Why Winter Causes Gaps in Your Hardwood Floors: The Role of Humidity

Gaps open between hardwood boards in winter because heating dries out the air in your house, and wood gives up moisture to dry air. As each board loses a little moisture it gets a little narrower, and the space between boards shows the difference. The cold itself has almost nothing to do with it. Gaps that appear during the heating season and close again by summer are normal for solid hardwood. You can keep them small by holding indoor humidity between 30 and 50 percent, and the thing to avoid is filling them.

Why hardwood floors shrink in winter

Wood is always trading moisture with the air around it. In damp air a board takes on water and swells across its width; in dry air it gives water back and shrinks. Given time in steady conditions, a board settles at a moisture level that matches the air. The USDA Forest Products Laboratory, the federal lab that has been measuring how wood takes on and loses moisture for about a century, publishes those matches. At room temperature, a board in 50 percent relative humidity settles at about 9 percent moisture. At 25 percent humidity, it settles at about 5 percent.

A drop of a few percentage points sounds small, but it happens in every board in the room at once, and each board's shrinkage shows up at its edges as a gap.

Why the air in your house gets so dry

Cold air holds very little water. January along the Monmouth County coast averages about 33°F, with most nights below freezing. That air can feel damp outdoors, but once your heating system warms it to 70°F, its relative humidity falls to somewhere around 15 to 20 percent, before cooking, showers and breathing add some moisture back. Outdoors, the same air is not especially dry for wood: the lab's data put wood left outside through a New York or Philadelphia winter at about 12 to 13 percent moisture. Warming that air indoors is what dries the floor.

The more cold air leaks into the house, the drier it gets. Every gap around windows, doors, the attic hatch and the fireplace damper lets in outside air that then has to be heated, so a drafty older house can run much drier than a tight one at the same thermostat setting. Wood stoves and electric heat tend to make conditions drier still.

Does wood expand in the cold?

Barely, and in a heated house the effect runs the other way. Completely dry wood does expand slightly when it warms and contract when it cools. But for wood holding ordinary indoor moisture, the shrinkage from drying out as it warms outweighs any expansion from the heat itself. A hardwood floor is at its widest in humid summer weather and at its narrowest in the middle of the heating season.

Are gaps in hardwood floors normal in winter?

Yes, within limits. Nearly every solid wood floor develops some separation between boards in fall and winter, and if the gaps close again when the weather turns humid, nothing needs repairing. That is the position of the National Wood Flooring Association (NWFA), the trade association that trains and certifies wood flooring installers and inspectors. As an industry group it has a reason to reassure people about wood floors, but here the physics backs it up: a board that loses moisture has to get narrower.

Seasonal gaps are thin, fairly even lines spread through the room. At the Shore they tend to show as the heating season settles in, reach their widest in the coldest weeks, and close through spring as the air turns humid again. Several other problems can look like winter gapping, and the table sorts them out.

What you see	Likely cause	What to do
Thin, even gaps between most boards in winter that close by summer	Normal seasonal shrinkage	Keep humidity in range; no repair needed
A wide gap every few boards, with tight boards in between	Finish has glued groups of boards together (panelization)	Humidity helps; prevent it at the next refinish
Gaps worst near a heat register, radiator, wood stove or sunny window	That area dries faster than the rest of the room	Humidify, redirect the heat, shade the window
Splits running along the grain on the face of boards	Very dry air	Raise humidity; have badly split boards assessed
Engineered boards with raised edges in midwinter	Dry cupping from low humidity	Raise humidity; call a pro if it persists
Gaps still open in late summer	Flooring installed too wet, or a past leak or flood	Professional repair
Gaps along with squeaks, soft spots or loose boards	Possibly a fastening or subfloor problem	Professional inspection

The opposite problem, boards whose edges rise in humid summer weather, is covered in our post on how coastal humidity makes hardwood swell and cup.

How wide should winter gaps be?

That depends mostly on how wide the boards are and how dry the house gets. Solid wood shrinks in proportion to its width, so a 5-inch plank opens about twice the gap of a 2 1/4-inch strip in the same room. The table below uses the Forest Products Laboratory's shrinkage figures for each species to estimate the gap at each board's edge for three common floors, assuming the house sat around 50 percent humidity in summer.

Indoor humidity in winter	2 1/4-inch red oak strip	5-inch red oak plank	7-inch flat-sawn white oak plank
20%	0.03 in.	0.07 in.	0.12 in.
25%	0.03 in.	0.06 in.	0.10 in.
30%	0.02 in.	0.05 in.	0.08 in.
35%	0.02 in.	0.03 in.	0.06 in.
40%	0.01 in.	0.02 in.	0.04 in.

For reference, 1/32 inch is about 0.03 and 1/16 inch is about 0.06. These figures are the most a board would move if it fully settled at each humidity level, and real floors usually move somewhat less: finish slows the exchange of moisture without stopping it, few boards are cut perfectly flat-sawn, and short swings in humidity mostly affect the surface of the wood. If your summers run more humid than 50 percent, as they can in a house near the beach without central air, the boards start the winter wider and the gaps come out larger.

