Hi
Ordinary lead acid release gasses when charging, VRLA valve regulated lead acid can release gasses when charging. AGM absorbed glass mat don't release gasses when charging, or for that matter any battery which is not attitude dependant. ie Can be used in any position, upright, on its side etc. Look at some of the posts on here and you will see people with racks of batteries laying on their sides. Also Ni Cad, Li polymer types, etc. don't leak
What is wrong with building a weather proof box about the size of a decent dog kennel out on your yard or at the bottom of the mast? I extended the roof of my shed and my batteries, only 330 AH are under that on a stand to get them up off the floor.
Brian