I thought I’d put some of the thoughts and learnings I’ve had on the topic of humidity control to paper, in the hopes that it benefits other piano owners/buyers, helps dispel any confusion, and generally spurs discussion. And as always, I’m sure there’s no 100% right answer, so I’m hoping to learn more this space, too.

1. What is humidity and humidity control?
Humidity is amount of water vapor in the air. Air isn’t just nitrogen and oxygen, it can hold significant amounts of water (think fog or rainclouds), and the amount of humidity, as measured by “%RH,” means how much water is in the air as a percentage of the amount the water the air CAN hold to become fully saturated in its present condition. It’s “relative humidity” because at given conditions, e.g., higher or lower temperature, air can hold more or less water, so the %RH will rise or fall with temperature, even if the absolute amount of water is the same.

2. Why is important for pianos?
To a large degree, acoustic pianos are made of wood. The case, the rim, the keys and action, and very importantly, the soundboard. Wood is at its heart a water retention and delivery mechanism for plants, so it is very sensitive to changes in humidity. Higher humidity causes the wood to absorb water and expand. Lower humidity causes it to release water and contract, and over time and many cycles of expansion and contraction, the joints start to loosen, glued parts start separating, and the wood itself begins to warp and crack. Even in the short term, changes in humidity can affect the fine tuning stability of a piano, and can cause the action to feel loose, or sticky/sluggish.

So, keeping a consistent, if not constant humidity, and reducing drastic swings in humidity, helps preserve the short-term performance and the long-term life of your piano.

3. What’s the best humidity to keep my piano?
Most piano makers recommend maintaining a relative humidity level between 40-60% year-round, e.g., Steinway, Bosendorfer. Yamaha in some literature says between 40-45%, in others 50-60%. Kawai claims 45% as ideal, and 35%-70% as an acceptable range. So, not every maker agrees to a tee, but you can see a general trend.

From this, it’s probably fine to aim for a range of 40-50% if possible, depending on your local conditions (some places are dry all the time, some are perennially humid, and others have mild or extreme swings between seasons).

If it’s impossible or impractical to keep a piano within a precise range, the next-best thing is probably to minimize the widest swings between highest and lowest humidity.

How do I determine humidity?
A simple hygrometer is all you need. Many are available online for less than $10, even fancy wireless ones that communicate with your smartphone and let you record and track long-range results. Here’s a generic amazon search: https://www.amazon.com/s?k=hygrometer

5. What about temperature control?
While you want to avoid extreme temperatures and drastic swings, one thing to keep in mind is how temperature affects the %RH that the air can hold. Warmer air holds more water, so the %RH will DROP as the temperature rises. Colder air doesn’t hold as much, so it becomes saturated faster and the %RH measurement will be higher for the same given water in the air.

Also note that a lot of room temperature control systems like air conditioning, furnaces, and fireplaces, can directly affect the humidity as well. AC systems and fireplaces both dry out the air, reducing RH.

6. What can I do to control humidity?
If you live in a low-humidity environment, you can use a humidifier. If you live in a high-humidity environment, you’ll want a dehumidifier. There are some units that will do both, depending on a target humidity.

These are available as stand-alone units, integrated units that can be built into your residence’s plumbing and air systems, or dedicated piano-units.

FAQ

1. Whole house, or standalone?
Generally speaking, whole-house units probably do the best job in the gentlest way. But they tend to be expensive and work-intensive to install (perhaps requiring permitting), and you may not be able to easily fine-tune between rooms of the entire residence.

Standalone units are effective, but must be sized appropriately for your room. A 20-30L dehumidifier is probably sufficient for a smaller music room, while a larger open floorplan room may need a 50L unit. It also depends on how much of a change you need to make. Note that dehumidifiers also tend to be very noisy (regardless of what the manufacturer says), and unless integrated with your plumbing, humidifiers need to be regularly filled, and dehumidifiers regularly emptied of water.

2. How fast does the humidity in the air change?
Assuming the properly-sized unit, almost instantly. My piano hygrometers start registering a change in %RH within 15-30 seconds of a 50L standalone dehumidifier starting up 16ft away in an unenclosed, open floorplan room. In my house, it typically takes 15 minutes to drop the humidity at the piano by ₃%.

