you are correct....open cell is the cheaper, less R and higher perm (allows more water through) then closed cell. Closed cell will be a added cost, but would probably be worth it in the basement at least. Closed cell, in general, will seal up your entire house better then open cell, as well as add structural rigidity to your 2x4 framing making your house stronger. Open cell does not. Closed cell is not water proof, as it seems your questions are hinting toward. Cellulose is just fine for the attic, and going with more closed cell is a waste of money IMO. An animal can rip apart open cell just as easy as getting into cellulose. Besides, if something gets in there you have bigger issues anyway! Also, your roof is suppose to be water proof....not your insulation. If your roof is failing, you do NOT want a water proof insulation anyway. This traps water and allows wood to rot, thus a possible structural failure in your roof without you really even knowing. You may also be money and r value ahead to practice air sealing your walls with caulk and then using a blown fiberglass or dense packed cellulose in your walls. A side note...spray foams are THE most expensive possible way to insulate a home, and most cases you will never see a ROI compared to dense packed cellulose and blown fiberglass with air sealing especially in your climate. (note, I never mentioned batts).

I"m researching this sealed attic concept for a renovation/addition in Virginia. It seems to me to make sense that if you seal your attic and your mechanical equipment is up there it will have less heat or cool loss - and the upstairs will be improved as it will be against a semi conditioned attic thus resulting in less heat or cool loss. I guess if you only insulated your ceiling - just the fact that your ac unit is running in a 130 degree attic or your furnace is running in a 30 degree attic - means you are losing cool/heat that you are paying for. A sealed attic (spray foam roof deck) would seem to be more efficient and result in lower energy usage. the closed cell/open cell decision and the the thoroughness of the sealing of the envelope would seem to be critical. Also - what are the real ramifications of roofing lifespan reductions due to hot sheathing. Will the sheathing really be that much hotter than when it isn't insulated under the roof sheathing? I suppose the heat cannot flow into the attic - but at some point doesn't the attic heat up to the point where there is no more heat flow and the roofing is as hot as it would be with a sprayed deck? HOw might metal roofing effect this consideration? What about solar PV or Hot Water above a sealed attic (seems like the solar hot water could only benefit - i guess the pv panels could get too hot if not spaced enough to 'cool' off. any thoughts?

have a foam company come and take a look. lots of times we cut thru a wall to allow access. if you only foam part of the roof, you aren't achieving a lot as you will still have air leakage from unfoamed area. if it is truly not acessable then you need to put a wall(..can be felt nailed to studs) between foamed and unfoamed part of attic. foam company can spray the 'wall' to make it air tight to the unvented portion. most companies should be able to figure it out and give you a better idea. shop around. best of luck.

"This method would keep the attic ventilated but cuts back on the radiant heat coming in from the underside of the roof sheathing". If you can use a straight edge, utility knife, and stapler, you can diy this for big savings using rolls of radiant barrier. The install is on the underside of the rafters using the rafter cavaties as channels to trap the heat gain and is pushed out by way of the soffit venting up and out the ridge venting. Here's a link to an outfit that offers different install methods and radiant types. I've used 50" wide x 1000" rolls one side foil faced. fiberglass mesh inbetween, with the opposing side kraft paper. You hold it up 3" from where the rafters meets the top plates and run it perpindicular to the rafters overlapping and foil taping the seams. You overlap at the ridge as well. Your soffit/ridge venting must be continous run meaning every rafter cavaty would be vented to allow the heated air to escape. The radiant blocks it from entering the attic. Very effective anvenue of keeping the attic cool. Here is a link that might be useful: radiant barrier