For soundproofing, use soundproofing materials--either constrained layer damping drywall, such as Quietrock brand or multiple drywall layers and Green Glue. Before the constrained layer drywall was invented, I used multiple layers on resilient channel. Lots of labour that can be easily short-circuited by just one wrong fastener, research shows.

Wrapping the exterior with 2" of rigid foam will prevent thermal bridging across framing members (up to 20% of the wall area!) and is well worthwhile. If you tape the rigid foam you'll also have a tight barrier against air penetration. A combination of 2" of foam on the outside of the wall and blown-in cellulose inside is a very cost-effective solution that will perform better than flash and batt. If the "flash" isn"t thick enough everywhere you can get a condensing surface inside the wall, which is bad. Cellulose is only corrosive if it's wet, and if your insulation is wet you have bigger problems than corrosion to worry about.

Down here we use open cell above grade and closed cell below Building Science articles do mention both cc and oc being used in attics, but they explicitly recommend cc. See "Unvented Roofs for All Climates" and "Understanding Attic Ventilation" (See Link below.) As I mentioned above, cc is excellent in attics as it acts as a secondary liquid water barrier. At the same time, it is permeable, allowing vapour to move both ways depending on the season. As for "hiding" leaks, I have seen commentary from foam appliers who handle all kinds of foam say that in case of leaks cc focuses the water, so it's easier to spot the source. Because oc foam is more permeable and holds water, the R value is degraded, which means the sheathing is more easily reaching dewpoint, which is not desirable. The same logic applies below grade. The one time I used oc foam, it was applied in cold weather and shrunk back considerably around the rim joists; this is recognized in the literature but brushed off as a minor quibble. When I've used cc foam in basements, rim joists, garage ceilings and attics (but not under the decking) I had none of those problems. Clint Eastwood's Dirty Harry had a famous comment about opinions--that I won't use here. That's why I try to reference mine to Building Science, whose principals are among the leading building scientists around; and who are also the source of much info at the US Energy Department --Energy Efficiency and Renewable Energy--Building Technologies Department. Here is a link that might be useful: Vented and Unvented Roof Recommendations

Thanks for the replys. Forgot to add that the firm that wants to do closed cell + fiberglass said it was a code requirement to cover the foam with what he called a code approved ignition barrier. I thought that any foam rated E-84 was OK by itself. Comments? One question I need to ask is why not just spray 4 inches of closed cell and be done it with. That should provide at least R-25. Compared to the inefective R-11 that was in there it would be a big improvement. One reason this may not be practical is it may require two passes as I'm not sure you can apply closed cell at a 4 inch depth in one pass due to excess heat generation and risk of self ignition. Two passes would add to the cost and most likely push it well above the open cell. As for converting the crawl space into a mini basement, I think the jury is still out on that one. If not done correctly I think the consequences are worse than leaving it the way it is now. I also think the cost would be much higher since you still have to insulate the foundation walls and completely seal the volume from the ground.