My potatoes sprout and my onions rot in storage
Because you are storing them together, and they want opposite conditions. This is the most common storage mistake there is, and separating them is free.
What actually works
Cure first at 50–60°F for 10–14 days to heal the skins. **Below 40°F the starch turns to sugar** — they go sweet and fry badly, though they're still food. Light turns them green and bitter, so dark means dark.
Nearly the opposite of potatoes. **40–65°F is the sprouting range** — the temperature most people's 'cool dry place' actually sits at, which is precisely why their garlic sprouts in January.
Apples give off ethylene, which sprouts potatoes. Onions and potatoes exchange moisture and both go off faster. Separate rooms if you can, separate corners at minimum.
A third set of conditions again — a cellar is too cold and too damp. A spare bedroom is usually right.
One soft potato ruins its neighbours, and no sensor you can buy will ever tell you about it.
Also searched as
storing potatoes · potatoes sprouting · onions rotting · garlic sprouting · root cellar conditions · how to store onions · squash storage · cure potatoes