TECHNICAL BULLETIN: WINTER AND HARDWOOD FLOORS: UNDERSTAND, PREVENT, PRESERVE
Objective: To provide consumers and professionals with a better understanding of hardwood behavior during the winter season, enabling them to apply proper prevention and maintenance practices that ensure the stability, durability, and aesthetic quality of wood floors throughout the year.
When temperatures drop and heating systems run at full capacity, our hardwood floors undergo significant environmental changes. Wood, a natural and living material, reacts to these shifts — sometimes visibly, sometimes more subtly. Better understanding these movements helps take the right steps to preserve the appearance, comfort, and durability of your floor.
Wood and Humidity: A Dynamic Relationship
Wood is hygroscopic: it absorbs or releases moisture depending on its environment. This phenomenon causes inevitable dimensional movements, both in width and thickness.
In summer: warm, humid air causes the wood fibers to swell, and in some products, show cupping (raised edges).
In winter: dry air extracts moisture → the wood contracts, creating visible gaps between boards in all product types (solid and engineered), especially in intensively heated rooms (wood stove, radiant heat, etc.)
Note: For engineered products, overly dry environments can cause shrinkage of the top lamella, giving a cupping appearance (raised edges).
These movements can vary depending on:
- The wood species (oak, maple, walnut, cherry…)
- The equilibrium moisture content of each room
- The product type (solid vs engineered)
- The installation method
NWFA Reference: The National Wood Flooring Association confirms that such movements are “normal and inherent to all wood floors,” though their extent can be reduced with proper environmental control (NWFA Installation Guidelines, section 1.2.3).
Visible Effects and Collateral Risks
When humidity drops too quickly or for extended periods, the consequences for the wood become more significant — and sometimes irreversible:
- Gaps between boards: typical signs of drying
- Surface checking: small visible cracks, often perpendicular to the grain
- Internal microcracks: not immediately visible but weaken the wood
- Through-cracks: can appear with thermal shock or extremely low humidity (< 30% RH)
- End checking: usually occurs at board ends
Some species, like maple, hickory, and white oak are more prone to surface checking, while others, like red oak, are more resistant.
According to the NWFA, these effects appear “when indoor conditions stray too far and too long from the recommended relative humidity range, causing excessive stress on wood fibers” (Technical Publication No. A100).
Relative Humidity: Your Best Preventive Tool
The ideal indoor humidity range for wood flooring is between 35% and 55% (NWFA). This level:
- Maintains the wood’s hygroscopic balance
- Reduces internal mechanical stress
- Preserves joint and finish integrity
Good tools:
- Humidifier: ideally integrated into the central heating system
- Thermo-hygrometer: for real-time monitoring
- Dehumidifier (summer) or air exchanger for seasonal regulation (air exchanger use must adapt to outside RH and airflow quantity)
Maintenance tip: Avoid sudden changes — e.g., never increase or lower heating by more than 2–3 °C at once, as it amplifies stress.
Radiant Heating: Optimal Comfort, But Watch the Controls
Radiant heating (heated floors) offers unmatched comfort, especially in winter, with soft, even warmth ideal for living spaces. However, combined with hardwood flooring, it requires specific technical precautions.
Impacts on wood:
Heat applied directly below the wood leads to faster fiber drying if ambient RH is not controlled.
Excessive or abrupt temperature changes can cause:
- Excessive contraction
- Internal or surface microcracks
- End checking
- Finish delamination in some species
Recommended best practices:
- Limit surface temperature: NWFA recommends never exceeding 26.7°C (80 °F) at the floor surface.
- Gradual temperature change: increase or decrease in 1–2 °C (5 °F) steps per day.
- Use precise controls: programmable thermostat with integrated temperature probe to avoid spikes.
- Stable relative humidity: even more critical with radiant heating, since heat increases drying effects.
Product compatibility:
- Engineered wood: strongly recommended with radiant floors for its better dimensional stability.
- Solid wood: possible in some cases (with suitable thickness and species) , but requires technical support.
- Narrow widths: preferred to limit visual effects of movement.
NWFA Reference: “Radiant heat can be used with wood floors if temperature, humidity, and product selection guidelines are strictly followed.” (NWFA Radiant Heat Guidelines, 2022)
Product Selection: Stability and Movement Tolerance
The behavior of a floor in winter greatly depends on the product type:
- Solid wood: more prestigious but more reactive; needs stable indoor climate
- Engineered wood: cross-layer structure offers better dimensional stability, especially in urban areas with large RH variations
- Prefinished vs oiled: some finishes may show more visible microcracks, but not necessarily affect structure
- Board width: wider boards are more prone to visible contraction
According to NWFA:
“Engineered floors are recommended in environments subject to significant humidity fluctuations.” (NWFA Wood Flooring Technical Manual)
Installation Method: A Key Factor
- Proper installation is essential to minimize winter effects:
- Floating installation: offers better tolerance to movement (provided there is no contact with fixed structures), though it tends to transmit more sound or wave-like effects
- Nailed or stapled installation: provides a rigid, more stable fastening, distributing the movement and stresses caused by environmental conditions.
- Glued installation: suitable for engineered wood installations where fasteners cannot be used and requires adhesives compatible with both the wood and ambient conditions.
- Expansion gaps: essential for all types of installations around the perimeter to absorb wood movement.
** For nailed or stapled installations using boards wider than 5”, refer to the glue-assist installation method.
IMPORTANT
All installations must be preceded by an acclimatization period for the wood (between 48 hours and 7 days depending on product, species, location, and normal living conditions).
Conclusion: Support the Wood, Don’t Restrain It
Hardwood flooring is not static: it lives with the seasons. But with proper care, knowledge, and maintenance, it can age with elegance and stability.
Best practices summary:
- Maintain relative humidity between 35–55%
- Monitor your environment with a thermo-hygrometer
- Choose a product adapted to your climate and intended use
- Entrust installation to a professional who follows NWFA and manufacturer guidelines
- Be informed, that slight seasonal variations are normal and expected
Official resources: To learn more, visit the NWFA website – Technical Resources for guides, infographics, and practical sheets for professionals and consumers.
