If there is one technical topic that every person working with reclaimed wood must understand, it is moisture content. More reclaimed wood installations fail due to moisture-related problems than any other single cause. Cupping floors, cracking beams, warping panels, mold growth, and joint failures — the vast majority of these problems can be traced back to improper moisture management. This guide explains the science, provides the numbers, and gives you the practical tools to get it right.
The Science of Moisture in Wood
Wood is a hygroscopic material, meaning it absorbs and releases moisture in response to the relative humidity of its surrounding environment. This is a fundamental property of cellulose and cannot be eliminated — it can only be managed.
How Wood Holds Moisture
Wood contains moisture in two forms:
1. Free water — Liquid water held in the cell cavities (the hollow centers of wood cells). Free water is the first moisture to leave during drying and does not cause dimensional change when it is removed
2. Bound water — Water molecules chemically bonded to the cellulose and hemicellulose molecules in the cell walls. When bound water is removed (below the fiber saturation point, approximately 28-30% moisture content), the wood shrinks. When bound water is absorbed, the wood swells
This distinction is critical: dimensional change occurs only when moisture content changes below the fiber saturation point. Above that point, wood can gain or lose moisture without changing size.
Equilibrium Moisture Content (EMC)
Wood constantly seeks equilibrium with its environment. In any given combination of temperature and relative humidity, wood will eventually reach a stable moisture content called the equilibrium moisture content (EMC).
Typical interior EMC values:
- •Portland, Oregon (year-round average): 8-12% depending on heating and cooling habits
- •Arid Southwest (Phoenix): 4-7%
- •Humid Southeast (New Orleans): 10-14%
- •Heated winter interiors (most regions): 5-8% due to low relative humidity from heating systems
- •Summer interiors (most regions): 8-12%
The key insight is that wood in service is always moving toward EMC, and in most environments, EMC varies seasonally. This means some dimensional change is inevitable and must be accommodated through proper design (expansion gaps, floating connections, breadboard ends).
Why Reclaimed Wood Requires Extra Attention
Reclaimed wood presents unique moisture challenges:
Unknown History
Unlike new kiln-dried lumber, which comes from a controlled drying environment with documented conditions, reclaimed wood may have experienced:
- •Decades of exposure to weather, ground moisture, or interior humidity
- •Uncontrolled drying after salvage (air drying in a warehouse or yard)
- •Exposure to water during deconstruction or storage
- •Widely varying moisture conditions across different pieces from the same source
This unknown history means you cannot make assumptions about moisture content. Every piece must be measured individually.
Moisture Gradients
Large reclaimed timbers often have significant moisture gradients — the surface may be dry while the core retains much higher moisture levels. This is because wood dries from the outside in, and large timbers can take months or even years to reach equilibrium through air drying alone.
A beam that reads 10% moisture on the surface may have 18-20% moisture at its core. If this beam is installed in a heated interior, the core will eventually dry to match the environment, potentially causing deep checking and splitting as the interior shrinks while the surface remains stable.
Re-Wetting
Reclaimed wood that was properly dried may have been re-wetted during storage, transport, or on the job site. Always re-check moisture content at the point of installation, regardless of what the supplier's documentation says.
Proper Drying Methods
Kiln Drying (Recommended)
Commercial kiln drying is the gold standard for reclaimed lumber:
- •Reduces moisture content to a precise target (typically 6-8% for interior use)
- •Equalizes moisture content throughout the piece, eliminating gradients
- •Kills any insect larvae or eggs present in the wood
- •Can be completed in days to weeks, depending on species, thickness, and initial moisture content
- •Relieves internal drying stresses through controlled conditioning cycles
At Lumber Portland, all of our reclaimed lumber is kiln-dried in our commercial kilns to the appropriate moisture content for its intended use. We verify moisture content with calibrated pin-type and pin-less meters before any material leaves our facility.
Air Drying
Air drying is slower but can be effective for less critical applications:
- •Stack lumber on stickers (spacers) to allow air circulation around all surfaces
- •Store under cover to protect from rain but with open sides for air flow
- •Allow approximately 1 year per inch of thickness for hardwoods to reach equilibrium
- •Air drying alone will not achieve moisture content below the local outdoor EMC — typically 12-15% in the Pacific Northwest, which is too high for most interior applications
- •Air drying should be followed by a period in a heated indoor environment or supplemental kiln drying for interior-use material
Measuring Moisture Content
Pin-Type Moisture Meters
These meters drive two metal pins into the wood and measure electrical resistance between them:
- •Accurate for surface and near-surface readings
- •Can measure at specific depths by driving insulated pins to the desired depth
- •Must be calibrated for the correct wood species (different species have different electrical properties at the same moisture content)
- •Leaves small pin holes in the surface
Pin-Less (Capacitance) Moisture Meters
These meters use electromagnetic signals to measure moisture without penetrating the surface:
- •Non-destructive — no holes
- •Measure a larger volume of wood, providing an average reading
- •Less accurate than pin meters for specific-depth readings
- •Can be affected by surface moisture that does not represent the core moisture content
- •Better for rapid screening of large quantities of material
Best Practice
Use both types: a pin-less meter for rapid screening and a pin meter for verification and depth-specific readings. Always take multiple readings across each piece and record the results.
Moisture Content Targets by Application
- •Interior flooring: 6-8% (must be within 2% of the subfloor moisture content)
- •Interior trim and millwork: 6-8%
- •Interior furniture: 6-8%
- •Interior framing: Below 19% (15% or lower preferred)
- •Exterior siding and decking: 12-15%
- •Outdoor furniture: 12-15%
- •Structural beams (interior exposure): 12-15% (lower if the building will be heated year-round)
The Acclimation Imperative
Even properly kiln-dried wood needs time to adjust to the specific conditions of its installation environment:
- •Bring material to the installation site at least 7 days before installation
- •Maintain the space at its normal living temperature and humidity during acclimation
- •Stack material with spacers to allow air circulation
- •Measure moisture content at the beginning and end of acclimation; proceed with installation only when readings have stabilized
Getting moisture content right is the difference between a reclaimed wood installation that brings decades of beauty and one that fails within the first year. It is worth every minute of testing and every day of acclimation.