What would be the single most important consideration in selecting a location for preserving archival materials, photos, tapes, letters etc.?
You need to select a place that is dry, secure, and safe from fire, without any possibility of the storage site getting worse. Generally a space somewhere in the center of a building (not in the basement nor in the attic), and not under any waste or water pipes. You need to have good air circulation and have reasonable protection from strong light. You need to have good security, making sure that readers are under a watchful eye and that they do not have direct access to the main collections. It would be nice to have HVAC but you should have it active all the time–better none at all than on only part of the time.