Thanks Dana.
Is it Ok to use the polyisocyanurate boards on the walls? The local Building inspector wants us to use a vapor barrier so I glued foil-faced polyisocyanurate boards on the walls instead of plastic sheeting on a wooden frame for fear of moisture buildup and eventual rot of the wood frame. I was going to leave a 1-inch gap between the polyiso boards and the wood frame to prevent condensation water that is likely to form on the polyiso boards in the warm months from contacting directly with the wood frame. I do not have a moisture block between the top of the foundation and the sill, but I think I am going to have to live with it. I live on a nice dry hill and have never had water in the basement - just moisture issues in the summer months that is controlled with a dehumidifier.
Good recommendation about talking with the designer. The person who did the heat calcs is the contractor's distributor. I asked the contractor if I should send along information about actual fuel use and the HDD numbers and he said that he would not need those. I do have the distributor's contact information, I will just have to figure out the best way to get that information to the distributor without torquing off the contractor...
Foil faced iso on the walls is somewhat risky, since it raises the moisture content of the concrete from ground moisture and is only able to dry toward the exterior on the above-grade portion. There are two issues that get created:
A. Efflorescence and spalling begins to occur on the above grade exterior portoin of the foundation a 100+ year problem from a structural degradation point of view, but it can be mitigated with a sacrafisial parge on the exterior.
...but more seriously...
B: It raises the moisture content of the foundation sill and possibly the band joist to a level where it rots out (sometimes a sub-decade strucutral degradation issue.)
The solution is to put only semi-permeable or semi-impermeable foam against the foundation, but make it thick enough that the temperature of the interior side of the foam where it meets the wood studs is always above the dew point of the room air, even in winter. The "perm rating" of the foam is ideally between 0.5 and 1 perms, which is acheivable with 1.5-2" of XPS (pink or blue) which delivers ~R7.5-R10, or 3-4" of unfaced EPS(bead board- usually white), delivering ~R12-R16. That way ground moisture doesn't wick up the foundation- it can dry toward the interior of the basement. Under NO conditions should you put a vapor barrier on the inteior of the studwall, not even kraft-facers on batts (which are about 0.4 perms).
A combination of R10 of foam and unfaced R11 or R13 batts in the studwall that it delivers~ R19-R20 whole-wall values (thermal briging of the studs & plates included), and has a stackup that will reliably keep condensation from building up in the wood in our climate zone. That's what all the hygric analysis stuff is about in the this document:
http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis
The best/most-cost effective solution is essentially case 8, figure 41, p.57 in that document. Read the moisture control paragraph, and print out the relevant sections for your inspector(!). The simulations were for a Minneapolis climate, which has a significantly cooler winter, with much higher condensation potential than in MA.
Note, they also simulated a foil-faced iso solution (case 9), which protected the studwall well, but note carefully the locations of the capillary breaks in the diagrams- unless your house has a metal or 10mil polyethylene sill gasket, you're stuck with problem B that I outlined above. If anything, errring toward a MORE permeable foam than the lower limit, say 2.5-3" of unfaced EPS (~R10-R12) would have been the ideal retrofit. If you're not changing out the 2" iso, make sure that the exterior of the foundation never sees snow buildup and that the whole foundation is well drained. If it's as well-drained below the footing as you think it is, you're probably going to be just fine. No house is built perfectly, yet they're still standing, but foil or poly vapor barriers against the foundation without a high-performance break at the sill isn't good building practice, in general.
FWIW: That inspector needs an education on this subject- insisting on a vapor barrier on foundation insulation is just plain WRONG (even though it's enshrined in the Canadian national building codes.) A vapor RETARDER, yes (XPS or EPS qualifies, as does closed cell spray foam at 1.5-2" thickness), but his insistence has no basis, and WILL create problems in somebody's house, hopefully not yours.
Whether it's 2" of iso or something else, as long as it's ~ R8 or more, it's well-worth the additional expense of unfaced R11 or R13 in the studwall, cutting the heat loss literally in half. Don't forget to put a capillary break (maybe your inchor so of foam you're putting over the slab) under the bottom plate of the stud too. (Even 1/4" of XPS is a sufficient capillary break, but if you put in right on the slab, put some poly underneath.)
In summer there's zero chance of condensation forming on the interior facer of the iso- butt the studs directly up against it- better yet, seal each bay with a bead of caulk before adding batts to maximize the performance of the fiberglass (which is very susceptible to convection and infiltration losses wherever there is a gap to convect into.) To get condensation the surface has to be below the dew point of the interior room air. If you dehumidify/air condition the basement to 60% relative humidity or less (recommended by ASHRAE- health professionals say 50%), even if it's 75F in the basement, that would put the dew-point at 60F. With ~50F sub-soil at the bottom of the foundation wall, with half the R-value in the foam layer, the place where it's 60F is inside the foam, protected from contact with the air. If the basement is 70F/60% RH, the dew point of the room air is 55F, but the facer is at 60F, still no condensation- you'll always win the summertime condensation game in summer anyway.
It's the winter, where at the above grade section will experience condensation for a random assortment of hours (the pre-dawn hours of the coldest winter days, typically), but never long enough to cause a problem as long as you have a sufficient ratio of foam-R to fiber-R. Even at 40/60 foam to fiber you'd be more than good in this climate, and center-cavity with R13 batts and R10 foam you'd be at 43/57. With R7.5 foam/R11 batts you're still at 40/60, and if you count the concrete as R1 (valid), R7.5 becomes R8.5, and you're at 39.5/60.5, which still has real margin if you use XPS or EPS, since the R value of those rise more than 10% when it's 0F on the cold side. (The rated R is the 75F number.) Iso drops in R value with temp, but iso rated R13@75F is still good for R11.5@0F. (The typical derated K-value for sub-zero use on iso is R5.6-R5.8/inch, down from R6-6.5 @ 75F.)
Before putting up the studs it's also better to air-seal the rigid foam by either taping the seams with FSK tape (2" foil tape often used to seal ducts) if foil-faced, or housewrap tape if XPS/EPS, or paint on some fiber-reinforced duct-mastic spread to ~1" either side of the seam.
If your iso extends all the way to the slab it's worth cutting back the bottom inch or so to ensure that there's no way moisture that might condense on or wick up through the slab wicks into the inteior of the iso, since there's no exit path for the moisture once it's between two foil facers. It wicks slowly, but it'll be semi-permanent if it ever gets in there.