3. What if I can’t control the humidity precisely?
In some areas or environments, it’s likely impractical or impossible to perfectly control humidity, or raise or lower it by such an amount to reach the “ideal” state. In these cases, your best bet is to aim to reduce the extremes as much as possible - bring it down from 85 to 70, or get to 30 rather than 20, if you can do so consistently and gently. If you can’t move the humidity appreciably, do what you can to keep it consistent (presumably, a piano spending its live at constant 85% humidity is going to be better off than one swinging between 40 and 80 regularly).

You can also look to different methods of maintaining humidity. For those who find constantly running. Whole-house or standalone climate controls system untenable or wasteful, a localized piano-based system like the Piano Life Saver may be an option. I don’t have any experience with this, but it’s supposed to be a way to keep the critical parts of the piano at a steady humidity, regardless of the state of the rest of the room/house.

4. Are aerosol humidifiers dangerous?
The EPA has a warning about “ultrasonic” and “impeller/cool mist” humidifiers. The Mayo Clinic does too, as do some other authoritative sources. In a nutshell, these humidifiers use a standing tank of water, and aerosolize the water to pump up into the air. People generally aren’t good about cleaning these water tanks (especially if you have to fill it 1-2x a day every day or so), and they end up pumping the mold, fungi and bacteria that can grow in those tanks, up into the air. There are other types of humidifiers that are better with this (steam and evaporator humidifiers).

5. Do I need both a humidifier and a dehumidifier?
This depends on your environment, of course. My house typically seems humidity levels between 40-70%, and on a few days a years it may drop into the mid to high 30s. I don’t bother with a humidifier, and just have a dehumidifier set up to keep the piano between 45-47%RH. Some people may live in perennially dry or wet environments, or ones that regularly swing between 10-90% depending on the season.

6. How accurate are consumer grade hygrometers? Do they need to be calibrated?
I've tested about a dozen “cheap’ hygrometers, and built a few manual sensors using commonly-available standalone components such as the DHT11, Bosch BME280 and Sensirion SHT-45. I’ve calibrated many of them with supersaturated solutions of NaCL (table salt) 50 75%RH, MgCl to 33% RH, and potassium chloride to 43%RH. And generally, they’re fine. You may hear that they’re inaccurate, or that they’re only accurate within a narrow range, and while I have found some outliers, overall I’ve found most consumer-grade hygrometers are more than good enough, usually staying within 2-3% RH of each other across a broad range of humidity. My reference sensor is the SHT-45, and the datasheet claims an operating range of 0-100% RH, +/- 1.0%RH between 20-70%, and max deviation of 3%RH between 90-100%.

That’s more than accurate enough for piano tracking.

Here's a recent reading from a few of my hygrometers (you can ignore the teal spikes, I'm running a couple of experiments on a new unit):. These are all within 1-2% of each other.

7. Where do I put the hygrometer?
Anywhere on the piano is fine. Just because they are so cheap and small, I have multiple units on the piano - one with a digital display on plate, another below the action, and one at the back of the beams under the soundboard. The dehumidifier has its own humidistat across the room. They all track each other to within a few %RH, and more importantly, move up and down consistently depending on the state of the AC and dehumidifier.

8. Where should I place the humidifier or dehumidifier?
It’s generally recommended to avoid putting the piano DIRECTLY in line of fire of a humidifier or dehumidifier, or any fan or cooling vents. I think this is especially important for humidifiers, that spray water up into the air (they can often pool/puddle a few feet away). I’ve found that changes in humidity propagate across a room fairly quickly, in less than a minute. So if you can place your humidifier in the room across from the piano, that may be good enough, and it will definitely help keep changes gentle and graduated.

9. What about digital pianos?
Most digital pianos don’t use a lot of wood, if any at all, and are for all intents and purposes impervious to the effects of humidity. There are felts and foams that can moderately swell and shrink, and some DPs have wooden keys or action parts, or even wooden cases/rims/soundboards. But given their relatively shorter lifecycles compared to an acoustic that can function as an instrument for 30, 50, or even 100 years, it’s typically not a huge concern. I've read reports that some wooden-key DP owners experience sluggish/stuck keys, as the wood around the balance pin can swell. But this can usually be address by minor regulation.

So, that’s about what I have. Any thoughts/questions/corrections?