The pattern matters more than any one number. Holding the house at 35 or 40 percent, instead of letting it sink to 20 or 25, cuts the gap at every board by about half or more.

How species and sawing change the gap

Species changes the numbers, though by less than you might expect. Using the same figures, a flat-sawn board of hard maple shrinks about 15 percent more than red oak for the same drop in humidity, white oak about 25 percent more, hickory about 35 percent more and beech about 40 percent more. Cherry moves about 20 percent less than red oak, and walnut and ash about 10 percent less.

The way the log was sawn makes a bigger difference. Wood shrinks about twice as much along its growth rings as across them, so quartersawn boards, cut with the rings standing upright in the board, shrink about half as much across their width as flat-sawn boards of the same species. White oak is often recommended for stability, and quartersawn white oak does move less than any flat-sawn board in the comparison above, but a flat-sawn white oak plank moves a little more than red oak. Our guide to the hardwood species and constructions that handle Shore humidity best goes further into choosing one.

How to prevent gaps in hardwood floors in winter

Solid wood will always respond to the seasons, but you can make the swing much smaller. Indoor humidity is what controls it, so most of the work is raising it, measuring it and keeping dry outside air from pulling it down.

What humidity should you keep for hardwood floors?

Keep the house between 30 and 50 percent relative humidity, and between 60 and 80°F, all year. That range comes from the NWFA, and Bruce, one of the hardwood brands we carry, gives the same range in its current installation instructions for both its solid and its engineered floors. Other products can list different figures (one engineered line's instructions call for 40 to 60 percent during installation), so check the care or installation guide for your own floor. Its warranty is written around that range.

In winter, other limits pull you toward the lower half of that range. The EPA advises against humidifying above 50 percent, because damp indoor air encourages mold and dust mites. And cold glass sets a ceiling. Water condenses on any surface colder than the air's dew point, the temperature at which its moisture turns back into water, and the colder it gets outside, the colder the inside of the window gets. The Fenestration and Glazing Industry Alliance, the window and door manufacturers' association that writes the industry's condensation tests, advises keeping indoor humidity at or below 40 percent when it is 20 to 40°F outside, and at or below 35 percent at 10 to 20°F. Its members make windows, but the advice follows from that physics and applies to any glass.

Moisture that works its way into cold walls causes damage you cannot see, which matters most in older houses. Allison Bailes, a physicist who consults on how houses handle heat and moisture, considers 30 or 35 percent reasonable for a leaky house in a cold climate, and warns that pushing up toward 40 percent can create new and potentially bigger problems.

Put together, a sensible winter target for a Shore house is 35 to 40 percent: closer to 40 in a tight house with good windows, closer to 35 in a drafty older one or on nights that drop below 20°F. The average January day here runs from the mid-20s to the low 40s, which keeps most of the winter inside the window industry's 40 percent limit. If you see water or frost collecting at the bottom corners of the glass, turn the humidity down a few points.

How to check the humidity in your house

A hygrometer, a small humidity gauge sold at hardware stores, is the only way to know where your house sits. Put it in a room with hardwood, at about table height, away from the humidifier, heat registers, the kitchen and bathrooms. Watch the trend over a week or two more than any single reading. Wood responds slowly, so a steady 35 percent does more for the floor than a humidifier that runs hard one evening and sits empty the next.

Choosing a humidifier

If your house has forced-air heat, a whole-house humidifier built into the furnace or air handler treats every room at once and is the simplest way to hold a steady level all winter. A heating contractor installs it. Homes heated by a boiler, baseboard or radiant floors often have no ductwork to carry the moisture, so portable humidifiers in the rooms with hardwood are the usual answer. A unit sized for a bedroom will not humidify an open living area and kitchen.

Portable humidifiers come in several types. The EPA's advice on choosing and using them:

Evaporative units, which blow air through a wet wick or pad, and steam units ("warm mist"), which boil the water, release little of the water's minerals into the air.
Ultrasonic and impeller ("cool mist") units send those minerals into the room, where they settle as a fine white dust. Use water labeled distilled in them.
Empty the tank, wipe it dry and refill it daily, and clean portable units every third day so mold and bacteria do not grow in them.
Keep steam units out of reach of children, since the steam and hot water can burn.

Keep cold outside air out

Because leaks are what bring the dry air in, Bailes puts air sealing first, ahead of any humidifier. Weatherstrip doors and windows, seal around the attic hatch and where pipes and wires pass through outside walls, and close the fireplace damper whenever there is no fire. A tighter house holds the moisture a humidifier adds, so the humidifier runs less.

