Great input for everyone, especially RS_not.
Our ranch is is the middle of South central Texas so precipitation (or participation for that matter as we are in the middle of nowhere) is rarely an issue (extreme heat, yes).
The new barn (metal) will be located a couple hundred feet from the house and will not have true HVAC, only a window unit for heating/cooling. It can of course be turned off during recording sessions. This will basically be a room within a room as the structure will be completely sheathed on all sides and top with plywood and then sheet rock on all sides. The top will remain plywood because I will use that space for storage. The side walls of the barn are 14" so there will be plenty of room above the studio room.
The room will also be used for jam sessions/practice sessions so I need to make sure the space is multi-purpose. I'm actually thinking of building space into a wall for my PA speakers so as to conserve floor space. I want to add a raised space along a wall for amps to provide easy access. Drums will be located on a small riser in the corner.
The barn also includes a 40X12 covered porch so the band (and the party) can move outside when the friends and neighbors gather at the pool.

Todd