Research indicates that for health and comfort, a relative indoor humidity of 40 to 60 percent is desirable.
Some say 35%-45% is the ideal level.
Less than 60% in the summer and between 25 - 40% in the winter.
Humid air makes you feel warmer.

This range will provide the best comfort for your family, while helping to protect your musical instruments, drywall, wooden furniture and other belongings or materials from the damaging effects of dryness or excessive moisture.

In winter, however, that level can drop to 10 percent.
Cold air can't hold much water vapor, and the colder the air, the drier it is. That means winter air that makes its way into the house through leaks, holes and combustion air ducts or is pulled in by ventilation fans is going to be dry. Heating that air only makes it drier, and the result often is low indoor humidity levels.

When outside temperatures drop below 20 F, even a 30 percent indoor humidity level can lead to window condensation. So you might not be able to run a humidifier when it gets that cold.

Also be cautious about adding too much humidity to the air. When humidity exceeds 55%, it's the perfect climate for bacteria, mold, and dust mites to grow. A good rule to remember is that there should not be condensation on a cold window or windowsill from the humidifier or vaporizer.

Water or ice will appear on windows, and if there's moisture on windows, it's also likely to be collecting, unseen, in wall and attic cavities. A few seasons of that, and building materials -- sheathing, studs and woodwork -- will begin to rot.

The following list, supplied by the Minnesota Department of Public Service, is based on a double-glazed window and an indoor temperature of 70 degrees. You will notice that the lower the outdoor temperature, the lower the indoor humidity should be.

  • If outside temperature is 20 to 40 degrees, humidity indoors should not be more than 40 percent.
  • If outside temperature is 10 to 20 degrees, humidity indoors should not be more than 35 percent.
  • If outside temperature is 0 to 10 degrees, humidity indoors should not be more than 30 percent.
  • If outside temperature is 10-below to 0, humidity indoors should not be more than 25 percent.
  • If outside temperature is 20-below to 10-below, humidity indoors should not be more than 20 percent.
  • If outdoor temperature is lower than 20-below, inside humidity should not be more than 15 percent.
Source: Home Energy Resource MN: Humidity

My experience with room temperature of 60° and double-glazed windows.
You will get condensation at:
30% humidity with outside temperature at 5-15°.
40% humidity with outside temperature at 20-25°.
45% humidity with outside temperature at 25-30°
50% humidity with outside temperature at 30-40°

If you are building a home or remodeling, demand high-quality windows. They should have U-values of 0.35 or less. Such windows are less prone to cold-weather condensation and icing, which means that indoor humidity can be higher.

Humidifiers and Colds:
I've run a humidifier in the bedroom when I had a cold and with the temperature at 65° the humidity gets up to 60% with a lot of condensation on the windows.

One of the first things most pediatricians recommend when a child comes down with a cold is running a humidifier to help ease congestion.

At Humidifiers Don't Do Lick Of Good Helping Colds : NPR they say they don't help.

Warm air holds more moisture.
Relationship between relative humidity (RH) and temperature.
Formulas
50°F   75%  55%
68°F   40%  30%
i.e. If the humidity is 75% at 50° and you increase the temperature to 
         68° the humidity will go down to 40%

My test
57°F    45%
70°F    32%
________________
48°F    48%
59°F    40%
Rule of Thumb : Relative humidity doubles with each 20 degree (Fahrenheit) decrease, or halves with each 20 degree increase in temperature.
Source: Temperature-Moisture Relationship - Utah State

Musical Instruments:
Most recommend humidity of 40-60% for wooden musical instruments. A range of 45-55% is ideal.

Wood swells and contracts depending on it's moisture content. It gets smaller as it dries out, parts of the instrument like the top are under tension, the perfect condition for the formation of cracks and failure of the joints and seams.
It can bend and deform (sometimes permanently) when it gets too wet.
Taylor Guitars claims that a guitar top will change in width about 1/8 inch with a 20% change in humidity.
Consistency is as important as actual actual level. This is difficult in the east where humidity can get up to 80% in the summer and indoor humidity in the winter can go down to 15%.
The worst thing you can do is change up the humidity frequently and quickly.
NEVER leave your violin in the car on a sunny day. Temperatures can reach 150 degrees!

See: Proper Humidity, Very Important For Your Instrument | BurgessViolins.com
and More than you ever wanted to know about humidity! | violinist.com

Hygrometers
They are notoriously inaccurate. Get one which you can calibrate (see below) if possible.
I got a Taylor Weather guide 1525 and it was 10% high (read 45% when it was 35%).
I did the salt test which should produce 75% humidity. The Taylor digital 1525 read 67% and taylor analog read 52%.

My Vornado humidifier which was supposed to shut down when the humidity hit a preset level thought the humidity was between 40 and 45% when it was 52%.

David Burgess of BurgessViolins.com says "I've tested probably 50 hygrometers, and ONLY ONE was within two percent of the correct reading."
See Hygrometers, Humidifiers, Humidity and de Humidifier Controls for musical instruments | BurgessViolins.com

How to test/calibrate a Hygrometer:
See Salt test

Recommended Hygrometers:
Product Review: Western Humidor Caliber 4R Digital Hygrometer and Thermometer - Steve Jenkins' Blog
Hygrometers - Caliber IV | BurgessViolins.com
Amazon.com: Hygrometers - Weather Instruments: Home & Kitchen
  Amazon.com : Caliber IV Digital Hygromter by Western Humidor 4.5 (179)
See Thermometers

Links:
Humidifiers in Products
Heat Index (Temperature Index)
Fixit: What is the ideal winter indoor humidity level | Star Tribune, Twin Cities, MN<

last updated 24 Feb 2015