When installing new flooring or refinishing existing floors, it is crucial to measure and monitor the moisture levels in the substrate (the surface that the flooring will be installed on). High moisture readings can lead to a variety of problems, including warping, cupping, gapping, and deterioration of the flooring itself. However, what constitutes a “normal” or acceptable moisture reading depends on a variety of factors.
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How moisture affects flooring
Moisture is present in every substrate, including concrete, plywood, OSB (oriented strand board), and existing flooring. Small amounts of moisture are normal. However, excess moisture can cause major issues with flooring installation and performance. Here are some of the most common problems caused by high moisture levels:
- Warping or cupping of wood flooring – Moisture causes the wood to swell, leading to an uneven surface.
- Bond failure with glued-down flooring – Adhesives rely on a dry, stable substrate. Moisture interferes with proper bonding.
- Growth of mold, mildew, and fungi – Wet conditions allow microorganisms to thrive underneath flooring.
- Adhesive breakdown with tile or vinyl – Trapped moisture degrades adhesives over time.
- Discoloration or degradation of flooring finish – Moisture can ruin the appearance and durability of flooring surfaces.
- Loose or hollow spots in floating floors – Moisture causes instability in the subfloor.
In addition to physical damage, excess moisture can lead to unhealthy indoor air quality and create conditions conducive to biological growths such as mold. For these reasons, monitoring substrate moisture is a key part of any flooring installation process.
Target moisture levels by flooring type
Each flooring material has an optimal moisture range for proper installation and performance. Going above or below this range increases the chances of failure and problems down the road. Here are target moisture levels for common flooring types:
Hardwood Flooring
For solid and engineered hardwood flooring, the wood subfloor should have no more than 12% moisture content on a moisture meter. Readings above 15% indicate a moisture issue that must be addressed before installation.
Laminate Flooring
Laminate flooring is highly prone to damage from excess moisture. For substrate moisture, readings should be below 3% on a Tramex-style concrete moisture meter. Wood subfloors should have no more than 12% moisture content.
Vinyl and Luxury Vinyl Planks (LVT)
Vinyl and LVT are vulnerable to adhesive breakdown from moisture. For concrete substrates, moisture testing with a Tramex-style meter should show levels below 5%. Wood subfloors should have no more than 12% moisture content.
Ceramic and Porcelain Tile
Tile flooring is durable, but excess substrate moisture can still loosen grout and degrade adhesives. Readings below 3% on a concrete moisture meter are ideal. Wood subfloor moisture content should be less than 12%.
Carpet
Although carpet itself is not damaged by moisture, wet conditions lead to mold, mildew, and adhesive failure. For concrete substrates, moisture meter readings below 5% are recommended. Wood subfloor moisture levels should be no more than 12%.
Sheet Vinyl
Like other glued-down floors, sheet vinyl relies on a dry, stable substrate. Target moisture levels are below 5% for concrete measured with a moisture meter, and 12% or less for wood subfloors.
Testing concrete moisture
Concrete naturally absorbs and emits moisture. New concrete slabs in particular often contain excess moisture from the curing process. For concrete substrates, using a specialized moisture meter is the best way to obtain accurate, actionable data. Here are 3 common options:
1. Tramex Concrete Moisture Encounter Meter
This meter uses electromagnetic wave technology to measure moisture levels up to 3 inches deep in concrete. It has plug-in ports for optional humidity and temperature sensors. Tramex-style meters produce results as a percentage of moisture content by weight.
2. Calcium Chloride Test
ASTM F1869 is a quantitative test that involves taping a test kit to the concrete and measuring moisture absorption over 60-72 hours. Results are given in pounds per 1000 square feet per 24 hours. This test requires a longer time but provides precise data.
3. Relative Humidity Probe
ASTM F2170 measures interior relative humidity using in-situ probes inserted into pre-drilled holes. After 72 hours, the equilibrium RH level is read as a percentage. This method gives a direct measurement of moisture in the air spaces within the slab.
