Do you really need to keep your relative humidity at 40% or higher if you have a hardwood floor?

Well, what does the science indicate?

Whatever relative humidity you keep a piece of wood in, will reach a moisture content equilibrium with that humid air - sometimes call acclimatizing.

Below is a chart using the following formula from US Forest Products Laboratory 

Moisture content (MC) is a measurement of the percentage of the water in the wood by weight. 

Different species are slightly different, but average saturation of the cellular materials is 28% by weight.  

If you look at the following chart, you can figure that wood kept in a room at 20 C with a 35% relative humidity (RH) will reach a moisture content of 7%. 

Therefore, if you install a piece of hardwood at 7% MC over a subfloor with 7% MC in a building that is kept at 20 C and 35% RH, you have to maintain 35% RH to maintain its stability. Higher humidity will cause swelling, while lower humidity will cause shrinking.

The wood you choose should be stored and acclimatized in the local climate where you are building.

Wood shipped from Ontario to Alberta and installed without reaching local equilibrium will cause you grief and you will have to maintain 40-50% RH.

Wood under a microscope

The cell walls full of water will hold 28% of moisture by weight. 

Once the fibers are saturated the voids begin to fill. 

Therefore a piece of wood that measures 50% MC will be saturated and have some moisture in these voids
1. Check with your local supplier as to the MC of the wood you are buying

2. Make sure you check that your building is stable and the subfloor is dry - use measurement tools!

3. Site finished will require more care and attention 

4. Don't be adding moisture before installing your flooring. Don't pour concrete or be taping & painting.

5. Deliver the wood early and allow it to reach equilibrium only once the building is dry and going to stay dry.

6. Use a data logger to record the RH the weeks prior to installing your hardwood, and adjust your plans accordingly.

7. Use a moisture meter and record the MC of the subfloor before installing the hardwood. 

8. To keep your hardwood stable, you need to maintain the moisture and humidity it was originally installed at.

9. If you are installing hardwood in your home make sure you design the home to handle the required humidity.

10. Install an HRV and a good humidifier. Also install a climate control that will measure and adjust the indoor humidity based on the outdoor temperature.

11. Install good triple pane windows if you install hardwood

Installing high MC hardwood on high MC subfloor in high humidity and you will have to maintain that high humidity.

In cold climates this can be a real problem due to teh risks associated with high humidty.
MAXIMUM HUMIDITY to prevent window sweating 

The above chart shows the maximum humidity levels at various temperatures

Below the recommended RH level should keep your windows from sweating too much.

These temperatures are weather dependent,  wind will require a lower RH, as it will cool the windows further.

Keep the blinds and drapes open and get the air in th e house to move over the windows - circulation.

During prolonger cold outdoor weather, turn down the indoor humidity. Lowering the RH for a week or 2 will not significantly affect the hardwood flooring

The higher the humidity is in the winter months the higher your risk of having moisture inside your walls or attic


Your house operates as a system. You can't simply have high humidity without addressing other issues. 

If you plan on hardwood, get better windows and a good ventilation system that can properly maintain the correct humidity.

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