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How to Prevent Wood Floor Gaps in Winter

By Craig DeWitt, Ph.D., P.E.
October/November 2010
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photo of gaps in wood flooringDuring the winter, even the most carefully installed wood floors tend to dry out and shrink. Customers begin to notice gaps between boards, and the phone calls begin. The floor behaves that way because of wood's relationship with moisture in the air (there's no accounting for how the customers behave, although educating them about gaps beforehand can help—more on that later). Air with a low moisture content, or low relative humidity (RH), causes wood to lose moisture. When wood loses moisture, it shrinks. What can we do about it?

To control winter-related shrinkage of flooring and the consequent gaps (what we used to call "cracks"), we have basically six options. Four deal with the wood itself, and two deal with moisture. Let's get the wood ones out of the way and then discuss moisture issues.

Wood Options: Use the Right Flooring

Engineered flooring is supposed to be more stable than solid wood. From a technical aspect, this should be true. But many engineered flooring manufacturers restrict the use of their products to a certain RH range. I've seen warranties that specify 35 to 55 percent RH or 40 to 60 percent RH as the acceptable range. If the flooring is exposed to conditions outside these ranges, the warranties are void. In my 30-plus years of experience dealing with indoor environments in the U.S., I don't know any location that will consistently maintain those RH ranges. So using engineered flooring may be an option for reducing winter-time floor issues, but check the manufacturer's recommendations and warranty.

Narrow boards will shrink less than wide boards for a given change in moisture content (MC). A 5-inch-wide plank will shrink twice as much as a 2¼-inch-strip. So the size of the gap between 5-inch boards will be twice as big as the gap between 2¼-inch boards. More joints means more places to distribute gapping.

Some species are more dimensionally stable than other species. For a given change in MC, a 5-inch-wide hickory plank will shrink more than a 5-inch-wide red oak plank. The U.S. Forest Service, and others, publishes dimensional change coefficients for different species. A second solution to excessive winter gapping is to use a species of wood that is more stable (one with a smaller dimensional change coefficient).

Along the same line of varying dimensional stability, quartersawn flooring shrinks about half as much as flatsawn flooring for the same amount of moisture change, so quartersawn flooring will have smaller gaps than flatsawn flooring under the same circumstances.

Therefore, from a wood standpoint, to have the smallest winter gaps, use quartersawn, narrow boards from a stable species.

Moisture Issues

The other approach to winter gapping is to address the moisture issues. Gapping and associated noises usually occur when the flooring dries significantly from its summertime high moisture levels. So, to reduce winter gapping, reduce the annual range of moisture levels. Or, more specifically, to reduce winter gapping, don't let the indoor RH drop too much. (Or don't let summer RH levels get too high—but that is another article.) A good annual range for the best flooring performance is a swing of 20 percent RH from wettest to driest. This means that in the Southeast, we may work in a range of 40 to 60 percent RH, while up north they may use 30 to 50 percent, and out west they may use 20 to 40 percent. All will work, as long as the RH range isn't too wide. But sometimes in the winter, the RH tends to dip too low.

There are two approaches to keeping winter indoor RH elevated.

Moisture Option 1: Reduce Ventilation

Table 1: Raising RHBecause of the relationship between temperature, moisture and RH, ventilation of a house in the winter tends to dry it out. When you bring cold outside winter air into a house and warm it up, the RH of that air drops significantly. For example, air at 30 degrees Fahrenheit and 50 percent RH when warmed to 70 degrees will be at 10 percent RH. To get the RH of this air back up to something respectable, we would need to add moisture. The more ventilation that is occurring, the more this dry air is drying out your customer's house, and the more moisture she needs to add. The solution to this part of winter drying is to reduce ventilation.

Ventilation of a house is measured in air changes per hour (ACH). As an example, a house that is 1,800 square feet with 8-foot ceilings has a volume of 14,400 cubic feet (1,800 x 8 = 14,400). Changing all the air in this house with fresh air once an hour would be one ACH.

Current building codes and standards recommend home ventilation rates near 1/3 ACH. Not all states enforce these codes or standards. Average homes have ventilation rates near 1 to 2 ACH, while some old, leaky homes are near 7 to 10 ACH. Weatherization and home energy audits typically measure ventilation rates. These programs can also pinpoint leakage sites and direct sealing efforts to reduce excessive ventilation rates. Old windows are often major leakage sites, as are recessed lights and other holes in ceilings and floors.

