Winter dry air and your epoxy table
When the heat comes on, indoor air gets dry. Wood responds by giving up moisture and shrinking across the grain. Cured epoxy doesn’t swell and shrink with humidity, so the wood next to it is doing all the moving. Here’s how much that can be, and how to keep an epoxy table happy through the heating season.
- Wood at 70°F reaches about 9.2% moisture at 50% relative humidity, but only about 6.2% at 30% and 4.5% at 20%.
- Using the Wood Handbook’s shrinkage coefficient, a 15 in. wide flatsawn walnut slab can shrink about 1/8 in. going from 50% to 30% humidity.
- The EPA suggests keeping indoor humidity below 60%, ideally 30-50%, which also suits wood furniture.
How dry wood gets
The USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282) (Table 4-2) gives the moisture content wood settles at for each temperature and humidity. At 70°F:
| Relative humidity | 20% | 30% | 40% | 50% | 60% |
|---|---|---|---|---|---|
| Wood moisture content | 4.5% | 6.2% | 7.7% | 9.2% | 11.0% |
How much wood moves
The Handbook’s Table 13-5 gives a dimensional change coefficient: the fraction a board changes across the grain for each 1% change in moisture content. For black walnut it’s 0.00274 tangential (flatsawn) and 0.00190 radial (quartersawn). For sugar maple, tangential is 0.00353.
Example: a 15 in. wide flatsawn walnut slab going from 9.2% (50% RH) to 6.2% (30% RH) changes 3.0 points. 15 x 0.00274 x 3.0 = about 0.12 in., roughly 1/8 in. The same change in sugar maple is about 0.16 in. Go down to 20% humidity and it’s more. For the full math, see how to calculate wood movement.
What you might notice
- Hairline gaps or stress where wood meets resin, especially at the ends.
- Small checks (splits) opening in the wood, often at existing defects.
- A slight cup in wide slabs if one face dries faster than the other.
Well-built tables are made from wood dried to the right moisture for its home, which limits this (see target moisture content by region).
Winter care checklist
- Use a hygrometer. The U.S. EPA: A Brief Guide to Mold, Moisture and Your Home notes they’re a small, inexpensive ($10-50) instrument at many hardware stores.
- Aim for 30-50% humidity, which the EPA recommends for homes. A room humidifier can help in very dry climates; don’t overshoot, since the EPA also warns to keep it below 60% to prevent mold.
- Keep the table away from heat vents, radiators and fireplaces, which dry one area faster.
- Don’t push a dry table against a cold exterior wall or window where condensation forms.
- Keep up normal care (see how to care for an epoxy table).
Related: seasonal wood movement chart for hardwood tabletops and wood moisture content by humidity.
Header image: Photo: “Oberfläche Wohnzimmertisch” by Martin Lorenz, CC BY-SA 3.0 (cropped).