Go easy on the heat

For the same outside air, warmer indoor air is relatively drier. Warm that January air to 62°F instead of 70 and its relative humidity comes out about 6 points higher, so a slightly cooler house is easier on the floor. Space heaters work against you here: the room a space heater warms is drier than the rest of the house, and the boards closest to it dry out first. Boards in front of a heat register, radiator or wood stove, or in a patch of low winter sun through a south-facing window, dry out faster for the same reason. When one area gaps more than the rest of the room, look for a heat source nearby.

Shore houses that sit empty in winter

If your place at the Shore is a summer house or an off-season rental, the thermostat setting while it sits empty matters to the floors as well as the pipes. The American Red Cross advises leaving the heat no lower than 55°F when you are away in cold weather, so the pipes do not freeze. That setting also happens to be kind to hardwood: the same January air that falls below 20 percent humidity at 70°F sits around 30 percent at 55°F, with no humidifier at all. Leaving the house unheated risks the plumbing and lets the floor drift toward outdoor moisture levels, above the 6 to 10 percent that flooring is installed at. A hygrometer that records its lowest and highest readings shows you what the floors went through while nobody was there.

What to do about gaps you already have

Give it until spring

If the gaps are thin and spread across the room, bring the humidity into range and let the season turn. Gaps that close by summer need no repair. In the meantime, vacuum with a soft brush attachment so grit does not pack into them, and see our winter floor care tips for dealing with the snow, ice-melt salt and slush that get tracked in.

Why you should not fill seasonal gaps

Wood filler, putty and caulk look like an easy fix, but they cause more trouble than the gaps. When the boards swell again in summer, they press against whatever is sitting in the joints. The NWFA's own magazine warns that filler put into gaps when they appear may permanently damage the flooring as the wood expands. Let seasonal gaps come and go on their own.

Gaps that do not close in summer

Gaps still open in August point to something beyond the normal seasonal cycle. Cracks develop when flooring takes on moisture before or after it is installed and then shrinks once the building is heated, whether the wood arrived too wet, sat in a damp house before installation, or later went through a leak or flood (our list of signs of water damage on hardwood floors helps you check). Even floors left with abnormal gaps this way can be repaired so the gaps very nearly disappear, according to the NWFA's magazine.

Repairs range from filling the widest gaps with thin strips of matching wood to replacing split or damaged boards, often finished with a sand and refinish so the work blends in. Timing matters: repairs should be done while the house is at its normal, controlled humidity, so the gaps are not filled at their widest and then squeezed when the boards swell. Our hardwood refinishing work covers repairs along with sanding and refinishing.

When the gaps come in blocks every few boards

If most of the floor stays tight but a wide gap opens every few boards, the finish has probably glued groups of boards together along their edges. Each group then shrinks as if it were one wide board, and the whole group's shrinkage lands in a single gap. This is called panelization, or side-bonding. It happens most with thin finishes that can flow down between the boards before they harden. Finishers prevent it by starting with a sealer made so it does not lock the boards together and by applying the finish in several thin coats. If your floor gaps this way, ask about it before your next refinish.

Installing new hardwood so winter gaps stay small

If you are planning a new floor, choices made before the first board goes down decide how much it will move every winter afterward.

Install the flooring at the moisture level it will live at. The Forest Products Laboratory recommends installing wood flooring at the moisture content it will see in service, which for most of the country, New Jersey included, averages about 8 percent, with individual boards between 6 and 10 percent. Flooring laid while it is too wet shrinks more once the heat comes on.
Acclimation is measured in moisture, not days. Bruce's instructions do not set a number of days. They call for acclimating the flooring, on or off the job site, until it meets the moisture requirements, checked with a moisture meter on both the flooring and the subfloor, and for the house to be held at 30 to 50 percent humidity and 60 to 80°F for 14 days before and during installation. Ask your installer whether they measure and record moisture before they start.
Wide planks need steadier humidity. For solid planks 4 inches and wider, Bruce notes in its instructions that seasonal shrinkage and cupping can be reduced by gluing the boards to the subfloor in addition to nailing them. Ask about it if you are choosing wide solid boards.
Engineered hardwood moves much less. Engineered boards usually put a real wood surface over a core of cross-layered plies, the same construction that gives plywood its stability along its length and across its width. They still need the humidity range: kept too dry for too long, some engineered floors dry-cup, with the board edges rising.
Plan for radiant heat. Radiant floor systems tend to lower indoor humidity, so a humidifier may be needed to keep the floor in range. Our post on choosing floors that work with radiant heat covers what to look for.

Our hardwood buyer's guide compares solid and engineered construction in more detail, and our hardwood catalog carries both.

When to call a flooring professional

The gaps are still open by late summer, or they are wider each winter than the one before.
The gaps are much wider than the table above suggests for your boards and your humidity.
Boards are splitting in many places, or engineered boards are cupping or coming apart.
The gaps came with squeaks, soft spots, loose boards or boards that shift underfoot.
The floor got wet from a leak, flood or storm and has gapped since it dried out.