Humidity is more an issue than heat. Wood, balsa included will swell and shrink across the grain with humidity changes. Every thing expands and shrinks with heat, some things more than others. But it is a more uniform size change. Green wood is the most prone to warping as it dries out and twist, mainly because it is stacked tight and looses moisture from one side prior to the other. Look at a stack of premium hardwood for furniture. It is stacked with cross pieces so 80% or 90% of the wood is exposed to air and the changes are more or less uniform.That keeps things almost flat.
I live on the central coast of California. Not a great deal of temperature changes compared to Michigan or New York where I lived before. We do get big changes over the matter of a couple hours. It is not uncommon to hit 90 during the summer day to wake up the next morning with a temp of 50, or even go to bed with that outside temp. and then reaching near 90 mid day the next day. This doesn't seem to bother my planes and they are in an unheated, un-cooled garage. What I did do for "ME" was cut 2" foam sheeting and pack it into the Garage Door. Winter days were awful for working in the garage and most summer days after noon were way to hot to work. Our garage door faces south west, and was a dark brown color, so the heat was a real problem. I packed in the Foam and painted the outside a light color and the world changed in my shop. There are some days when the heat has been high for a couple days and it get uncomfortable to work, and then in the winter, I have to run my little electric heater for a few hours before I go out to work, but just insulating the big door really changed things for the positive for working in the garage year around. And it is not an air tight door.
My stacks of balsa sheets still lay flat year around.