OK, so I found this useful site. Looks like I'm going to need something like 18 inches? But I'm still not sure how I measure how much area I have to cover?

http://www.energystar.gov/index.cfm?c=diy.diy_index

Blackcats-
The insulation rolls always state how many square feet they cover. You're going to need to take rough measurements to determine how many total square feet you have to cover, and divide that number by the area per roll to get numbers of rolls. Since you're probably going to have insulation piled higher than the joists, you might consider buying insulation that is just thick enough to bring the depth up to the top of the joists, and then lay a second layer of insulation crosswise to it so you don't have a bunch of gaps in the spaces above the joists.

Unfortunately, you provide no info about where you live (i.e., what your winter heating demands are, or your summer cooling needs, if any). Regardless, you don't need as much insulation as you're talking about. A total of 10" would give you at least R-30, which means if you add 6" or so to what you have, it'll be plenty. And don't get too hung up on R-values. I won't bore you with the physics of heat transfer, but the first inch of insulation saves the most energy, and each additional inch saves, but saves less. By the time you get to 10" you're saving almost all the energy you can, and several more inches will do very little more. If you were to put 18" of insulation in that attic, you would have a hard time identifying a savings for that compared to 10". I'm all for energy conservation; it's an area I worked in for years. But there is a certain point where huge amounts of insulation don't end up doing much more for you.

We are in Chicago. It's the rough estimates I'm having trouble with. I suppose figuring out the size of the space below it would suffice? Thanks so much for all the info! I'm glad to hear we don't need as much as thought, and will take your advice to avoid gaps.

Yes, this is not a high precision calculation. If you can calculate the area of the space below, that will do it. Most places will allow you to return unopened rolls for credit, so, do an estimate, buy a little more than you think you need, and return any excess. For example, if you calculate you need to cover, say, 520 square feet, and the roll says it covers 49 square feet, you'd buy 11 rolls (520/49 = 10.6). Just make sure you know what your stud spacing is, because fiberglass batts come in a couple of widths.

This this place is so hard to access, you might consider blown in cellulose insulation. You can just run the hose up through the window and then spray away.

Stud spacing, OK. I think this is why we haven't done this project yet. The couple of times we went to the store were just kinda overwhelming.

If we were to do blown in, is there anything to consider when we replace the roof? It's very old and will need to be done within the next couple of years, along with proper ventilation, etc.

We were also thinking about getting a whole house fan. I haven't researched this AT ALL yet, so I know absolutely nothing about it, not even where or how it is installed, other than a vague idea that it's on the roof or in the attic or something.

Forgot 1 other question I had. If I were to install the fiberglass batts on top of whatever is already there, is it the unfaced that I want?

For the most economic R values to aim for, the US Department of Energy--Energy Efficiency and Renewable Energy has guidelines for specific Zip Codes.

Fiberglass batts in walls, as typically installed, lose up to 28% of their stated R Value. I would spray in cellulose on top of your existing fg batts. If your roof is leaking or inadequately ventilated, fix it before you reinsulate. Water mixed with either cellulose or fg insulation reduces the R value and encourages the growth of mould.

When you put insulation on top of existing insulation, you never want it to be faced with any sort of barrier. The barrier is there to stop moisture. You don't want moisture getting stopped in the middle of your insulation.

Putting more fiberglass over fiberglass is a bit of a pain and involves more measuring and cutting than it sounds like you are comfortable with. It is MUCH easier to gets blow loose material on top of everything. All you need to know is the total dimension of the space (length and width) and how deep you want it to go.

In terms of replacing the roof, as long as it isn't currently leaking water into the area, it shouldn't be a problem. You can save yourself some headaches later by adding some markers on the existing woodwork that will stick up over the final level of your insulation. That way, if you do have to do any work up there, you'll know where it is safe to walk.

For a whole house fan, you need the fan to be venting into someplace with very good ventilation. If the attic area is currently unventilated, you can't discharge a fan into it. That air needs someplace to go, so you would have to fix the overall ventilation before you consider a fan. However, if you are considering that as part of your long term plans, it would be a good idea to address the electrical before you bury everything in insulation. You can do it later, but it is a lot easier to do when everything is exposed.

Thanks guys! That some good info to consider. I just found a place in the area that will do the energy audit w/blower door etc for $400, so we are going to do that ASAP and then do the insulation.