While extreme heat can warp the celuloid and dry 
the paint out, cels need to be protected from cold 
too, because the cold will temporarily harden the 
paint and make it crack or break away from the 
celuloid, so the fridge is a bad idea.  
It's better to keep them at room temperature of 
around 70-80 degrees faranheit with a slight 
humidity and away from sunlight.  
Think of how it feels comfortable to you, because 
your skin reacts much the same way.  Too cold and 
dry makes your skin tight and flaky, while too hot 
and humid makes your whole body feel sluggish.
My cel books hang on brass hangers in my hall 
closet that borders with the bathroom.  I always 
keep my condo at about 75 degrees.  I'm happy and 
my cels are happy.  ^_^
.