How to Test the Relative Humidity Prior to Painting

Mechanical hygrometers provide a cheap and easy way to test relative humidity yourself.

High humidity levels cause paint to take longer to fully solidify and harden. Different types of paint have different tolerances for humidity and temperature, so you will need to read the instructions on the paint container to determine what the ideal conditions to paint in will be. In general, it is best to paint in mild temperatures with low relative humidity. Testing relative humidity is easy using a hygrometer, however, you will also need to calibrate the hygrometer to ensure accuracy.

Things You'll Need

• 1/2 cup table salt
• 1/4 cup tap water
• Coffee cup
• Spoon
• 1-gallon zip-closure plastic bag
• Hygrometer

Instructions

Calibration

1. Read your hygrometer's documentation for instructions on how to adjust its settings and how to read it. If your hygrometer can't be adjusted, you will still be able to calibrate it.

2. Put the tap water and salt into the coffee cup.

3. Stir the mixture with a spoon for several seconds.

4. Put both the coffee cup and your hygrometer into the plastic bag. Avoid spilling any saltwater on the hygrometer.

5. Seal the bag. Place the bag and its contents in a draft-free area that is out of direct sunlight.

6. Check your hygrometer's relative humidity reading after 8 to 12 hours. It should read about 75 percent.

7. Adjust your hygrometer so that it reads 75 percent, if it can be adjusted. If it can't be adjusted, record the difference in its reading. Whenever you take a reading, you will have to adjust for this difference.

   For example, if your hygrometer reads 70 percent instead of 75 percent, you know that your hygrometer gives a reading 5 percent lower than the actual relative humidity, so add 5 percent to any future readings.

Testing Relative Humidity

1. Place your hygrometer in the area you plan to paint in. Don't set your hygrometer in front of heat registers, chimneys, or any other location where it could be directly exposed to heat.

2. Wait two hours to get an accurate reading. Most hygrometers take some time to adjust to new locations.

3. Check your hygrometer's reading, and compare it to the threshold for the type of paint you're using.

References

• Canada Mortgage and Housing Corporation: Measuring Humidity in Your Home
• Sherwin Williams: Painting Application FAQs
• United States Department of Agriculture: Why House Paint Fails