Wood in a heated interior expands when humidity rises and contracts when humidity falls. This is not a defect — it is the physical behavior of a hygroscopic material. Specifying wood interiors that perform through freeze-thaw seasons and wide humidity cycles requires calculating the expected movement and designing the installation to accommodate it. Designing against wood movement always fails. Designing with it produces systems that last.
The Mechanism: Moisture Content Drives Dimensional Change
Wood dimensions change in response to its equilibrium moisture content (EMC), not temperature directly. EMC is the moisture level the wood reaches when in equilibrium with the surrounding air at a given relative humidity and temperature. As RH rises, EMC rises, wood fibers absorb moisture and swell. As RH falls — in a heated winter interior — EMC falls, fibers release moisture and contract.
The key numbers:
- A 1% change in moisture content produces approximately 0.25% dimensional change across the grain (tangential, in flat-sawn lumber)
- Radial movement (in quartersawn or rift-sawn lumber) is roughly half: 0.12% per 1% MC change
- Movement along the grain (longitudinal) is negligible — less than 0.01% per 1% MC
For a typical heated mountain interior in Colorado at 2,200 meters elevation: winter occupied RH might be maintained at 30-35%. Winter unoccupied RH in a weekend home might drop to 10-15%. The range is 20 points or more. At 20-point RH swing, the EMC of most hardwoods shifts by roughly 4-5%.
Calculation for a 150mm wide flat-sawn white oak board:
- EMC change: 4%
- Tangential shrinkage coefficient: 0.00268
- Movement = 150 x 4 x 0.00268 = 1.6mm
That 1.6mm per board, across a 3-meter floor with 20 boards, is 32mm total movement — requiring 16mm of clearance at each side wall if the floor is installed symmetrically. Standard floating floor installations specify 10-12mm at perimeters; this calculation shows that a 10mm gap is inadequate for a wide-plank floor in an extreme dry climate.
Freeze-Thaw and the Envelope Connection
Outdoor freeze-thaw cycles do not directly affect interior wood — the wood responds to the interior conditions it is exposed to. But freeze-thaw cycles affect the building envelope in ways that alter interior conditions.
Air infiltration paths. Thermal cycling causes building materials to expand and contract at different rates. Sealants at window frames, floor-wall junctions, and penetrations fatigue and crack over multiple freeze-thaw cycles. These gaps allow cold, dry outdoor air to infiltrate, creating localized drying events near the envelope. Wood surfaces adjacent to these infiltration points — window sills, walls below windows, floors near exterior doors — experience accelerated moisture loss relative to the rest of the room.
Vapor drive reversals. In shoulder seasons (spring and fall), the vapor drive direction reverses. What was outward vapor drive in winter becomes inward vapor drive in spring as outdoor humidity climbs above interior humidity. Assemblies designed only for winter vapor drive conditions can experience condensation events during spring reversal. Wood panels in these assemblies need the ability to absorb and release moisture from both sides.
Species Shrinkage Data: Choosing Correctly
Every wood species has published shrinkage coefficients from green to oven-dry, and from this data, the in-service movement at any given MC range can be calculated. Some key values (from the USDA Wood Handbook):
| Species | Tangential (%) | Radial (%) | T/R Ratio |
|---|---|---|---|
| White oak | 9.0 | 5.6 | 1.6 |
| Red oak | 8.9 | 4.0 | 2.2 |
| Ash (white) | 7.8 | 4.9 | 1.6 |
| Douglas fir | 7.6 | 4.8 | 1.6 |
| Walnut | 7.8 | 5.5 | 1.4 |
| Hard maple | 9.9 | 4.8 | 2.1 |
Lower T/R ratios indicate more stable movement — the wood moves similarly in both directions, reducing the tendency to cup. Species with high T/R ratios (red oak, hard maple) are more prone to cupping in wide flat-sawn boards and require rift or quarter-sawing for floor and wide-panel applications.
Detailing for Movement: Floors, Walls, and Thresholds
Floors. Perimeter expansion gaps sized to the full expected movement range, covered by base molding or a floating base profile. For wide-plank floors (boards over 100mm), a floating installation (no adhesive, click or secret nail) is preferred because it allows the floor to move as a unit. Glue-down wide-plank floors at altitude need a moisture barrier below and carefully controlled adhesive selection.
Wall panels. Shadow reveals or open butt joints between panels allow each board to move independently. Detail the reveals to be consistent at both the installed and contracted dimension — a 4mm reveal that reads as 2mm in winter and 6mm in summer is a failure of detail. Size the reveal at the midpoint: 4mm at design EMC, 2mm at max expansion, 6mm at minimum contraction.
Thresholds and transitions. Transitions between wood and hard materials (stone, concrete, tile) need a compressible expansion joint — not a rigid sealant bead. The wood moves; the stone does not. A rigid joint at this transition cracks or delaminates.
Stair treads. Face-fastened stair treads in a cold climate must be fastened with screws at both ends — adhesive alone fails over time as the tread moves against the stringer. Pre-drill oversized pilot holes to allow for tread movement without tearing the fastener through the wood.
Próximos pasos
Wood movement in cold-climate interiors is fully predictable from published data. The calculations are not complex, and the detailing solutions are well-established. The failures that appear in winter — cupped floors, splitting panels, cracked thresholds — are almost always the result of skipping the calculation, not of any material deficiency.
Conoce el método de MÉTODO to understand how we build wood interior specifications from climate data and movement calculations rather than from convention alone.