I have many folks asking what is the correct humidity/temperature for
my piano/organ/musical instrument?  The answer is it varies with your
area.  The most important point is that the humidity and/or temperature
not swing widely.

No, the piano does not have to be in a heated or cooled room and set
to laboratory conditions and stable 24/7.  We in the pipe organ tuning
business are very aware of temperature.  I tell churches to set the
thermostat to never let it get lower than 55 F. and never higher than 85.

We heat or cool the building to service or concert temperature from the
time we start tuning until we finish completely.  This is usually
around 70 F. but varies with the church.  Once we leave, the air system
goes off for a week between Sundays.  When the room returns to service
temperature the pipes or piano strings go back to 99% of the correct
tuning at which they were set.

We try to get the church to install a computerized thermostat that will
set back the temperature without turning completely off.  If the church
can afford the power, the heat should set to not be below 60-65.  If
money is a problem, it can set back to 55 or even 50.  The same goes
for cooling in summer.  The farther away from service temperature the
room goes the less well the pipes return to correct tune.

Doing this is found to be more dollar efficient than letting it cool
down with the heat completely off.  That way it usually takes a day
or two for the heat system to get the walls, ceiling, and furniture to
heat up enough so that the unit is not kicking on constantly.  Money is
saved if the room is kept warm enough to heat back up to normal in only
a few hours.

It can be disastrous to have a piano or organ in a very damp room or
basement for long periods of time, but if  you can keep it short of a
mildew garden it would be most helpful.  It is better on the piano if
it can be in a very dry climate.  The most important thing is to have
the humidity stable without wide variations during a week period.  If
summer is one reading and winter is a very different but stable reading
that is more acceptable.  One swing per year does less damage than one
swing per day.

You do not want humidity to be very much below 30% unless it is that
way normally year round.  When the humidity gets to be in the teens or
lower, the soundboard cracks open up.  When recrowning soundboards,
we put them in a dry room of around 10% humidity or less and all the
cracks open up for repair.  This is the only way to make sure the
cracks do not open back up once the piano is finished with restoration.
Most pianos do not see very low humidity under normal use.

Ideally you do not want humidity too far above 75%-80%.  If it is
normally around that high, be looking for signs of damage like mildew
or condensation on the sides of the piano.  I suggest you purchase a
good hygrometer either dial or digital.  I prefer digital for around
$50, but a calibrated dial unit would be fine as well.

Any hygrometer of the old fashioned kind may be calibrated yourself.
Take a dish towel and wet it.  Squeeze it out to not drip and wrap the
hygrometer in it completely.  Leave it about 15 minutes and check the
gauge.  It should be 100% humidity.  If it is not open up the unit and
there should be an adjustment inside.  If there is not you can usually
pull the needle off and press it back on to point at 100%.  Reassemble
and remove towel.  When the unit goes back down it should read the
correct humidity for your room.

D.L. Bullock
http://www.pianoworld.us/