I'm going to jump in here.....I am trying to decide nearly the exact same thing in nearly the exact same area. Our 64x40 with 16' walls is well insulated with R19 batts in the 6" walls, R50 cellulose 20" in the ceiling, and R10 rigid foam under the 5" cement floor. We are putting liner panels in next, and there is a 20'x40' loft at one end. No windows and only one man door, with only one 14'x16' insulated overhead door. We are installing a 100" ceiling fan any day now as well.
It has been at this stage insulation-wise for the last 2 winters, which admittedly were milder than normal for this area. However, with no heat input at all and minimal direct sunlight (building is well-shaded by another large barn 30' to the south) it never dipped below 32 inside. In the summers on 100 degree days inside only got up to 84 degrees. Very happy with that, insulation definitely worth it.
I have tried to do the Manual J load calc's here but am no professional. With minor variation on the inputs I get cooling btu's from 6946 up to 10771. And heating btu's are closer at 96654 to 104400 but I am still very unsure of what I actually need.
I like the idea of the MrCool 36,000 with two 18,000 head units. I am doubtful on its ability to heat our building on an average 20-30 degree winter day. Most of our winter days fall in this range. Cooling isn't as big a concern for us as simply keeping the humidity under control, which I believe it can manage.
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