It would help to know know the materials rather than the contractor claims. If one is closed cell foam and the other is open cell foam the difference in cost should be considerable.

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

Hi, I think that the approach of spraying foam under the roof rafters, and making the attic space part of the conditioned space of the house is a good one if done carefully. Seems like it would be a time consuming job to block all the current attic ventilation carefully. If you use closed cell foam, I don't see why there would be any moisture or condensation problem with the roof sheathing. Some of the people who make roof shingles don't like insulation right under the sheathing because it makes the sheathing and shingles run hotter and may reduce their life -- some people think this is bunk. If you have ducts or AC equipment in the attic, moving the insulation line up to the roof rafters will make your AC and duct system more efficient. The FSEC has some papers on what kind of saving results from this. Have you thought about just blowing additional cellulose insulation on the attic floor? Seems like this would be a lot cheaper, and may accomplish about the same result? http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/97/970504.html Gary

Are you installing this or hiring someone? Typically bays are hung these days via turnbuckle system (cables or chains), so that necessitates that the soffit above the bay be cut open to accommodate. Stacking a couple pieces of foam board on top the head board would not be a bad idea, although again, most good units come with insulated head and seat boards.