Cupped Wood Flooring and Rotting Subfloors -- Causes and Solutions

Lots of raised floor homes in our warm, humid climate develop cupped wood flooring and subfloor moisture problems and there is conflicting information about what to do. There is not a one-size-fits all remedy, but understanding the combination of factors that cause the problem reveals the most cost-effective solutions.

In a nutshell, raised floors get wet in summer because of the combination of cool air conditioning and impermeable floor finishes in a humid climate. No A/C, no problem, but that's not how we live today!

Understanding The Causes:

Moisture moves from warmer to cooler, and from wetter to dryer areas. So, air conditioning your home in hot, humid weather creates a strong water vapor drive from outside to inside. Cold air conditioning can also cool the subfloor below the outdoor dew point temperature, which causes moisture in the air to condense on the cool subfloor. That amplifies the problem.

The colder the A/C, the worse the problem. Hot, humid air outdoor air picks up additional moisture that evaporates from soil, then condenses on the cool subflooring and tries to move inward. Typical insulation methods (such as fiberglass batts) make it worse because the subfloor stays cooler (from the A/C), and both air and moisture go through it to the subfloor.

Flooring cupping or buckling occurs and a subfloor moisture problem worsens when there is impermeable flooring like vinyl, laminate, or a typical polyurethane finish on wood. When the floor finish is impermeable, moisture can’t get through it, so the subfloor just gets wetter and wetter all summer.The wet subfloor wets the bottom of wood flooring, which causes the bottom of the planks to swell, causing cupping in summer.

Conditional Solution Options:

1. If you keep the house warmer in summer (higher A/C setting above the outside dew point – so at least 76 degrees, preferably 78 degrees), you would likely be OK with just a ground cover (plastic sheeting) under the house, good drainage around the house and normal crawl space vents.

2. Or, if you don’t keep it too much cooler, you might be OK if you have all floorings that are moisture permeability (carpet, unglazed ceramic tile with unsealed grout, hardwood (not engineered wood) with a permeable finish such as high-perm water-based polyurethane) and good drainage.

Note: With a ground cover, it’s important that rainwater not flow under the house and pool on top of the plastic sheeting. The ground cover can be 6 mil plastic sheeting secured with stakes or gravel on top; no need to tape or seal it.Dr. Reichel does NOT recommend running a fan under the house.Bringing more humid air to cold subfloor materials can actually worsen the problem in humid weather – which is pretty much all summer in Louisiana.

Best Solution Options:

The subfloor systems that are best to avoid moisture problems in our climate, and allow you to have any type of flooring, and set you’re A/C as cold as you like, are either:

1. Fire rated, foil-faced rigid foam boards (Polyiso board, 2 inches thick for R-13 insulating value) attached across the floor joists, with seams taped and all edges and penetrations sealed airtight; or

2. Closed-cell spray foam insulation (2-3 inches) sprayed on the subflooring between joists and, if the crawl space is not completely open air, add a light spray foam coating on the joists.

Both of these subfloor treatments provide an airtight, insulating vapor barrier system that prevents moisture problems both in summer and winter.

Be aware however, that some pest control companies object to foam insulation systems because it can reduce their inspection view, so may disallow it in their termite contract terms. Firms have a right to set their own parameters for their services. However, many firms recognize that they can clearly inspect for termite evidence beneath the foam, so the treat that similar to how they deal with not being able to see inside walls.

Caution! Never insulate while the subfloor and flooring is wet.

One option is to time the insulation installation at the end of winter, after heating your home in cold weather which causes the floors to dry outward. Otherwise, you can temporarily seal your crawlspace with plastic sheeting (on the ground and around the perimeter) and run a commercial dehumidifier in the crawlspace until the subfloor and wood flooring moisture content stabilizes (preferably 12% moisture content or less, measured with a quality wood moisture meter). Once all is dry, then you can insulate with one of the recommended systems above.

If you have cupped wood flooring, do not sand it down flat until all is dry. Cupped planks will flatten as they dry unless they have splits, so wait for them to stabilize.The time to refinish your wood flooring is after it’s flattened as much as it will.If you sand off the cupped edges before it flattens, you could end up with a scalloped floor in winter after it dries.

The LSU AgCenter has a publication available online ( ) and print copies at LaHouse Resource Center that describes the results of a south Louisiana study of the moisture performance of various insulation systems in raised floors.

Also explore LaHouse Resource Center on the LSU campus and its website ( ) for more insights on this and many other housing topics.The demonstration house, with its cut-aways, displays, feature signage, lots of free publications and exhibit room is open for self-guided touring M-F from 10-4:30 as a public service of the LSU AgCenter.

2/5/2020 4:38:59 PM
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