Moisture Option 2: Add Moisture

Table 2: Household MoistureAs I mentioned above, bringing in 30-degree air at 50 percent RH, then warming it to 70 degrees causes its RH to drop to 10 percent. To raise the RH, we need to add moisture. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) publishes charts showing moisture and air relationships. Using these charts with our example house from above, we need to add about 3.3 pints of water per hour to raise the inside RH to 30 percent. If the ventilation rate is higher, we need to add more water. If it's colder or warmer outside, the amount of water needed changes. This same house, if located in Duluth, Minn., would need almost 5 pints per hour during common winter conditions. If we want the RH to be even higher, we need to add more moisture. Table 1 shows some examples.

As you can see from Table 1, colder outside air requires more moisture. Higher ventilation rates require more moisture, and higher target indoor RH levels require more moisture. Since the ventilation rate and moisture needed are related, an economical approach is to reduce ventilation rates, then add moisture.

Moisture is added to indoor environments from normal household activities and use. When this moisture is not sufficient to meet the needs, a humidifier can be added. Table 2 shows some typical indoor moisture sources and amounts.

According to Table 2, a family of four contributes about ¾ pint of moisture per hour. This number is likely smaller than that shown, because people aren't home all day and don't clean every day. So I would suggest ignoring household sources when determining moisture needs.

Adding moisture then boils down to using humidifiers. Humidifiers can either be stand-alone or attached to a central forced air furnace. Typical residential systems can provide up to about 6 pints per hour. This is an important number: 6 pints per hour, maximum. Table 1 shows that more than 6 pints per hour are necessary to get to 40 percent RH when it is real cold outside in a relatively tight, 1,800-square-foot house. We can't even get to 30 percent RH in a somewhat leaky house when it's moderately cold outside, or in a larger, moderately tight house. (By moderately cold, I mean the kind of weather I have in South Carolina. By real cold, I mean the type of weather in Minnesota or New Hampshire.)

To make matters worse, moisture output from some humidifiers depends on furnace air temperature. According to Aprilaire, a large manufacturer of whole-house humidifiers, their humidifiers produce a maximum of about 3.6 pints per hour when connected to a heat pump. With that number, we can't even get to 30 percent RH in a moderately tight, moderately sized house in a moderate climate.

Adding Too Much Moisture

Let's say we can get the humidity up in the winter. Now we need to worry about hurting the building and causing condensation or, even worse, mold. Condensation typically forms first on windows. Condensation will form on a typical double-pane window at a RH above 40 percent when the outdoor temperature is zero degrees or below. For single-pane windows, condensation can occur at a RH above 30 percent when outdoor temperatures are below about 30 degrees.

Honeywell, another large manufacturer of humidifiers, recommends an indoor RH no higher than 35 percent when it is 20 degrees outside, 30 percent at 10 degrees, 25 percent at 0 degrees, 20 percent at -10 degrees and 15 percent at -20 degrees. According to Consumer Reports, when the outside temperature drops below 20 degrees, even 30 percent RH may be too high. What all this means is that if we add too much moisture to a house, we can cause condensation and possibly mold on windows. In some cases, walls can rot.

Realistic Solutions

So what do we do? First, we can't fault wood for being wood or doing what wood does. Second, we can't change the laws of physics and make cold, dry air magically wetter but not hurt houses. Therefore, we are left with a few options to prevent large seasonal gaps.

Solution 1: Go back to the basics. Use narrower boards, more stable species of wood, quartersawn boards, or a combination of those features. Or consumers can accept seasonal 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 Chicago will likely have large seasonal gaps.

Solution 2: Use a product that can handle low-RH environments. Solid wood flooring has been used in those environments for years while, based on warranties, much engineered flooring and some factory-finished flooring apparently are not designed for those environments. Pick a flooring material that can handle the normal, local environment.

Solution 3: Change the building design and/or operation. This solution isn't up to flooring professionals. Builders and building owners can take steps to reduce ventilation rates, and/or add humidifiers. Humidifiers do need routine maintenance, as often as every month during the heating season. And we all know how good we are at routine maintenance. So be prepared for some gapping complaints a couple years down the road.

Bottom line: Winter weather dries out wood flooring, causing gaps, possibly increasing squeaks and opening surface cracks. Wood will be wood. Physics will be physics. Inform your clients of these facts, and don't rely on humidifiers or other sources of moisture to prevent normal winter conditions. Humidifiers can help some, as can choosing the right wood flooring for the right situation, but only to a certain extent. If your client wants 7-inch hickory plank flooring with minimal winter gapping, major changes to the house may be necessary. If hairline gaps aren't acceptable, even 1½-inch quartersawn oak strip flooring may not work. Establish proper customer expectations, and everyone wins.

HF Connect logo For more infomation:

To find dimensional change coefficients, go to chapter 13 of the revised Wood Handbook, available for free download.

