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How to Prevent Cupping and Worse in Summer Months

By Craig DeWitt, Ph.D., P.E.
April/May 2011
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During the summer, even the most carefully installed wood floors tend to expand and sometimes cup or even buckle. These events occur because of wood’s relationship with moisture in the air. Air with a high moisture content (MC) or high relative humidity (RH) causes wood to gain moisture. When wood gains moisture, it expands. The result can be distortion, cupping and buckling. What can we do about it?

To control summer-related expansion of flooring and associated issues, we can deal with the wood itself, or in some cases deal with moisture. Wood flooring basics, like proper acclimation, narrow boards, stable species, quartersawn configuration and possibly engineered flooring can help prevent summer moisture issues. This article discusses options for the environmental control of summer moisture issues.

Summer Moisture

Some of the characteristics of air that we deal with every day are:

1) Warming air lowers its RH

2) Cooling air raises the RH

3) Cooling air too much causes condensation

4) Outside air contains moisture.

Over the course of a typical day, mornings start out cool and humid. Sometimes mornings are cool enough to form dew on grass and car windshields. But by afternoon, the air warms, the RH drops and the dew has evaporated. At night, the air cools again, the humidity goes up and dew forms.

The amount of water in the air over any given day stays relatively constant. But, because of the relationship between temperature and RH, as the temperature changes, the RH changes. The term “dew point” is an indicator of the amount of water in the air. Dew point is the temperature at which air at a specific temperature and RH will become saturated (100% RH) and condensation will form. On a typical summer day, the dew point may be near 45°F in the Southwest, 55°F in the Northwest, 65°F in the Northeast, and in the upper 70s along the Gulf Coast. (By contrast, winter dew point temperatures can be well below freezing.)

Building Temperatures

The next aspect of summer moisture involves the temperature of the building. Buildings are commonly cooled with air conditioning (AC), making the inside temperature lower than outside (at least in the afternoon). Over the years, indoor summer temperatures in air conditioned buildings have dropped from the mid to upper 70s to nearer 70°F. I’ve been in several buildings recently that have been cooled to 65°F or 68°F.

Soil under a building is often cooler than the interior. This cool soil helps cool slabs, basements and crawl spaces. Ductwork in basements or crawl spaces also helps cool the basement or crawl space.

So we now have warm, humid summer air and cool indoor surfaces. Then add some moisture entry through crawl space or basement foundation walls, from exposed soil in a crawl space, or from activities in a building. Combine that with the fact that cooling air raises its RH, and we have a significant potential for issues with our hardwood floors.

Ventilation as a Moisture Source

Our time-honored solution has been to ventilate these spaces to help control moisture levels. But there is a serious flaw in that thinking.

When warm outside summer air enters a house and cools, the RH of that air increases. For example, air at 90°F and 50% RH, when cooled to 70°F, will be at 100% RH. (This air has a dew point of 70°F.) To get the RH of this air back down to something respectable, we would need to remove moisture. The more ventilation, the more moisture is getting into the house, and the more moisture we need to remove. So, reducing ventilation can be beneficial.

Current building standards recommend home ventilation rates near 1/3 air changes per hour (ACH). Average homes have ventilation rates near 1-2 ACH, while some old, leaky homes are near 7-10 ACH. Weatherization and home energy audits typically measure ventilation rates, and can pinpoint leakage sites. Old windows are often major leakage sites, as are recessed lights and other holes in ceilings and floors.

Using charts published by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), I created Table 1 showing how much moisture needs to be removed from a house with different air and different ventilation rates. According to this table, about 7.4 pints of water per hour needs to be removed from an 1,800-square-foot house to lower the inside RH to 50% when the outside air dew point is 70 degrees. If the ventilation rate is higher, even more water needs to be removed. If it is colder or warmer outside, the amount of water to be removed changes. This same house, if located in Duluth, Minn., may not need any moisture removed during the summer because of the cooler, drier summer air. A house in Las Vegas may even need to add moisture in the summer because of its hot, dry air.

As you can see from Table 1, warmer outside air requires more moisture removal. Higher ventilation rates require more moisture removal, and lower target indoor RH levels require more moisture removal. Since the ventilation rate and moisture removal are related, an economical solution is to reduce ventilation rates before attempting to remove moisture.

