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As far as we can tell, the attic is laid out like so - first is the 2nd floor ceiling, then whatever material is right above that, then the attic joists, then there is plywood nailed to the joists, creating a storage area in the attic. There are a couple of cut throughs in the plywood for recessed lights in the bedroom upstairs, and from what I can tell, there is a minimal amount of insulation between the joists (between the plywood decking and the ceiling of the 2nd floor. It is definitely not completely filled with insulation, there is also a large amount of air space between the joists. I would estimate the joists maybe being about 6 inches high.

Is it effective to roll out batts on top of the plywood? I was most likely going to go for R-19 batts, and do it in two layers, at right angles to each other. Will this help, or is there a more effective way to do this that does not involve taking out all the plywood?

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What sort of climate do you live in? Do you know if there is any sort of vapor barrier? Probably not as there is little insulation. R-38 is a huge improvement, but it may be a little overkill or not adequate, depending on climate. –  bcworkz Nov 21 '12 at 19:22
    
The house is zone 4 from energystar.gov/… . I don't know about a vapor barrier at all, but in the parts I can see through (around the lights) it just seems like some loose fill. –  Aaron Nov 21 '12 at 19:40
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Since you are in zone 4, R-38 is a reasonable insulation level. That doesn't tell us how much an issue a lack of vapor barrier is. It is less of an issue in dry climates. What a barrier does for you is prevent moisture from condensing inside the insulation, significantly reducing it's insulating value. Ever notice how hypothermia victims have invariably gotten themselves wet? Unfortunately, properly installing one can be very difficult (read: expensive). As it happens, plywood acts somewhat as a barrier, depending on it's construction. Thus, you may need to just focus on gaps and openings.

Another possible make-do vapor barrier is a good heavy coating of latex paint on the ceiling. You may already have this! Truth be told, the best way to insulate would be to remove the plywood and blow in the needed insulation. This gets sealed around all the joists and other ins and outs nicely and makes effective use of what little insulation is already there. However, it may not be worth the effort.

You can still get a good insulation job by layering it on top as you suggest. The draw back is the dead space under the plywood effectively becomes part of your conditioned space, greatly diminishing the value of what's there. Even a plaster ceiling alone is worth something. And you've increased the top story volume you're heating 5-6%. Another issue with this approach is the plywood becomes your vapor barrier. What are the ramifications of moisture condensing on the under side? There's poor air circulation, so it will not dry easily. It could be the cause of a mold colony gaining a foothold, which can have serious health consequences. Or it may not be an issue if you're in a dry climate.

Whichever way you go, try to ensure what ever passes as a vapor barrier is a complete as possible. Ensure any recessed light fixtures are not insulated over, and that no insulation contacts the cans. It may seem more efficient to cover them, but you greatly increase the chance of them overheating and starting a fire. Insulation is little good if your house burns down! A truly efficient ceiling has no recessed light fixtures for this very reason. Note that there are supposedly fixtures rated to be covered, but I don't think I've ever seen one.

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If you install recessed lights around insulation, they must be IC-rated cans. You can also get air-tight IC cans (Insulation Contact). Keeping the insulation away from the cans is not a solution. Worst case is a fire, semi-worse case is water dripping from the cans due to condensation. –  Steven Dec 24 '12 at 0:35
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