As you can see from the chart below, foam greater than 2 - 3" thick isnÂt cost effective considering the minimal heat loss and the high cost of foam.

Follow the link to the recommendations from the US Department of Energy for the most cost effective insulation levels for your area. You can refine it down to your zip code. You are closer to the recommendations than the contractors. Unlike fiber glass, foam loses very little of its R Value over time. I am surprised by how high an R factor is recommended. The attic is above our minimum R40 in a cold climate (Toronto, Canada).

Here is a link that might be useful: Recommended insulation levels

I'm in Charlotte, NC, and wanted more than the 2" and 2.5" that our Spray Foam Contractors do, so we used 2x6 walls and put unfaced fiberglass over the 2" of foam.

If I had it to do over again, I'd start with something Worthy has shown pics of before, where he had foam board sheathing put on the exterior, taped at the seams. Then I'd use blown cellulose inside the walls and a spray foam on the underside of the roof.

If you're past the stage of doing 2x6 walls or foam sheathing, then you can still see about blown cellulose over the 2" foam.

Thanks all for the replies. bdpeck, we are close to being dried in. We did OSB exterior sheathing so no insulation value there. I have 2x6 exterior walls on the main level where I'm planning on using damp sprayed cellulose to R-20+ and the majority of the attic will be blown in cellulose to R-38. The area I'm concerned about is 900 sq ft of finished attic in which I have 2x4 walls surrounded by vented attic and a vaulted ceiling with 2x6 rafters. The finished attic is the area I'm wanting to use foam. I'm trying to get close to the same R value as I have in the rest of the house. My contractor is saying I need 2" of foam in the walls and 2.5 in the vaulted ceiling. This won't give me any where close to the desired R value I want. The insulation contractors tell me and some information and graphs I've seen and read suggest the R value of closed cell foam doesn't tell the whole story as to its effectiveness as an insulation. I'm just not convinced that 2.5 inches of foam in the rafters in the attic is enough. The reminder of the attic that is unfinished is just to large and costly to seal off and foam.
Thanks for the input!

duckwhacker- I'm guessing that, if you can spray foam on the backside/ceiling of your finished attic, you could also do cellulose there. If you're doing the kind of air sealing detailing that'll limit the dust problems noted in another thread on cellulose, it's probably an as-good-but-cheaper solution. And it'll have better R value.

We have a bonus room/loft in our old house in ATL. Had the walls sprayed with foam from the attic side, but mainly because the walls are 60-year-old paneling that's leaky as heck. The ceiling has JM fiberglass to R-38.

There are a lot of reasons why cellulose is probably a better solution for the ceiling. When we originally started putting in FG, I knew I was going to do a lot of rework down the road and wanted relatively easy access w/o a huge mess. That said, the work that was done later pretty much trashed a lot of the FG.

We used open cell, which of course requires a thicker application. We filled out the 2x6 ceiling in a 24x24 foot media room (over garage) with it. Our walls are ICF, so we don't have the issues yo will with your walls.

I'm glad we did. When I shoot the 9 ft ceiling with an infrared thermometer, on a 100 degree day outside, it's mostly 79 (and setpoint is 77). The room next door, with a foamed attic floor above it, is mostly 77. Now, that may not seem like much, but it can be once a lot of electrical gets going. (Mid-wall to floor in both rooms is 77F).

Also, you are going to get heat transfer via the 2x6's from the roof deck through to the sheetrock to the ceiling. So, it seems to me that the more 2x6 that's covered in foam, the less it will radiate that heat into the airspace and sheetrock beside/below it. Just some observations to consider.

And above all, make sure you have adequate return ducts in that room, or it will never be comfortable.

If you're not using the spray foam on the rest of the house, then you won't get good value out of it on this small area. Using foam all around to get a much higher level of air sealing is the real benefit to a spray foam.

In your case, I'd use a fiberglass batt on the 2x4 walls, then cover the 2x4s and insulation with another fiberglass batt (unfaced) to get an R-20+ wall (and lessen thermal bridging). And I'd use the thickest fiberglass possible for the cathedral ceiling. You could still spray the roof there and use some unfaced fiberglass under it, but with 2x6, you're not going to get much better than R-20. Depending on headroom, you could also fur down the ceiling to get more insulation.

I'm surprised that more isn't written that the installation of closed cell (new construction) being applied to the osb! and according to what I've found is that when the bricks are installed having 1/2 to 1" air gap this will actually further enhance the R value of the foam. Thats what I intend to have done and to me seems the ultimate way to go when using cc foam. I've built on a 5 block high concrete block crawl space foundation and will have the foundation sprayed down to the ground....any comments??

Thanks
BL

If you want to know the truth, read this.

http://www.monolithic.org/stories/r-value-fairy-tale-the-myth-of-insulation-values

I have personally been in a potato storage building in Minnesota when it was well below zero and 30 mph winds outside. The building had only 2" thick sprayfoam in walls and 3" in the roof. I was interested in the foam before I had ever seen it in use but was curious about real world apps. Let me say that I was truly impressed. I got to talk to the owner while there and he told me that it literally took half the heat/cost to heat that building than the other one right next to it that is the same size. He told me that the next summer he would be re-insulating the other building with the sprayfoam. ( It had 10" thick fiber insulation)

So, thicker isn't better. I will be building a domehome next year and I guarantee it will have sprayfoam.