Equilibrium moisture content (EMC) is the moisture content at which wood is neither gaining nor losing moisture in the surrounding air. It is set by the relative humidity and the temperature of that air. The chart below gives the published values for 40 to 80°F (4.4 to 26.7°C) and 20 to 80% relative humidity. It is context for reading a moisture figure, not an installation limit: the limits for a floor come from the installation guidance, the adhesive maker and the flooring order. See our engineered oak flooring specifications guide for where moisture sits in the full specification.
The chart
| Relative humidity | 40°F(4.4°C) | 50°F(10.0°C) | 60°F(15.6°C) | 70°F(21.1°C) | 80°F(26.7°C) |
|---|---|---|---|---|---|
| 20% | 4.6 | 4.6 | 4.6 | 4.5 | 4.4 |
| 25% | 5.5 | 5.5 | 5.4 | 5.4 | 5.3 |
| 30% | 6.3 | 6.3 | 6.2 | 6.2 | 6.1 |
| 35% | 7.1 | 7.1 | 7.0 | 6.9 | 6.8 |
| 40% | 7.9 | 7.9 | 7.8 | 7.7 | 7.6 |
| 45% | 8.7 | 8.7 | 8.6 | 8.5 | 8.3 |
| 50% | 9.5 | 9.5 | 9.4 | 9.2 | 9.1 |
| 55% | 10.4 | 10.3 | 10.2 | 10.1 | 9.9 |
| 60% | 11.3 | 11.2 | 11.1 | 11.0 | 10.8 |
| 65% | 12.3 | 12.3 | 12.1 | 12.0 | 11.7 |
| 70% | 13.5 | 13.4 | 13.3 | 13.1 | 12.9 |
| 75% | 14.9 | 14.8 | 14.6 | 14.4 | 14.2 |
| 80% | 16.5 | 16.4 | 16.2 | 16.0 | 15.7 |
Source: U.S. Forest Service, Forest Products Laboratory, Wood Handbook (General Technical Report FPL-GTR-190), chapter 4, Table 4-2. Rows 20% to 80% and columns 40°F to 80°F of the printed table; the printed table also covers 5% to 95% and higher temperatures. Later editions of the Wood Handbook revise the chapter, so check the current edition before quoting a value in a specification.
How to read it
Pick the column for the air temperature, then the row for the relative humidity, and read the moisture content wood would reach if it sat in that air long enough. Two points follow from the chart itself:
- Humidity does most of the work. At 70°F (21.1°C) the chart runs from 6.2% at 30% relative humidity to 9.2% at 50% and 13.1% at 70%. Moving from 30% to 70% adds 6.9 points of moisture content.
- Temperature matters much less. At 50% relative humidity the chart gives 9.5% at 40°F and 9.1% at 80°F, a difference of 0.4 points over a 40°F (22°C) range.
A worked example: a building is held at 70°F (21.1°C). In a dry heated season the air sits at 30% relative humidity and wood in the room tends toward 6.2%. In a humid season at 60% the same wood tends toward 11.0%. The gap, 4.8 points, is the amount a board gains or loses as the seasons turn, and it is why a floor is described by the humidity range of the building and not by one number.
What the published values are based on
The Wood Handbook states that for most practical purposes these values may be applied to wood of any species, and that they were calculated from an equation. The underlying measurements were made mainly on Sitka spruce, under conditions the handbook describes as midway between adsorption (wood gaining moisture) and desorption (wood losing moisture). The handbook also notes that wood takes up less moisture when gaining it than it holds when losing it, with a mean adsorption to desorption ratio of about 0.8 near room temperature. In plain terms, the chart is a practical middle line, and an individual board can sit a little above or below it depending on its history. It is not a measurement of oak.
Common mistakes
- Using the chart as a pass or fail limit. It tells you where wood settles in given air. It does not say what a floor may be installed at.
- Reading moisture content on the wrong basis. Moisture content here is water mass divided by ovendry wood mass. Meter readings and oven-dry tests can differ, so state which method a figure comes from.
- Using outdoor or warehouse humidity. The relevant air is the air of the finished building, with the heating and air conditioning running as they will in service.
- Treating an engineered board as a plain piece of wood. The chart describes wood. How a board built from several glued layers moves is a construction question, covered in our guide to cupping and crowning and in the note on seasonal movement of solid oak.
What to write in an RFQ or specification
- The indoor temperature and relative humidity range of the building in service, taken from the HVAC design or a logger, not from this chart.
- The moisture target for the boards at shipment, agreed with the factory and confirmed in the quotation.
- The installer and adhesive maker limits for site conditions, which they set.
On our side, lumber is kiln-dried and checked against the moisture target for the order, and moisture is checked again before packing, as shown on the Production page.
To send a project with its site conditions, use the Specify page. We normally reply within one working day.