Removing Moisture

Moisture is removed from indoor environments by two main mechanisms. First, ventilation with dry air can help dry out a house, but this air must be cold to be dry enough to be helpful. A dew point of near 60 degrees is about the cutoff where ventilation with outside air starts adding moisture to your house. In other words, if the dew point is below 60 degrees, venting will help dry your house. If the dew point is above 60 degrees, venting will add moisture to your house. So except for some arid or cool parts of the country, venting to dry a house in the summer is not a feasible solution. If you are lucky enough to live in one of these parts of the country, summer venting can help dry your house, but winter venting may really dry your house.

The main option for removing moisture is to use a mechanical system. These systems can either be air conditioners or dehumidifiers. A typical air conditioner uses about 25% of its energy to remove water from the air. A 3-ton air conditioner can remove about 9 pints of water per hour, when it’s running wide open. But—and it’s a big but—an air conditioner only removes moisture when it is running. A properly sized AC runs wide open about 1% of the year. The rest of the time, it won’t remove 9 pints per hour.

On a hot day, the AC may run 90% of the time in the afternoon, but only 10% of the time first thing in the morning. The outside dew point is the same, so the same amount of moisture needs to removed both morning and evening to maintain the same indoor RH. But in the morning we may only remove 1 pint per hour, because the AC isn’t running much. (Ever wonder why your house feels muggy in the morning, but fine in the afternoon?)

I mentioned that a properly sized AC runs wide open only about 1% of the time. One problem common to an AC is oversizing (that is, installing one bigger than needed). An AC is controlled by temperature. Once the indoor temperature is good, the AC shuts off. But moisture is removed only when the AC runs. An oversized AC will cool the house quickly, but because it doesn’t run very long, moisture removal isn’t good.

AC manufacturers have helped this situation by making variable-speed equipment that runs longer. More run time means more water removal, but in many situations in the Southeast, this extra removal still isn’t enough.

Other Moisture Factors

It isn’t enough to just think about the AC and the size of the unit. You also need to take these factors into consideration: 

AC fans: Another quirk with AC systems is that there are actually two parts: the fan, coils and ductwork that blow cold air throughout your house, and the outside unit that makes the cold. Typically, when the thermostat says “house needs cold,” the outside unit and the indoor fan turn on. As indoor air goes through the coil, the air is cooled to the dew point, and water condenses on the coils and drains out. This is how an AC controls humidity.

Thermostats have a switch that lets you set the fan in the “ON” position. This keeps the fan running even when the outside unit is not making cold. When the outside unit stops making cold, the coils warm up. Condensation that is still on the coils evaporates back into the house air, rather than draining out. A bunch of your humidity control is lost, and RH levels can go up. 

Dehumidifiers: The best solution for real humidity control in many buildings is a central, or whole-house, dehumidifier. Good systems put close to 100% of their energy into removing moisture, and can handle up to around 6.5 pints per hour, every hour, regardless of mild weather or cool mornings when an AC won’t run much. They will also shut down when an AC can handle the moisture loads. And since dehumidifiers turn on and off because of humidity levels, oversizing an AC or even leaving the AC fan in the “ON” position becomes less of an issue.

Portable dehumidifiers can also help, but they typically don’t remove much more than about 2 pints per hour, and they use more energy than a good whole-house dehumidifier. Since they are usually located in one room, humidity control in other rooms or the whole house may not be good. 

Basements: Since lots of surfaces in basements are cool, an AC won’t run much. Many times I can’t even find a small enough AC to get good run time in a basement. So basements are prime candidates for dehumidifiers.

Crawl spaces: Crawl spaces are like short basements, only sometimes they have more ventilation. Venting very often makes a cool crawl space not just humid, but wet. Temperature doesn’t matter in a crawl space, so an unvented crawl space with a dehumidifier is a great, efficient way of controlling humidity levels below wood floors that are over a crawl space.


Go back to the basics. Acclimate the flooring properly. Use narrower boards, more stable species of wood, quartersawn boards, or a combination of those features. Deal with concrete slab moisture by using appropriate flooring-approved vapor barrier membranes or systems.

