Hay barns are tall poles with a roof and open sides.
The hay needs air circulation. If the hay has been treated with sodium ascorbate, acetic acid, or proprionic acid as a preservative in damp or improperly cured hay, increased urination of the animals will become a problem if water intake is not increased. Much below that is expensive straw and it really doesn't matter if it is grass hay or legume hay. Molds and dust can still be a problem even if the hay is too dry, don't be fooled by dryness. To test for moisture content, do what the livestock people do.