Dry and room temperature is usually good. And they need airflow. Maybe in an unused bedroom or a closet?
Sometimes the failure comes from fungus they pick up while they're growing. For best results you are supposed to spray them, but I never have. I always take out the ones that had grown a seed head first and dehydrate them. You'll know which ones they were because they'll have a stiff hollow center stem (after drying) and sometimes have two halves connected. If I start losing any, I'll dehydrate all that are left that don't look big and perfect. I like to watch for onions to go on sale at the store and dehydrate those too. That is usually why they are on sale, because they aren't expected to keep. And they are so handy to cook with when you have them already cut and you can just toss a handful in.
