each.
Tools and materials
- Moisture meter (pin/pinless), hygrometer, infrared thermometer (optional)
- Pry bar, oscillating multi-tool, tapping block/puller for click LVP
- Utility knife, floor scraper, 100-lb floor roller (for glue-down reinstall)
- Dehumidifier and 1–2 air movers; wet/dry vac
- PPE: gloves, eye protection, N95 or P100 respirator
- Plastic sheeting, painter’s tape
- EPA-registered fungicidal cleaner or diluted disinfectant; HEPA vacuum
- Underlayment with integrated vapor barrier (for floating floors) or 6-mil poly over concrete as allowed by manufacturer
- Two-part epoxy moisture vapor barrier (if slab RH/MVER too high)
Fixing it: Floating LVP/LVT
- Stop the source
- Tighten/replace leaking fittings; replace a toilet wax ring; call a plumber if unsure. Don’t proceed until the leak is resolved.
- Remove flooring in the affected zone
- Start at a doorway or transition and unlock planks back to and past the wet area. Label rows for reassembly.
- Dry and clean
- Extract standing water with a shop vac. Run dehumidifier and air movers 24–72 hours. Clean subfloor with a fungicidal cleaner and HEPA vacuum once dry-to-touch.
- Verify dryness
- Wood subfloor should be back to baseline and ≤ 12–14% MC. Concrete must be at/under your product specs.
- Prevent recurrence
- Over concrete: install an underlayment with a vapor barrier rated for LVP, or 6-mil poly if the manufacturer allows. Maintain expansion gaps; don’t caulk the perimeter shut.
- Reinstall or replace
- Reuse planks that are flat and undamaged. Replace any swollen, cupped, or delaminated pieces. Tap-lock carefully to avoid chipping.
Time: 1–2 days plus drying. Cost: dehumidifier rental $30–$50/day; air movers $20–$35/day; replacement planks $2–$5/sf.
Fixing it: Glue-down vinyl or sheet vinyl
- Stop the source (as above).
- Lift and remove affected material
- Cut back to dry, well-bonded areas. Expect to replace sections; reusing glue-down often fails.
- Remove adhesive residue
- Use a floor scraper and adhesive remover if needed. Follow safety data sheets for ventilation.
- Dry, disinfect, and test
- As above. For concrete not meeting specs, apply an epoxy moisture vapor barrier.
- Patch and reinstall
- Feather with patch/leveler as needed. Use the adhesive specified by the vinyl manufacturer and roll with a 100-lb roller.
Cost: new material plus labor $3–$8/sf; epoxy MVB $2–$4/sf.
Safety considerations
- Electricity and water don’t mix. Shut power to affected circuits if water reached outlets.
- Wear gloves, eye protection, and an N95/P100 during removal and cleaning.
- If visible mold exceeds ~10 sq ft or occupants have health sensitivities, hire a remediation pro.
- Ventilate when using solvents or epoxies.
Tips for best results
- Maintain indoor RH 30–50% with a dehumidifier in humid seasons.
- In bathrooms, after fixing a leak, caulk 3 sides of a toilet base and leave a gap at the back to reveal future leaks.
- On slabs, verify moisture with ASTM tests before reinstalling; don’t guess.
- Keep expansion gaps per manufacturer and use proper transitions. Blocking gaps traps moisture and causes buckling.
Common mistakes
- Drying the surface but not the subfloor.
- Reinstalling over a slab that still exceeds RH/MVER limits.
- Reusing swollen or warped planks.
- Sealing the perimeter of floating floors with caulk, preventing expansion and trapping moisture.
- Skipping the 100-lb roller on glue-down installs.
When to call a pro
- You can’t find the leak (plumber or leak detection).
- Mold area >10 sq ft, strong odors, or recurring health symptoms (remediation contractor).
- Concrete slab won’t pass RH/MVER and needs epoxy moisture mitigation (flooring specialist).
- Large glue-down or sheet vinyl replacement where seams must be heat-welded or patterned.
Stopping the moisture source, drying thoroughly, and verifying subfloor conditions are the keys. With the right tools and a methodical approach, most homeowners can handle small to moderate areas and prevent mold from returning.