During the winter, even the most carefully installed wood
floors tend to dry out and shrink. Homeowners begin to notice gaps between
boards. The floor behaves that way because of wood’s relationship with moisture
in the. 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. 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. 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-inchwide 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-inchwide 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
Because 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
As 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. The 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. 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. 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, we mean the kind
of weather in South Carolina. By real cold, we 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. A 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. 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 the homeowner 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.
No comments:
Post a Comment