Accept some gaps. This takes consumer education by establishing proper customer expectations. Make sure you explain clearly, and hopefully in writing, that the 7-inch hickory flooring for a house in Florida will likely have seasonal gapping if you install it so that cupping does not occur in summer months.

Use a product that can handle high-RH environments. Solid wood flooring has been used in variable environments for years while, based on warranties, much engineered flooring and some factory finished flooring apparently aren’t designed for those environments. Don’t set your client up for automatic failure, or an extra unknown expense.

Deal with the building environment. These solutions (reducing ventilation, controlling water in crawl spaces or basements, adding dehumidifiers or modifying AC systems) aren’t up to flooring professionals. Building owners can take steps to reduce ventilation rates, reduce moisture intrusion and/or add mechanical dehumidification. But don’t expect them to install $20,000 worth of extra climate control or building modifications for $2,000 worth of flooring.

HF Online logo

Read the author’s companion article to this one about reducing gapping in wood floors in winter.

Read more on how wood flooring responds to moisture.

For more from author Craig DeWitt, read:

How the air in buildings affects wood floors.

and his Troubleshooting column about crawlspace moisture wreaking havoc on a solid wood floor.

        Installing wood flooring    Wood floor acclimation        Buckling    Cupping    Moisture               


Thank you, this is what I deal with all year long! I will print this and give it to the folks that ask for 5-8 inch flooring and think my insight is a lie to blow the price up for extra care under the home. We get mold here very bad so the RH is key. Be safe,
wayne Lee  Floorman in south  4/16/2011 9:33:18 PM

I have 3 1/4 in wide, 3/4 inch solid teak hardwoods installed over a crawl space, and it is cupping after a few months. The flooring company states that venting the crawl space is REQUIRED for their warranty purposes, and mine has their recommended amount of venting. Now I am concerned, based on this article, that I have been ill-advised, but they will challenge this, of course. What do you suggest? Also, how does a homeowner know if their AC unit is too big for their home? Had a new one installed last fall, larger than the one put in by the builders 15 years ago, could this be a problem, and how would I know? Thanks so much.
leah b  homeowner  8/26/2011 6:37:54 AM

Leah b, Don't just run out and close your crawl space vents assuming you will solve the problem. Closed spaces need the same attention as the interior of your house to prevent air and moisture intrusion and vented crawl spaces ARE NOT typically designed that way. Point being that the cawl spaces need work to seal it completely (and insulate etc) before they operate properly even with a dehumidifier. They become part of the building envelope.
neil moss  director of technical services (retired)  8/26/2011 10:48:58 AM

It is hard to say if your A/C is too large but one can get a bit of an idea by determining how often it cycles. If it only comes on for a couple of minutes, then off for a few and continues this type of cycle it is likely it is oversized. Unfortunately for years A/C contractors have intentionally oversized. It makes them a bit more money and the selling point is that reduced running time equals less electric billing. This method of sales overlooks the dehumidification part of the equation and focuses only on the cooling aspect. The result is that a short cycle results in cool damp air v. cold dry air. One can often save quite a bit of money with an undersized unit as it may actully draws less current overall. Why? One can typically RAISE the thermostat in the summer 2-4 degrees because cool dry air FEELS cooler.
neil moss  director of technical services (retired)  8/26/2011 10:50:43 AM

One can sometimes find the source of the high humidity that causes cupping, or at least get some direction by installation of an area rug. When you have a floor that is cupping look under the area rug and see if it is doing the same. If one is not in place put one in for a week or two. If it is the interior dew point or humidity the rug becomes an insulator while reducing air movement so you don't get the "flash" dewpoint problems. Consequently the area with the rug often will have no cupping whilst the surrounding area does. In addition, I have found that if there is no change under the rug (still cupped) the likely source of the moisture is from below.
neil moss  director of technical services (retired)  8/26/2011 10:56:00 AM