For more on how wood flooring responds to moisture, read "Under the Microscope"

For more from author Craig DeWitt, read:

"The Comfort Zone," about how the air in buildings affects wood floors.

and the Troubleshooting column "Look Out Below," about crawlspace moisture wreaking havoc on a solid wood floor.


NWFA Connect:

NWFA Technical Manual A100: Water and WoodNWFA Connect

For more information on how moisture affects wood flooring, read the NWFA's Technical Manual A100: Water and Wood.



        Installing wood flooring    Moisture testing wood floors    Wood floor acclimation        Cracks    Gaps    Moisture               


This was a well written and extremely informative article. Very Helpful!. I've been searching for the information I need, but until now hadn't found anything that was thorough enough for me to make a decision. Thank you!
KLM    10/1/2012 3:15:59 PM

Thanks for this! My building put in beautiful wood floors in my apartment, in July in New York City. No one ever said anything about gaps occurring so when I felt the floor's cracks under my bare feet I really thought something was wrong! Now I get it. I will be interested to see how the gaps change when we get our high humidity levels in summer.
sk    5/3/2013 3:44:13 PM

This is well written and jjust what I was looking for to see how to treat the flooring of a house that was vacant in the winter.
C Nelson   retired  5/12/2013 5:47:12 PM

I had hard wood put in and feathered into existing. I was never warned about gaps. It was installed in August. Now I have numerous gaps in the new and existing. I had numerous issues with the install. So with the money I spent I didn't know this would happen. Is an install issue or will it go back with the warm weather?
PRL    12/15/2013 2:05:23 PM

I'm in the same situation as PRL. Does anyone know what will happen after winter?
NBNY    12/16/2013 7:15:59 PM

It will go back in the summer as the wood regains moisture. Might creek for a bit and then settle back down.
Doug    1/22/2014 10:35:16 AM

Great article. We have a two bedroom high rise condo that is laid out extremely long. We installed an engineered bamboo floating floor throughout the entire unit and laid the floor boards lengthwise down our entire condo unit. But now we see gaps beginning to form around the middle of our hallways. 0.5 inch gaps have opened up in some areas. The gaps are not evenly spaced out throughout. After reading the article, we will reduce the amount we ventilate in the winter. and increase our relative humidity levels. Question: Would you also recommend adding moisture directly to the floorboards by giving them a moist mopping once every week or so in the winter? Thanks!
Thomas  PM  1/24/2014 11:02:18 PM

Thomas, reducing ventilation can only be so much help. With the extreme cold seen in several areas this winter, even minimal ventilation won't stop the long-term drying effect. But if you have a leaky condo, reducing ventilation can help. If nothing else, it may reduce the amount of moisture you need to add. 0.5 inch gaps don't sound right, even for bamboo. Bamboo is funny stuff, at least from a flooring standpoint. It really behaves differently than hardwood flooring. In any case, moist mopping once a week or so probably won't help a bit. Moist mopping ever couple hours might help but you risk causing other problems if too much water is put on the floors. I have a friend who is putting over 4 gallons per day into his house using a humidifier, and he doesn't see a difference. You may have other issues at work.
Craig DeWitt  Forensic Engineer  1/27/2014 5:22:27 PM

If the floor is stained and sealed for the first time in non controlled air, does that permanently damage the wood floors?
Mark   Home Builder  1/28/2014 9:53:47 AM

Mark, This is one of the common misunderstandings about wood floors. The answer to your question is "No, no harm is done." In reality, the air in a house is truly controlled only during the winter when the is running and during the summer, if A/C is installed and running. During much of the spring and fall, temperatures are such that neither heat nor A/C are running. And in much of the north, many houses don't have A/C, so there is no true control in the summer. On top of that, wood floors have worked fine for hundreds of years before the use of A/C or central heat. Acclimation is the process of getting the flooring to a moisture content and size that is somewhere in the middle of the expected annual range of temperature and humidity in the house. That condition usually occurs in the spring or fall, when heat or A/C may not even be needed. As long as the floors are properly acclimated, it doesn't matter if the climate is actually controlled at the time or not. BUT, subfloor moisture and other conditions in the house should still be as required for the flooring to get teh best performance out of teh flooring.
Craig DeWitt  Forensic Engineer  1/29/2014 3:13:40 PM

