Furnace in an unvented attic

I am building a new home and sealing the entire envelope and roof deck with closed cell spray foam insulation. I believe that I can only put a high eficiency non combustible furnace in the attic as the whole attic is non vented. My HVAC contractor wants to instead build a closet or small room around the furnance and vent just that room to the outside while keeping the attice totally nonvented. He states that he prefers to put an 80% efficiency furnace in an attic. Any thoughts? Also If I use spray foam over batt insultaiton how much can I cut back on heating BTU's and air condition tonage. My HVAC contractor wants to put in the same units that he would as if the house was insulated with batt insulation in a vented attic.

First, I would never put foam over batt fiberglass insullation. One of the benefits of foam insulation is that it creates an good air barrier as well as creating a good R-factor.

Before you pick any furnace, a heat load analysis must be done on the house. This takes into account the orientation of the building, the amount of insulation, the size and orientation of the windows and doors. Then, and only then, can you choose the size of the furnace required. The type of furnce is immaterial. Once you know what size you need, then you can make choices about the type. In your situation, I'd consider a high efficiency, closed combustion system. This could probably vent directly out the wall and gets its combustion air from outside. They often have only minimal clearances required. Other than the sound (they're usually fairly quiet, but would be annoying in say a bedroom which might not be allowed by code anyways), closing it off into a closet with appropriate room for maintenance, it should not impact the space much.

I should clarify what I wrote. I don't mean spraying foam over the batt insulation, I mean spray foam instead of batt insulation. Additionally, The HVAC contrator wants to build the small closet around the furnace in the conditioned attic and just vent the attic closet to the outside with an 80% efficiency furnace in this closet in the attic. You state the type of furnace is immaterial. Are you suggesting that I can put an 80%efficiency combustible furnace in a nonvented attic. Where would it draw its air from for combustion?

You would have to provide the required amount of combustion air.

No, I wasn't clear...the type of furnace you choose is immaterial, the first thing you need to determine is how big it is. Then the type you select will determine how it must be installed. The better furnaces out there today are closer to 95%. With today's costs of energy going nowhere but up, an 80% furnace probably isn't a good idea. Also, check with your local utility company, they often have rebates for selecting more efficient units, making the difference smaller. Depending on what you choose, you may qualify for federal tax credits as well. None of that would be available with an 80% unit, and you'll be paying for it the rest of the time you live there. Some mortgage companies will let you get a larger mortgage if the house is particularly efficient, knowing that you'll spend less on utilities.