Originally Posted by
John Donhowe
A few things come to mind. First, dollar per R-value, it's hard to beat fiberglass insulation, esp. for a DIY project, and in a mild climate. Since heat rises, most heat loss is from the ceiling, so put as much there as you can. Also, since roofing material is usually a darker color than siding, and facing more skyward, roofs absorb more heat in the summer, so more heat would pass into your ceiling (per sq ft) than the walls.
As far as ceiling height, I'd personally go with higher ceilings. First, it would give you a lot more storage space- you can store long pieces of lumber and plywood vertically, if you wish, and install taller shelves or cabinets. Second, it makes it easier to wrestle around tall or long projects (say a wall unit/tall bookcase) without worrying about hitting the ceiling. The change in heat loss due to increased height is probably not all that great- again, the ceiling heat loss would be the same, so the only difference would be the increased area from the perimeter walls.
You mentioned that your overhead door was 15 feet, which I assume is the width. If it's the door height, though, you're obviously locked into a 15+ foot ceiling to clear the track, or else drop the ceiling behind the track, which seems overly complicated.
Installing a ceiling is better for insulating than leaving open and insulating between the rafters. If you do so, make sure you ventilate the "attic" above the ceiling well. To reduce heat gain in the summer, really consider installing an attic ventilating fan in the roof.
As far as siding, others have already recommended finding the water damage source, and ideally enclosing the building with sheathing and house wrap. That is a pretty big undertaking, and it might not be worth the effort, if you can otherwise eliminate the cause of your water damage. There are a lot of factors to consider, such as weather exposure and climate. Do you get a lot of wind-driven rain, that could penetrate the siding and cause internal damage? One reason for house wrap such as Tyvek is to act as a moisture barrier to outside water, while still letting water vapor to escape from the interior. I'd say if you're going to pull off the siding anyway, go ahead and sheath and house warp. One alternative would be to consider the current siding as a sheathing of sorts, wrap over it, and put a new siding over the wrap.
It sounds as though you live in a mild climate, so water vapor escape isn't an issue because: 1) water vapor is a problem in cold climates, since in will condense (and freeze) within the walls, causing water damage and reducing insulating capability; most household water vapor is generated from activities such as cooking, showers, laundry and the like, which you're not likely to do in your shop.
If you plan on covering the interior side of the walls, I think it's pretty hard to beat plain old drywall in terms of dollars/square foot. Good luck- I'd love to have that much space for a shop!