Stephen Thomas Piano Tuner
Piano tuning and humidity.
Extreme swings from hot to cold or dry to wet are harmful to your piano. Dryness causes the piano's pitch to go flat; moisture makes it go sharp. Repeated swings in relative humidity can cause sound boards to crack or distort. Extreme dryness can also weaken the glue joints that hold the soundboard and other wood portions of the piano together. Moisture may lead to string rust. A piano functions best under fairly consistent conditions which are neither too wet nor dry. Optimally at a temperature of 68 degrees F and 42 percent relative humidity.

Using an air conditioner in humid summer months and adding a humidifier to your central heating system will reduce the extremes of high and low humidity. Room humidifiers and dehumidifiers, as well as systems designed to be installed inside pianos, will control humidity-related disorders.