Think about what's in the room above your heirlooms, too. Heat causes damage. Don't hang valuable objects over radiators, heat-producing appliances or the fireplace. Books you want to read 20 years from now shouldn't be shelved on the mantle, the window sill or the radiator.
Light causes fading and other damage.
Don't put valuable books and paper where they'll get direct sun or bright light of any kind. Hallways or other rooms without windows are best. Install and use shades and heavy curtains where you can't avoid windows. Indoor pollution is a growing problem in energy-conscious spaces with good insulation, and causes rapid damage to paper. The glass or plastic glazing of a frame will keep pollution and dirt away, and the item's edges will not be damaged by handling or tacks.