Moisture in wood & wood floors:

This article explains the effects of moisture on wood floors and reviews the proper moisture levels necessary when working with or installing a wood floor.

Moisture Content in Wood Affects Wood Floor Shrinkage, Swelling, Gaps, Finishes. Need for Acclimatization of Wood Flooring Before Installation.

This article series discusses and provides a best construction practices guide to the selection and installation of building interior surface materials, carpeting, doors, drywall, trim, flooring, lighting, plaster, materials, finishes, and sound control materials.

Guide to the Effects of Moisture on Wood Flooring: Best Wood Floor Installation Practices

As described in the book, Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) Chapter 5, Interior Finish:

Understanding and controlling moisture levels is the key to success with wood flooring.

The conventional wisdom of acclimating wood flooring to job-site conditions can cause more harm than good if the job site is not sufficiently dry when the flooring arrives.

[Click to enlarge any image]

Wood flooring installed in very dry conditions and later exposed to high moisture levels can cause problems such as cupping, particularly with wider planks.

In extreme cases, the swelling planks crush the wood fibers along their edges, leaving a permanent “compression set.”

Gaps appear when the flooring returns to its normal moisture content.

Moisture Content in Wood Affects Wood Floor Shrinkage, Swelling, Gaps, Finishes

Wood is a hygroscopic material, meaning that it picks up or gives off moisture to the air until it reaches equilibrium with the relative humidity.

While there is wide variation among wood species and among individual boards, Simpson & TenWolde put the shrinkage percent from green to oven-dry moisture content in Coastal Douglas Fir at

1.8% radial,

7.6% tangential, and

12.4% volumetric. - (Simpson 1999 US FPL)

As it absorbs or releases moisture, the wood swells or shrinks (see Figure 5-6).

Finishes and sealers on the wood slow this process, but do not stop it.

Most hardwood flooring is kiln-dried and delivered with a moisture content (MC) of about 7.5%, which is approximately the equilibrium moisture content for wood at 70°F and 40% relative humidity—typical indoor conditions for most of the U.S.

Need for Acclimatization of Wood Flooring Before Installation

While much has been written about acclimating wood flooring to the job site before installation, in most cases it is the job site that should be dried out before the wood is delivered.

If dry wood flooring is brought onto a wet job site, the flooring will swell as it adjusts, creating unsightly gaps when it shrinks back to normal levels.

Our photo of an exploded wood floor in an old gymnasium (below a roof leak) demonstrates that the effects of excessive moisture on an installed wood floor can be extreme.

Details are

at FLOOR WOOD, DAMAGE DIAGNOSIS. - DF.

Before the flooring is delivered, the building should be closed in, and all concrete, masonry, drywall, paint, and other wet work should be thoroughly dry.

The basement should be dry and the ground sealed in any crawlspaces.

The goal is to have the indoor relative humidity and the moisture content of the subflooring close to the levels they will be after the home is occupied. To sufficiently dry out the site, it may be necessary to run the heating or air conditioning for a week or more prior to delivery of the flooring.

As a rule of thumb, the subflooring moisture content should be no more than 2% over the maximum normal level for that region based on the map in Figure 5-7, and the flooring and subflooring should be within 2 percentage points of each other.

A moisture meter is necessary to determine these levels. Checking the relative humidity with a hygrometer is also a good idea.

With the exception of extremely humid regions such as the Gulf Coast, or extremely dry regions like the arid Southwest, wood delivered at 7.5% moisture content will be suitable for installation in a dry home.

If the flooring needs to be acclimated, unbundle the boards and spread them out in the rooms where they will be installed until they reach a moisture content within the range shown in Figure 5-7 above.

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Wood Floor Moisture Research

• Doggett, Steven M., "Unexpected Expansion of Fiber Cement Panels", [PDF] The Building Exnclosure (website), (14 Feb 2012), retrievedf 12/21/2014, original source: https://builtenv.wordpress.com/2012/02/14/ unexpected-expansion-of-fiber-cement-panels/
• Falemara, B. C., J. M. Owoyemi, and B. Olufemi. PHYSICAL PROPERTIES OF TEN SELECTED INDIGENOUS WOOD SPECIES IN AKURE, ONDO STATE, NIGERIA [PDF] Journal of Sustainable Environmental Management (JSEM) 4 (2012): 16-23.
• Skaar, C., Wood Water RElations, 1988, Springer-Verlag, New York 1988
• Simpson, W.T., ed., "Dry Kiln Opeator's Manual", Agriculture Handbook 188, USDA Forest Service
• Simpson, William, TenWolde, Anton, PHYSICAL PROPERTIES AND MOISTURE RELATIONS OF WOOD [PDF] from U.S. FPL, Forest Products Laboratory, Wood Handbook - Wood as an engineering material", 1999, Gen. Tech. Rep. FPL-GTR-113, Madison WI, U.S. Department of Agriculture (USDA) Forest Service, FPL. - retrieved 2024/02/07 & 12/20/2014, original source: woodweb.com/Resources/wood_eng_handbook/Ch03.pdf
  
  Excerpts:
  Wood exchanges moisture with air; the amount and direction of the exchange (gain or loss) depend on the relative humidity and temperature of the air and the current amount of water in the wood. This moisture relationship has an important influence on wood properties and performance. This chapter discusses the physical properties of most interest in the design of wood products.
  
