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[quote=Anonymous]This is a complicated question. Even people in the construction business have trouble with this.

There are two ways to do attics, conditioned and unconditioned. Conditioned means that the attic is part of the heated and cooled part of the house, unconditioned means it isn't. The way that you do insulation is completely different for the two types of attic.

The short answer is you insulated the floor for an unconditioned attic and the underside of the roof for a conditioned attic, but there's a lot more to it. Here's are the biggest concerns for each type: If you attic is unconditioned, it's essentially outdoor space. If your HVAC equipment is there, it's going to lose the heat and cooling that you're paying to produce. So you have to figure out a way to insulate your HVAC equipment separately.

If your attic is conditioned, your biggest concern is ventilation. In cold weather moisture tends to condense on the underside of the roof. Over time this causes the framing and sheathing of the roof to rot. There are two ways to prevent this: either ventilate the underside of the roof, or seal the underside so tightly that humid air can't come into contact with it. Sealing is typically done with spray foam.  Venting is done with channels between the insulation and the roof that connect to vents on the ridge and eave. If you just staple up batts of fiberglass your roof will rot out and fail.

Just from the information you've provided it's impossible to know whether a conditioned or unconditioned attic is right for you. You should really ask this question at a forum that specializes in building science, my favorite is Green Building Advisor: https://www.greenbuildingadvisor.com/qa

As I said, a lot of people in the construction business -- even in the insulation business -- get this stuff wrong. You need to know enough so you can tell whether the person you're dealing with knows his stuff. [/quote]

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