stryped said:
I plan on seling everything with mesh tape and matic before insulating. I actually thought of wrapping with r 15 bats, unfaced, then using the silver relectix radiant barrier as a vapor barrier.
It is a 30x30 building. I would have a 14 inch trunck about 20 feet long. I would have 4-8 inch supply ducts coming off of this trunck to 4 diffusers in the ceiling, spaced 7.5 feet from each wall if that makes sense.
I understand the description, but this seems to be more complicated than it needs to be for the average workshop. Unless you need very slow moving air, very quiet operation, or very uniform distribution, it is probably not necessary to run a header through the middle of the shop.
I'd encourage a couple of options for the OP, adding to what jrmichler was showing.
If your shop is open, with no partitions, at 30' square, a single discharge will probably be fine. To make this a little better, I'd suggest putting the air handler against the middle of one wall with a ductboard header set on top (to the ceiling), maybe 6-10 feet long. Get three/four (aimable) registers/grilles, large enough you won't have airflow issues or excessive noise. Aim them as you need them to distribute air across the shop, and put in a return where it makes sense--usually you'd stagger the header to the opposite side of the return on the floor. This AC "air dump" or "plenum dump" is very common in work areas, where HVAC systems lack of obtrusiveness is not required, or overhead equipment is an issue. A short duct system like this will tend to be noisier, but for me, I am running tools/machines/etc., and I'd rather be cool than in silence (plus I have a stereo with a volume knob). It also has the benefit of only taking up a very small part of the overhead area in the shop.
I had a choice when I did my current shop (30'x30', with 9' ceilings), to either put in a package unit, or install a mini-split. I chose the mini-split, and it has run admirably. It has been cheaper to operate than the three large fans I kept in the garage in the Florida summer. It runs at variable load, and always (almost) circulates air, so the garage is very uniform in temperature. I really only mention this because It is a 2 ton heat pump with single low velocity discharge, and it warms and cools the shop just fine with zero ductwork, and it is VERY quiet. There are some downsides to mini-splits, however.
I make these suggestions, because ductwork is expensive and it can cost money long term--(hot) attic duct will be a heat/money loss. Every time the unit shuts off, all the attic duct reaches the attic temperature (or close enough to it), and the unit will blow hot air until all the old air has been cleared. Also, attic duct leaks can be a substantial energy loss. Minimizing duct in the attic is not a bad thing.
PS--My previous shop was 35x30 with a 30x20 upstairs, 2.5 ton heat pump, and it only had a single dump downstairs (14"x8") with the return, and worked well for the 35x30x10' high work area. The unit didn't run very much, no variable speed, but it was very comfortable for the gulf coast. The upstairs had 4 registers, but it was a half story with attic on three sides.
Sorry if post has gotten too long...had to stop before I really got going.