Relative Humidity

Relative humidity (RH) is a term that piano owners are concerned with because too much or too little is not good for a piano, being made mostly of wood. Excessive humidity can cause metal parts to rust, and wood to swell. Too little humidity in the air and the wood parts can become dry and brittle, and crack or split, and the glue joints can fail.

As piano technicians, we advise that the piano be kept at 42% RH, but it’s actually the wide swings that can cause more damage.

RH can be a difficult concept to grasp, as witnessed by a recent exchange I had on a piano tuner’s forum where some posters were actually quite angry with me for saying that cold air can be more humid than warm air.

I understood their reaction becasue this is counter intuitive; common experience says that cold winter air is drier than warm summer air.

Before explaining my comment, let’s take some time to discuss exactly what relative humid is.

What is Relative Humidity?

Air can hold water. Hot air can hold more water than cold air. Relative humidity is the amount of water in the air, relative to amount of water the air can hold.
See Wikipedia article HERE

Consider an air mass that has a constant amount of water in it, at different temperatures.

The fraction of Actual Water to Maximum Possible Water is clearly higher in the cooler air.

In other words, cooler air has a higher relative humidity than warmer air with the same water content. Counter intuitive, but true.

Take a look at the data I recorded in a simple experiment. I took a hygrometer, which measures temperature and relative humidity, and put it outside on a warm day. I recorded the temperature and humidity and then brought it inside our air conditioned home. If the cooler air conditioned air, did indeed have less moisture than the outside air, then common knowledge says the cooler air should be drier.

Clearly the cooler air was more humid. (Any error in the exact RH measurements is moot since we are looking at the relative measurements, not the absolute measurements.)

Here are photos of the hygrometer outside in the warm air, and inside in the cool air. Time between photos: about 15 minutes.

Air Conditioned Air

Let’s consider the case where we use an air conditioner to cool down the outside air before it enters a home.

For this situation, we must discuss Dew Point.

Dew Point is the temperature at which the air has been cooled so much that, if the temperature goes lower, the air will not be able to hold the current amount of water, and the water will condense out of the air. We can see this water as dew on the cool morning grass after a cool night.
See Dew Point.

Air conditioners have a drain pipe to export the condensed moisture outside, when the air is cooled below the dew point. Just because AC units have this drain pipe, does not mean that the air will always be cooled below the dew point. Let’s consider the situation where an air conditioner is used to cool outside air, but the air is not cooled below the dew point.

In this case, it is easy to see that, for all inside temperatures lower than the outside temperature, the cooler air inside the home will always be more humid than the outside warmer air, if the inside and outside air contain the same moisture.

But what about when the AC unit cools the air below the dew point and condensation occurs?

In this case, there is a point of equal RH where the air is cooler, has less absolute moisture, but has the same RH as the warmer outside air. Any place in the home that is above that temperature, will have more humid air than outside. Lower temperature areas will have drier air. Again, counter intuitive.

The implications of this data have to do with our possible misconceptions with respect to RH.

It is true that pianos are recommended to be kept close to 42% RH, and that swings in humidity are not good for pianos.

But with this new information, some interesting conclusions regarding maintaining a piano’s RH can be deduced.

For example, if a piano is in an air conditioned home, keeping the piano close to the AC unit with the idea that the colder air will be drier and therefore protect it against the outside humid air, may actually increase its RH instead of lowering it.

Similarly, piano owners concerned that leaving their pianos in a cold garage over the winter will dry it out, can be relieved knowing that the cooler garage air may actually be more humid than the warmer inside air, even if the furnace has a humidifier. (There is a chance of wood damage if the air gets below freezing; the moisture in the wood may freeze and expand.)

I was told this stuff by a friend of mine who is an accomplished high end piano rebuilder and technician, and also a mechanical engineer.

When he first told me, I was confused because it went against my common knowledge. But because of my technical training, he was able to easily explain to me why this was true.

I hope that this article, with the links to Wikipedia, and the graphs I produced, will make it easier for you to understand this subject as well.