  Moisture content of wood is defined as the weight of water in wood expressed as a fraction, usually a percentage, of the weight of ovendry wood. Weight, shrinkage, strength, and other properties depend upon the moisture content of wood. In trees, moisture content can range from about 30% to more than 200% of the weight of wood substance.
  
  In softwoods, the moisture content of sapwood is usually greater than that of heartwood. In hardwoods, the difference in moisture content between heartwood and sapwood depends on the species.
  
  The average moisture content of heartwood and sapwood of some domestic species is given in Table 3–3.
  
  These values are considered typical, but there is considerable variation within and between trees. Variability of moisture content exists even within individual boards cut from the same tree.
• Simpson, W.T., ed., "Dry Kiln Opeator's Manual", Agriculture Handbook 188, USDA Forest Service
• Skaar, C., Wood Water RElations, 1988, Springer-Verlag, New York 1988
• Stamm, A.J., Loughborough W.K., Thermodynamics of the swelling of wood, Journal of Physical Chemistry 1935, 39(1):121

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-06-02 by danjoefriedman (mod) - diagnose cause of recurring dark spots on woode flooring

When you replaced the flooring, were there indications of water or other leaks on the surface below?

Tose spots are more or less in the center of boards so I don't think this is somethign seeping up from below. It looks more as if it's coming from deposits onto the floor.

Is this an engineered wood or laminate floor covering or is it conventional thick (say 3/4" thick) wood ton gue and groove boards?

On 2020-05-31 bny Danny

I have had an issue with my floor with black spots about 3 years ago. The floor is about 10 years old. The spots are just in two areas. We have had portions of the floor removed and replaced, but the spots returned about a year latter. The flooring is on a concrete slab. I live close the ocean in Ventura County, Ca. We don’t know what to do.

On 2018-10-20 by danjoefriedman (mod) - why are black spots appearing on my flooring?

No, Diane, though if you attach some photos I may be able to comment further.

From what you've said so far it doesn't sound as if the issue is water below the floor.

On 2018-10-20 by Diane

My apt wood floor (12 years old) is glued to plywood and plywood nailed to cement floor.3-4 inch black spots and markings started to appear. Mostly in exposed areas, very few under rugs. The floor is not cupping and some marks have a silvery tint. Nothing rubs off when wiped. The marks look like burn marks. Any ideas what this is from?

Question: how wet does drywall have to be to require removal -?

Jan 30, 2014) Phil said:
What % of moisture or water penetration into Drywall can it reach before it can not be dried and has to be removed. Also, if there is a time limit on when you have to start the drying process.

Reply:

Phil, that's an interesting question and I haven't thought about wet drywall quite that way. Let's try this:

If drywall is wet above 18% or so, even if just measured on the drywall surface, then conditions are ripe for mold formation; Generally the rate at which mold will grow on wet drywall depends on moisture levels and temperature;

While there are molds that will grow under just about any condition, in buildings drywall mold of any of about 40 common genera/species will show up in 24-48 hours.

Wetter drywall grows different species (such as Stachybotrys chartarum) than less-wet but still too-wet drywall (18% say) (such as Aspergillus sp.).

So I would say that regardless of how wet the drywall is, if we could get it dry - say at 12% or less throughout its thickness - in 24-48 hours and if we don't already see mold growth on it, we're probably OK.

See WET, DEFINITION OF  where we describe the critical moisture level for various building materials

and

see DRYWALL MOLD TESTING

Also use our on-page search box to earch InspectApedia for

Mold on Walls, Drywall, or "Sheetrock®"

where I will re-post this question and where you will find more detail

Thanks for asking

Question: high utility bills with radiant heat floor

(Dec 12, 2014) Margo said:
I live in a small, single-level slab on grade duplex rental in Superior, WI built in early 2012 with water-based radiant heat as the only heat source and no cooling system. What could be causing the following:

1. Oozing of a substance that looks and feels like wet sand from between the seems of the flooring during warmer months

2. Worms coming from either the seams or the exterior edges of the home in warmer months

3. Warping and cupping at the seams of the flooring, including a couple of spots where a perfect circle can be seen in the flooring (about the size of a baseball)

4. Visible gaps at some seams of the flooring (about 1/8 inch each)

5. Utility bills are about twice as high as one would expect, going over $300/month in winter (around $80 in summer)

Thank you!

Reply:

Margo it sounds to me as if water is leaking under your floor and that there are various infestations that need to be addressed as well, perhaps, as structural damage.

If the leak is in a radiant heat floor that could explain abnormal heating bills.

See inspectapedia.com/Energy/Radiant_Heat_Use.htm

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Continue reading at FLOOR, WOOD INSTALLATION GUIDE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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• BUCKLED WOOD FLOORING
• FLOOR, WOOD RADIANT HEAT
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• WOOD FLOOR DAMAGE REPAIR - home
• FLOOR, WOOD INSTALLATION GUIDE
• SIDING FIBER CEMENT COEFFICIENTS of EXPANSION

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