Leah B, Where do you live? In many parts of the country, outside air used to vent a crawl space is a significant source of moisture, and makes the crawl space wetter. If your flooring company requires vents, its possible that the flooring wasn't acclimated correctly to allow being installed over a vented crawl space. Closing your vents may be necessary to eliminate that source of moisture. But if your crawl space is wet, you may need a dehumidifier there to dry it out, or even keep it dry in the long run. HVAC contractors are supposed to do heating and cooling load calculations in order to pick the right equipment. Ask for that documentation. Much of the new equipment has less issues with being too big, but can blow more air and make things colder. So they can be issues as well.
Craig DeWitt  President, RLC Engineering, LLC  8/26/2011 1:20:59 PM

If the sub floor is above 14 percent, and the RH is above 50 percent the warranty is void. So Mr. Neil how do you fix it?
Wayne lee    8/26/2011 9:39:10 PM

Neil is spot on when he speaks of oversized A/C units. In MN it's a common problem, I see interior climate controlled near 65% with A/C's running. If sized correctly the A/C should run longer and keep residence dryer just as he states. For our region 50% would be the norm if sized correctly. Wayne you can lay a floor anywhere, from the desert or on a yacht where there is no climate control. The key is acclimation for your geographical location, even a yacht. If you are running 14% on subfloor it must take a month or two to acclimate your flooring to 12%. Of coarse I forgot many folks like the plank look and WILL experience more cupping than a 2-1/4 or 3" strip floor will. During the 1900's with no climate control 1-1/2" strip was chosen for a reason, it's only twice as wide as it is thick, almost impossible to cup unless you hit it with the hose. You maybe selecting a floor width which has a slim chance of success for your area or not acclimating it correctly?
Roy Reichow  Flooring Consultant  8/27/2011 8:47:56 PM

Leah B, Craig has highlighted many valuable points to consider. Like Neil has mentioned moisture is the source and you may want to retain a third-party ( NWFA flooring inspector) to determine causation/source of the moisture. Blogs are a great place to start but without site investigation makes it impossible to diagnose, the possibilities are endless. Go to to find an inspector near you. The inspector will identify the source and responsible party. Good Luck
Roy Reichow  Flooring Consultant  8/27/2011 8:59:34 PM

Wayne, I think RR answered your primary question. What warranty would you be asking about however. If the product is unfinished generally the only manufacturers warranty is going to be on grade and milling. If it's factory finished then there are multiple warranties, some of which would not be affected in my opinion.
neil    8/28/2011 2:06:31 PM

These are the kinds of things I covered in my "Summer Moisture" and "Crawl space" webinars recently. A/C units dehumidify only when they are running. A bigger one does not run as much as a smaller one, so you get adequate temperature control but less humidity control. Even a correctly sized A/C unit is right only a couple hours a year. The rest of the time it is over sized. And a floor is affected by moisture on both top and bottom. In this case, we have a crawl space underneath. And we don't have enough information to say where it is too humid on that side of the floor (nor the cause), or whether the top side is too humid for that matter. I agree with Roy that Leah should have the floor inspected. It would need to be more than just a checklist inspection if you really want to know the cause of the problem.
Craig DeWitt  President, RLC Engineering, LLC  8/30/2011 7:13:10 AM

In a situation where I have 3 1/4 x 3/4 oak flooring over a conditioned crawl space [non-vented] and am getting minor cupping and have refrained from running dehumifier due to $$$. However, I thought applying a clear wood sealer to the crawl space overhead sub-flooring this winter might be beneficial as it would be a barrier to moisture getting into underside of the finished floor during the warm summer months. Any thoughts on or experience with would be appreciated.
Bob R  Homeowner  9/22/2011 8:43:08 AM

We have 3 1/2 x 3/4 oak and only one room is cupping, and its primarily around a central ac vent. Its over a crawl space. What role does insulating the ceiling of the crawl space play? Should it be insulated? The crawl space is vented, but we still have cupping.
TJ    6/18/2012 12:21:38 PM

