Hardie products are a composite of cement and fiber. They are non-combustible, rot resistant, bug resistant and warrantied by Hardie for 50 years of exterior use. Most of the products come pre-primed for paint, and one of the things I like best about them is that they take paint beautifully. Here in Florida, they are considered to be an upscale improvement.
T-111 is the worst product we can use in Florida -- may do OK elsewhere. It rots in a few years. Most of the T-111 homes around here have a band of cedar about 30" from the bottom where the T-111 was replaced near the ground. I've used HardiPlanks to cover the T-111 on my commercial property and my daughter's house.
Hardi does recommend an underlayment - OSB, plywood, etc., but it can also be installed directly onto studs with a max of 24" on center spacing. It requires a weatherproof barrier like Tyvek. It's kind of brittle, so I wouldn't install it directly over studs without a backing anywhere it might get poked, like with a FEL. Any backing is better than none, so I'm thinking about putting it on over rigid foam or builder's board. You can use foam up to 1" thick.
The only real problem comes in cutting it. If you use a saw, it should be a diamond blade. It will create a LOT of dust, and the dust can eat up the bearings on a power tool. It's best to do it outdoors. If the dust gets wet, it's a bear to clean up. If you're doing a big job, consider buying a hand-held electric shear, like the one's made by Snapper. I have two Snapper shears, and they work really great - no mess, no dust, no silicosis. The only problem is they are expensive. I've seen some a little cheaper, lately, but I paid $450 each for mine at a full service lumber yard (not the cheapest way to buy, but the most convenient).
Unlike vinyl or aluminum lap siding, the stuff looks like real wood once it's installed and painted. Use HardiBoard for corner and opening trim for a truly long life installation.
The real problem for someone with girts is that they go the wrong way for Hardiplank - you need vertical studs. My own barn will be stick built with studs 16" on center. I don't like pole barns in Florida; the water table is so high I don't think the poles will last unless they are marine grade, and I don't trust them, either.
4'x8' sheets of HardiPanel are an option for girts, but I think the joints between the sheets are problematical, both for sealing and for aestehtics. They don't overlap like the T-111 does. They look great with battens to give a board and batten look.
Here's a link to the installation page for HardiPlank:
Hardie installation . From there, you can navigate to other sections dealing with the panels, painting, etc.