Thanks for getting back to me. Our two bedroom condo is about 1,000 sqft and our second humidifier just arrived. I will be running that as an experiment. I also bought a humidity meter to help keep track of humidity levels. The bamboo flooring installers/manufacturers suggest that the long length of our unit and the way in which the bamboo boards were laid (lengthwise to complement our apartment's layout) is reason why gaps are opening up now. Again, the gaps are not appearing evenly throughout the unit, but appear only in the hallway where the run of the bamboo boards are the longest. For each "lane" of bamboo board, a one 0.5 inch gap has opened up. They have suggested that we install T-molds in certain parts of the hallway to break up the length of the run and prevent future gaps. We don't like this idea for aethestic reasons. Question: are there any seals? sprays? treatments that could be applied to engineered bamboo flooring to help them retain moisture longer? Is there a glue that could be applied in the short ends of the bamboo boards to reduce the probability of gap formation? I am going to also give the boards a damp mop every other day or so (with just a little bit of water) to see if the extra moisture might close up some of these gaps. Any other thoughts, ideas? Thanks! PS we also thought about nailing the boards to the subfloor, but we cannot because we live in a highrise condo building.
Thomas  PM  1/29/2014 10:12:36 PM

PSS Never mind about the damp mopping. It is probably a bad idea...
Thomas  PM  1/29/2014 10:21:10 PM

Thomas, Sounds to me like you just have boards that have shifted or came apart rather than shrinking. The reason for using t-molds is legitimate. We install alot of floating floors without the transition pieces knowing that we run the risk of end or butt joint separation. We do use an adhesive to prevent the boards from coming apart with very good success. To get the joints that have come apart back together you can put floor jacks at each end of the run and then use a rubber dead blow to draw them tight. A bead of glue in each gap will prevent those from separating in the future.
Trever Mulrony  Owner Home Hardwood Floors  2/1/2014 2:05:47 PM

I had engineered wood floors installed in several rooms of my house in August 2012. When my salesperson called to schedule installation she said that the wood would be delivered several days before installation so it would become acclimated to my house. It was not delivered so I called and was told they would probably bring it to their office to become acclimated to inside temperatures. I asked the installer where he picked up the boxes and he said the warehouse. I am sure it was really hot at the warehouse. Installation took several days so I had the installer bring the boxes in my house. He said he had never heard that the wood had to be acclimated . I have noticed that the wood has gaps. Is this due to winter weather or improper installation?
VSR  Retired  2/9/2014 12:13:27 PM

VSR, There are really too many possibilities to tell what is causing the gaps from your description. Wood floors can be expected to gap some in the winter. If they close in humid periods, they would be considered normal gaps. If the gaps do not close, they would be considered abnormal, and worthy of finding out why. It is probably time to have an inspector looks at your floors.
Craig DeWitt  Forensic Engineer  2/11/2014 5:02:00 PM

Mark, I have read your article on gaps and also cupping. You do a grand job of explaining issues. However, we have been in our home for just a year and are experiencing both! The builder (in January) said the problem was too little humidity and to pump the humidity to 60% for a month. We had not been using a humidifier, but bought hygrometers and they have been 35-37 without adding anything. A professional flooring inspector said our problem was too much humidity in the bamboo floating floor over a concrete slab (11.4-16 evasive and 21-25 non-evasive). He said not to add moisture! We have significant cupping and some of the boards have pulled 1/2 inch away from the wall and there are some 1/2 inch gaps mainly in doorways. The inspector also said he thought the installation was poor. From your experience, what do you think, Craig? Is there any hope for our floor?
BB  Homeowner  2/17/2014 6:02:51 PM

Oops. My message should have been addressed to Craig, not Mark! Sorry.
BB  Homeowner  2/17/2014 6:13:50 PM

We had an engineered, Brazilian Walnut floor installed four years ago. Everything was fine until a month ago. We now have 34 boards in a 400 sq. ft. area that are split, crazed, or cupping. The supplier says this was a tough winter and the low temps and humidity caused this. I understand that this can happen, even in a humidified house. Questions: #1. If this happens during a colder than average winter in the Midwest, why would this product be offered? Do they sell it in the Northern states in spite of the 30% RH minimum warranty? My HVAC contractor says the 30% is virtually impossible to maintain and undesirable when the outside temp is below 20 degrees. #2. Is it normal to see it happen in so many boards? #3. Would this have happened to this extent with solid wood rather than engineered? #4. Is repair or total replacement the better option. Thanks
Kevin  Homeowner  4/5/2014 2:00:51 PM

Thanks Craig, this is an excellent article and really makes me feel better about my floor. I live in Chicago and this last fall had a Brazilian Bloodwood floor installed. At the time of installation, the installer used wood filler for any tiny gaps between the boards. During the winter with board shrinkage, the filler looks bad (all cracked up). I imagine this summer when the boards swell the filler might pop out in some places. Aesthetically, is it best to just leave the gaps alone or use some type of filler? Is there a filler that is pliable enough to withstand shrinkage and swelling? Jeff
Jeff  Homeowner  5/17/2014 1:35:22 PM

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