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

with foam the install is crucial. inexperienced untrained installers do not get the air sealing that makes for an air tight seal. we use open cell here, full rafter depth and faces of 2x rafters covered. this is a hot humid climate. ask to see installs of jobs they have done for others or to visit sites where jobs are ongoing. insulation should not dip in and out in rafter bays but be mostly even. 6" of foam is 6".. not 3" to 6". open cell has less R-value than closed cell. and all should be installed to specific R-value spec' best of luck

I would say that the level of difficulty depends completely on your knowledge of what the job requires and your ability to do the work on a sloping roof. 1. If you lay something down on the roof and it slides off to the ground then the roof is probably too steep for an amateur to be working on. 2. Ripping the old skylight out is the relatively easy part, just don't tear up any of the adjoining roof or you will have to repair that also. 3. Covering the resulting hole with sheeting is pretty straight forward just make sure it is supported underneath by 2x4's all around the hole . Use at least the same thickness of sheeting that is on the existing roof. 4. You will have to carefully lift up the shingles and slide felt (tar paper)under them at least one foot all around, more would be better. 5. Depending on the age of your roof matching the new shingles to the old may not be possible as far as color goes. You have to weave the new shingles into the old ones. If all that sounds easy enough to you then go for it, otherwise have some one that knows to do it, do the job

Ichabod, we're in the desert southwest, but I probably should have expanded on that comment! Our annual average rainfall here is under 10". Snow does happen but is not common. Ice dams are not a problem here, for example, so local code does not require ice and water shield at the eaves, either. We recently had a wet 6" snow but that is fairly remarkable for us, down here in the southern part of New Mexico. Also to you and tlbean2004, thanks for your thoughts on the metal flashing. We may request metal valley flashings, with ice and water underneath. Metal flashings got the roof through 17 years with no problems, so I'm not sure why to abandon them now...unless the technology of modern underlayments has rendered metal valley flashings obsolete. I thought that was what the roofer was implying, and we weren't sure if that was correct. I'll admit I am also a bit more worried about this roof than I would be if it was not such a low pitch. So you folks are a big help. klem1, thanks for your explanation. Nope, no interest here in removing the 2x2 trim to try to treat the rafter tail end grain (DH would leave me if I suggested that) so it looks like the consensus is to keep a very wide metal flashing. I guess it will be less distracting when painted to match the wood. Our house was built in 1965 and I sure don't want to make uninformed decisions that lead to damage...all for the sake of looks. (As much as I dislike the look of the big drip edge.) I guess a smaller drip edge and rotted wood would not be very pretty, either. I need to figure out which ice and water shield product will work in our hot temperatures, if we want it under the metal valley flashing. One bid specifies GAF Weatherwatch (but the contractor isn't local.) I don't know how heat resistant that product is. I know Grace Ice and Water has the best reputation, but I was surprised to see the Grace website says that only their most expensive product, Grace Ultra, should be used in the desert southwest: Grace IWS Contractor's Guide So if the esteemed Grace Ice & Water Shield (regular product) will not stand up to desert southwest heat, I wonder how any of the (much less expensive) GAF ice and shield products would work. There are a few scary stories in some roofing forums about "bitumen bleed" when ice and shield products are used in higher temps than recommended. (Roofing is not for sissies, have you noticed?)