I am remodeling my 1959 unvented flat roof home in Nebraska. The existing insulation is 6" x 24" fiberglass batts between joists with no vapor barrier. I would like to add recessed lighting and in-ceiling speakers. My plan is to drop the ceiling 4" and put in new fiberglass insulation, then put the lights/speakers below that. Looking for advice on how to do this, or what I should do differently. Thanks!

Flat roofs are best insulated from above, ESPECIALLY if you're going to poke a bunch of holes in the ceiling for can-lights & speakers! This is best done when it's time to re-roof.

To do a code-compliant unvented roof in US climate zone 5 (all of Nebraska is zone 5) it requires R49 total, with at least R20 above the roof deck or as air-impermeable insulation (foam) in direct contact with the under side of the roof deck, with the fiber insulation in direct contact with the foam ( or roof deck) above, per section R806.5 as outlined in TABLE R806.5:

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%208.html

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2011.html

So, you probably have crummy R19s between 2x6 rafters, which perform at only R18 when compressed to the 5.5" rafter depth. To meet code from an R-value perspective would then require R30 above the roof deck, but if you just went ahead and installed 4" of roofing polyiso (R24-ish) under a new membrane roof you'd probably meet code performance on a U-factor basis, and would have huge dew point margin at the roof deck. To take the sting out of the cost (including environmental cost) using reclaimed roofing foam from commercial demolition at 1/4-1/3 the cost of virgin stock works. This reclaimer in Lincoln seems to have a bunch in stock:

https://lincoln.craigslist.org/mat/5934411311.html

Wow! That wasn't the answer I was hoping for! We have a rock roof on the house and I don't see us replacing that anytime soon.

Local code says I can get away with R-30. Is there anyway to do that from below?

You'd need to have at least 40% of the total R as closed cell foam directly on the roof deck, with the rest cheaper fiber. That's really not too bad- 2" of closed cell foam runs ~ $2 per square foot and would deliver R12. You can then add 2" edge strips of foil-faced polyiso to the bottom side of the rafters to accommodate R19s (which compressed to 5.5" are R18). Then a layer of half-inch gypsum long-screwed to the rafters, painted with standard latex primer delivers the Class-III vapor retardency.

Framing edge strips can be cut fairly easily from 2" foil-faced rigid foam by sharpening the edge of a 4" taping knife. You can cap-nail the strips in place to keep them there while adding the fiberglass, but it's the long-screwed gypsum board that will ultimately hold it all together. A bead of foam-board construction adhesive between the foam & rafter edge makes it even more secure. For 2" foam it'll take 3.5" bugle-heads 12-16" o.c. If that's too much of a PITA you can use 1.5" foam and half-inch plywood strips through-screwed to the rafters (and glued to the foam) and hang the gypsum board on the plywood using shorter fasteners.

Don't punch any holes in the gypsum or you may still convect enough air-transported moisture to the surface of the closed cell foam to be a problem. Put the can-lights and speakers on a suspended ceiling below. If you prefer to make the gypsum the finished ceiling, use surface-mount LED fixtures to achieve the recessed-lighting look, air sealing the electrical boxes to the gypsum with the appropriate foams or caulks. (Not sure what you can do about air-tight speaker fixtures.

Thank you so much for your detailed response! Very easy to understand. I have looked into low-profile led lights and haven't ruled them out. I may have to ditch the speaker idea though.

Chud, These folks can hep you with cut to size "thick" pieces of various types of foam: http://www.universalconstructionfoam.com/

They use factories all over the country so shipping is easier. Using these folks makes it so each piece of foam is cut to the size you specify, so you don't need to cut (much) on site. You also don't have to have a bunch of 2" slabs as they can make each slab almost to any thickness you'd ever need.

The rock roof should not be a problem. I've done it over a rock roof in the past by tossing off the big rocks. The gravel that is embedded in the hot roofing is typically no problem to leave.

Hope this helps!

A cheesy short instructional video on how to cut ~2" rigid foam cleanly with a taping knife lives here:

https://www.youtube.com/watch?v=KuFWRTEULD0

Thanks Roger, but I hope to not touch the roof at all and do as much as I can from underneath. It isn't the best solution, but the cost is much more reasonable.