Our home was built long before A/C existed. It is a two story frame house built on piers and beams some 6' above the ground on the Texas Gulf Coast. Four years ago, we installed 3 1/2" x 3/4" white oak flooring with a heavy felt mat over a wooden subfloor in all of the first floor rooms. Beneath the house between the piers and beams, we installed 4" fiberglass insulation, held in place by sheets of 4' x 8' plywood. Each summer when our central A/C is struggling to keep the inside temperature in the low 70s and the outside temperture is in the high 90s, our hardwood floors have severely cupped, buckled and even split. When repaired, the felt mat beneath the flooring has been damp to dripping wet. The A/C keeps the inside humidity in the 50% to 60% range while the outside humidity most often is in the 80 to 90% range. A heavy foam 4" x 4' x 8' mat insulation under the house has been recommended to greatly reduce or eliminate the condensate forming under the flooring due to the outside heat contact the bottom of the cold floors. Will it work? If not, will anything?
Ed Bluestein  Retired Homeowner  8/22/2012 5:00:02 AM

Is this a picture of what the wood floor should look like, or is this a picture of a buckled floor in this article.
Carl   Woor Floor   9/17/2012 9:46:30 AM

Carl, that is a badly cupped wood floor.
Kim Wahlgren  Editor  9/17/2012 9:52:57 AM

TJ, Most insulation materials help stop heat, but do not stop water movement. So insulating your floors won't have much impact on the cupping. You might have some success by insulating the AC vent, boot and duct if the insulation on those isn't correct. Ed, You are running into condensation problems on the underside of your subfloor. Moisture will move through the plywood sheets (and gaps between them) and through the fiberglass insulation and condense on the underside of the cold subfloor. The foam sheets can help, but the joints and seams need to be caulked, taped or sealed very well. Dry things out well before you install the foam, or you will have a real mess.
Craig DeWitt  RLC Engineering, LLC  9/17/2012 3:27:41 PM

My husband recently installed 5 inch by 3/4" hickory hardwood floors on our main floor. The living room has crawl space beneath it and the floor is mildly cupped. He did not allow any room for expansion of the floor when he installed it saying that "nailing" won't let the wood move so he discounted the notion that it needed room for expansion. We now know that was a mistake. He pulled the base boards and removed an inch of the wood flooring. Should this help the floor to relax and become flat again? We live in Ohio where humidity is a problem most of the summer, now that we are into fall weather I'm hoping the floor will dry. We did wait a week to allow the floor to acclimate but it was installed during winter months. What are your thoughts on our situation?
PVanDuynhoven  home owner  10/6/2012 3:32:34 AM

We have a new custom home that is nearing completion. The floating wood floor has been buckeled since the day it was installed. We have a concrete slab floor. Our contractor wants to receive the final payment for the house and assures us that the floor will "relax" over time. We think it can and should be fixed now. Any suggestions?
David  Home Owner  11/11/2012 11:01:46 AM

We are getting ready to install 2 1/2" wide, 3/4" thick pre-finished hickory floors. We are in south Louisian near the gulf and sometimes 100% humidity. Our house is on the bayou and is 5' off the ground. Our contractor says to use tar paper under the wood floors. The flooring company sold us thermal insulated padding. I have to find the right choice ASAP as the floors will be delivered next week to acclimate to our house once the a/c unit is installed. We also flood a few times a year but the water never gets closer than 2' from the house. PLEASE HELP!
Denise  Home Owner / Remodeling  6/23/2013 9:53:47 PM

We recently built a home , we used oak tongue and grove flooring on some of our walls which have buckled in various places on every wall . We have had record rain fall this year , have not installed an air conditioner yet ,and after reading about how to install wood flooring for walls I now know they were installed incorrectly . Our builder has decided to fix this problem he will create the gaps now by running a skill saw down each seam..........that worries me..........what's your thought ? p.s. My email is down I hope you cam post answer on your web site.
kim  home manager  8/15/2013 1:40:04 PM

Have ordered and will have reclaimed barn wood in 4", 5" and 7" widths flooring laid next month. Getting so many comments from people that "know" something on exactly how to lay it. Screw it, glue it, staple it, nail it and/or all of the above. Being told we will get cupping, crowning, etc. if we don't install it a certain way. But reading sites, it seems that we wont have those problems if we leave a gap (for movement of the wood), acclimate the wood to the house and use a vapor barrier. Thoughts??
Erin  Home owner  12/21/2013 4:44:06 PM

I have a question. Can hardwood be used in a summer home that is unoccupied and unheated in the winter? Would a very hard bamboo floor that is a floating installation work?
Sal   Summer home  5/7/2014 7:54:34 AM

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