Coastal humidity
Coastal Humidity & Hardwood Floors Wilmington NC | Why Floors Cup Here
Morning humidity on this coast averages around 85% and never drops below 80% in any month. A wood floor wants to live between 30 and 50%. In 15 years of working in that gap, we at Wilmington NC Hardwood Flooring have found that nearly every flooring failure in this region starts in it, and that most of them start three feet below the floor in a crawl space nobody has looked at.
We measure rather than assume, which is why our meters come out on the estimate visit. Across more than 4,500 completed flooring projects we have seen the same pattern repeatedly: boards that cup every August, gaps that open every January and close again by June, and floors sanded flat while still wet that dry into permanent crowning. This page sets out what the humidity actually does and what holds a floor flat here.
Coastal Humidity And Hardwood Floors In Wilmington
Wilmington's morning relative humidity averages around 85% and does not drop below 80% in any month of the year. A wood floor wants to live between 30 and 50%. That gap is the single most important thing about owning a wood floor on this coast, and it is why floors fail here for local reasons rather than because of anything the installer did wrong.
Wood is hygroscopic. It takes on and gives up moisture until it reaches equilibrium with the air around it, and then it moves: swelling across the grain as it gains moisture, shrinking as it loses it. Everything below follows from that.
What That Means For Your Floor
Cupping is a crawl space problem
Edges higher than centres means the underside is wetter than the top. In this region that is almost always the crawl space, not anything that happened on the surface.
Winter gaps are usually normal
Heated indoor air in January can drop below 30%, boards shrink and gaps open. If they close again in summer that is seasonal movement, not a defect.
Acclimation is a measurement, not a number of days
Material conditions inside the finished, running house until the readings agree. Three days from a brochure means nothing.
The shoulder seasons are the risk
Air conditioning dehumidifies only while it runs. In spring and autumn the thermostat is satisfied, the system never runs, and the house drifts up to match the outside.
Width amplifies everything
The same percentage change is a bigger gap on a 7 inch board than on a 2¼ inch strip. Wide boards need engineered construction and a controlled house.
Do not ventilate to dry a wet floor
Opening the windows brings in 85% air. Close the room, run a dehumidifier and move air across the surface with fans.
How To Hold 30 To 50% In A Wilmington House
- Deal with the crawl space first. If your house has one, this matters more than everything else combined. A sealed, lined crawl space is what keeps the floor above it flat.
- Run the air conditioning for humidity, not just temperature. A thermostat with a humidity setpoint, or a dehumidistat wired to call for cooling, is the cheapest intervention available.
- Add whole-house dehumidification if the shoulder seasons are bad. Close to essential in a second home that sits empty.
- Accept that winter is the opposite problem. Do not fill seasonal gaps in February; the filler gets crushed out in July.
None of this makes a wood floor a bad idea here. It means measurement, substrate work and a controlled interior are the difference between a floor that lasts forty years and one that gets sanded twice in ten.
What Humidity Does To Each Part Of The Job
{cards([ ("Before installation", "Material has to condition inside the finished, running house until " "the readings agree with the subfloor. Delivered into a garage, it is " "conditioning to the wrong air.
"), ("During installation", "Expansion gaps at every vertical surface are not optional here. A " "floor with nowhere to go lifts at the seams the first humid " "summer.
"), ("Choosing a width", "Movement scales with board width. The same house that gaps a 7 inch " "plank will hold a 3 inch strip tight all year.
"), ("Choosing a cut", "Quarter-sawn and rift stock move roughly half as much across their " "width as plain-sawn. On this coast that is worth the premium.
"), ("Refinishing", "Never sand a cupped floor flat. Dry it, let it come back, then sand, " "or you convert a reversible problem into permanent crowning.
"), ("Living with it", "A humidistat and a dehumidifier do more for a wood floor in " "Wilmington than any upgrade in species or finish.
"), ])}A Rough Guide To The Year
Spring. The risk months. Mild enough that the air conditioning rarely runs, humid enough outside that the house drifts upward. This is when a dehumidifier earns its keep.
Summer. Humid outside, but the air conditioning is running constantly and dehumidifying as it goes, so interiors are often better controlled than in spring. Crawl spaces, however, are at their worst.
Autumn. The second risk window, for the same reason as spring. Mild, humid, and the system satisfied on temperature without ever removing moisture.
Winter. The opposite problem. Heated air can drop below 30% and boards shrink, which is when seasonal gaps appear. They close again in summer and should not be filled.
Humidity & Wood Floor Questions
What humidity should I keep my house at for wood floors?
Between 30 and 50% relative humidity, at 60 to 80°F. In Wilmington that needs active management rather than good intentions.
Why do gaps appear in my floor every winter?
Heated indoor air in January can fall below 30%, so the boards give up moisture and shrink. If they close again in summer it is normal seasonal movement, not a fault.
My floor is cupping. What do I do first?
Find the moisture before touching the floor. Put a hygrometer in the crawl space in summer. Sanding a cupped floor flat while it is still wet turns a reversible problem into a permanent one.
How long does flooring need to acclimate?
Until it reaches the target moisture content and sits close enough to the subfloor. That is a measurement, not a number of days, and in this climate guessing wrong is expensive.
Is a dehumidifier really necessary?
In many houses here, in the shoulder seasons, yes. Air conditioning only dehumidifies while it is running, and in spring and autumn it often is not.
Get your floor and crawl space checked
Moisture readings on the floor, the subfloor and the crawl space, free with every written quote.