Based on no scientific measurements -- but hauling lots of pianos
and reed organs out of unheated storage areas (garages, wash houses,
unused out houses, etc.) and having to store some of my own inventory
in unheated areas in east central Illinois -- I have concluded the
following:

The big problem is the moisture, not the temperature.  The glue joints
on reed organs that have been stored in dry unheated conditions are
usually good.  If stored under a leaky roof you had better take a
shovel and a broom.  Pianos that have been in dry unheated storage for
some years all have many bad glue joints.

I feel the main cause is that in the Spring, overnight we may go from
cold weather to warm humid weather.  You have this large cast iron
heat-sink (plate) that is cold.  When warm humid air hits a cold object
you get condensation.  Notice the condensation on a cold can of Diet
Pepsi on a warm humid day.

A rule of thumb formula is that, when you change the temperature 20
degrees Fahrenheit, you double or halve the relative humidity depending
on which way you are going.  When we get one of these sudden spring
warm ups, I have seen so much condensation on farm machinery that you
could see the water drip off.

I do not believe wood changes much in dimension with temperature,
but we know it does change a lot with change in humidity.  One way to
control the relative humidity is to control the temperature.  Raising
temperature is much easier than trying to remove water from the air.

For Larry Toto: if you decide to store the piano in the unheated
garage, have you considered building a box around the piano and putting
a small electric space heater with a thermostat in the box to maintain
a more even temperature.  If the box is well insulated (several layers
of old garage sale blankets, carpets, etc.) a couple of "damp chaser"
rods and a humidistat instead of the space heater might work.

My thoughts for what they are worth.

John Dewey