Figure 5-6: (C) J Wiley, S Bliss Effects of Moisture on Wood Flooring

  • FLOOR, WOOD MOISTURE - CONTENTS: Flooring, effects of moisture on floors. Moisture Content in Wood Affects Wood Floor Shrinkage, Swelling, Gaps, Finishes. Need for Acclimatization of Wood Flooring Before Installation
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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.

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.

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

Figure 5-6: (C) J Wiley, S BlissAs described in the book, Best Practices Guide to Residential Construction 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

Buckled wood flooring (C) Daniel FriedmanWhile 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

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.

Figure 5-7: (C) J Wiley, S Bliss

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.


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