Any of these 3 testing methods can provide useful moisture data. Be sure to take readings in multiple areas, since moisture levels often vary across the slab. Testing concrete moisture is the only way to avoid surprises after flooring installation.
Testing wood subfloor moisture
For wood substrates like plywood and OSB, using a moisture meter to measure moisture content is recommended. Here are 2 common methods:
1. Wood Pin/Probe Meter
This type of meter has thin pins that penetrate the wood surface to give readings at various depths. They can test moisture in wood from 5% to 30% moisture content.
2. Non-Invasive Pinless Moisture Meter
These meters use electromagnetic wave technology to measure moisture levels in wood without damaging the surface. They have a moisture content range of 5-55% MC. Pinless meters provide rapid results for both hardwood and softwood.
As noted above, target moisture levels are no more than 12% MC for wood subfloors. Be sure to take readings below the subfloor surface, where excess moisture is more likely to accumulate over time.
Other tips for measuring moisture
- Test in multiple locations around the installation area. Take at least 20 readings per 1000 sq. ft.
- Check manufacturer’s specs for recommended moisture limits and test methods.
- Test at appropriate depth – up to 40% moisture can be trapped below the surface.
- Test on bare substrate for accurate results.
- Keep records of all readings for liability protection.
- Stop all moisture sources before testing and installation.
- Let surfaces dry out until safe moisture levels are reached.
- For concrete, test both top and bottom surfaces if installed over a moisture barrier.
Dealing with moisture issues
If moisture testing indicates a problem, there are several options to reduce moisture to safe levels:
- Run dehumidifiers to actively dry the air and substrate.
- Use moisture mitigation systems – sealers, primers, or epoxies designed to block moisture.
- Allow more drying time for new concrete slabs before flooring installation.
- Replace damaged pipes, fix leaks, and eliminate other moisture sources.
- Install subfloor ventilation if moisture is migrating from below.
- Apply floor leveler products to create a new moisture barrier.
Addressing excess moisture ensures flooring will last its expected lifetime. No installer wants a callback for floor failure due to moisture-related problems. While optimal moisture levels depend on the flooring type, following the manufacturer’s guidelines is key.
Moisture and indoor air quality
In addition to damaging floors, excess moisture promotes mold growth. Mold spores and VOCs released by damp materials can cause respiratory issues and allergic reactions. Chronically wet surfaces also lead to rot, odors, and infestations. Even without visible mold, high humidity creates an unhealthy indoor environment.
Flooring installers have a responsibility to check for moisture not only to ensure a successful job, but also to protect the client’s indoor air quality. Alerting homeowners to any underlying moisture issues provides a chance to correct them before covering over with new flooring and trapping in moisture problems.
Conclusion
Measuring substrate moisture is a critical step before installing any floor covering. While optimal moisture levels vary by flooring type, going above manufacturer recommendations means risking flooring failures, adhesive breakdown, and moisture-related damage. For concrete slabs, Tramex-style moisture meters give accurate quantitative data. For wood subfloors, measuring moisture content with a pin or pinless meter is recommended.
Catching and resolving excess moisture issues in advance prevents major headaches down the road. Moisture testing protects installers from callbacks, helps flooring achieve its expected lifespan, and improves clients’ indoor air quality by identifying dampness issues before they are buried under new floors.
| Flooring Type | Target Moisture Level for Concrete | Target Moisture Level for Wood |
|---|---|---|
| Hardwood | Below 3% with Tramex-style meter | Less than 12% MC |
| Laminate | Below 3% with Tramex-style meter | Less than 12% MC |
| Vinyl | Below 5% with Tramex-style meter | Less than 12% MC |
| Tile | Below 3% with Tramex-style meter | Less than 12% MC |
| Carpet | Below 5% with Tramex-style meter | Less than 12% MC |
| Sheet Vinyl | Below 5% with Tramex-style meter | Less than 12% MC |