I have heard for some time that fine art can be damaged by temperature variances on exterior walls. And think about it. Your art is essentially providing a thermal break between the warmer air inside your house and the very cold temps outdoors.
But we have insulation! Yes, you probably do. And it might be well placed and sufficient. But a thermal break is just that and moisture can develop.
Case in point. The bottom of this image is the warm bed frame in a master bedroom.
On the wall above the bed was hanging a large mirror. Cooler temperatures are defined by lavender and purple. You can see the defined cooler lines along the edges and on the corners at the bottom where the mirror was hanging. As the mirror hung the lower half and corners touched the wall.
The top of the lavender/purple area is less defined because, we know, as something hangs on the wall the top is further away. Things hang at an angle.
Two surprises -
1. The purple line on the left is a wall stud and is 46F!
2. The back side of the mirror had mold on it! Not a lot, but mold nonetheless. And this mirror was hanging over the bed!
If you live in a climate with wide temperature variances, hot or cold, and have fine art, don't think it cannot be damaged by hanging on an exterior wall. It can! This was a mirror, not fine art, but BEWARE!
My recommendation: place your finer art pieces on interior walls that do not experience wide temperature swings. It is less likely to be damaged by condensation, warping, or mold.