We did our house in 2006 with Sealection 500 open cell foam. The attic space is insulated not open, and we have a metal roof. I am very pleased with the product thus far.
As a home owner, there is one thing you have to watch out for. Every subcontractor (including the general contractor and architect if you have one) is focused on their piece of the puzzle. As such, they have two goals. They want to get a bigger piece of the money pie, but they also need to compete with other subcontractors on price. You are the only one viewing the project as a system and you are the only one who can weight trade offs.
We built with 2x6 wall studs. The foam contractor bid the job as spraying an average 3.5" of insulation. The roof trusses were made from 2x6's, and he bid 5.5" in the attic. He knew that the a goal of the house was to be energy star certified. I didn't care about that, but the design-build group that did the house wanted it, and as long as it didn't cost me anything I said OK.
As you may know, there are two ways to have the insulation in the house approved. Prescriptive is when the amount of insulation in the walls and ceilings has a certain R value. The other way is to do a full analysis of the house and make sure it's OK. A program that is online called "resi-check" (I think...) gets fed your square footage, wall area, window U values, window square footage, and other things and spits out a number. If the number is below 1200, you are good to go.
So with that background, here's what happened. The insulation contractor specified the above thicknesses of insulation because it was cheapest to install. There would be little to no cutting of foam in the walls, and in the ceiling he could just fill the spaces and not guess about depth. The resi-check program let him get a 1200 score, and that was all he had to do as his part of working towards an energy star rating. He had a bid of $18,000 to do the house. For grins, we got a price for fiberglass insulation and it came in at $12,000. This is a good sized house and home office, so look at the percent change more than the numbers. Cost per square foot are kinda silly here, since it's the surface area to be insulated more than the floor space.
The problem was that we had a lot of money spent in the bigger walls (I did 2x6 walls on 16 centers for better strength), lots of money spent on high dollar good windows (argon filled, low E, Anderson stuff), and the design of the house had a reduced number of windows (fenestration ratio, it was 12%). All this extra money allowed him to do a lot less with the insulation and get a low bid, but his proposed low bid would have neutralized the effective energy savings and money spent on walls, windows, and less window area. I caught this by accident when I was playing with the resi-check program after I found out about it.
It took a lot of discussion with him to figure out what he was proposing and why. So I ended up asking for a quote for an average of 5" in the walls (with no point less than 4", big reduction in cutting vs. a full fill), and 7" in the ceiling. The $18K went to $24K. We also got a single-coat spray, about 1.5 to 2", under the master bed room and between the 1st and 2nd floors for sound and heat. The resi-check number went from 1200 to 900. This is a unit-less number, and I suspect it's like R values and is logarithmic, so things got better and I had the above average insulated house I wanted. He ended up cutting about 1/3 of the walls (the top and bottom where he reversed direction). He started the ceiling in the hardest part of the rafter to get to, and I climbed up in there and saw that he was spraying 5.5" and then a top coat, we had a discussion, I put some nails in to show where 7" was and the rest of the job went OK.
So the effective R values in the walls was 19, and in the ceiling it was 25. The only thing I'd change is I'd take the ceiling to 30. The combination of the metal room reflecting so much of the sun's energy and the insulation is great. On a 102 degree day during construction (the floors were going in) I had the AC on with the house at 73 degrees. The attic was at 85 degrees.
The other argument for the original quote was that the foam seals so much better than fiberglass. This is very true, and is (IMHO) the main win with the foam. This is especially true in "the bands", the area where the floor trusses meet the outside of the building. I think there is a trend to try to use this better sealing as justification for lower R values in order to make the product more cost competitive with fiberglass. I view this this way: Foam is the "perfect" product for using R value because all air leakage is eliminated and the R value is pure R value not diluted by leaks (if that makes any sense). So yeah, not leaking is great but I still want a high R value.
There is often talk in many areas of construction about payback periods. Sometimes this makes sense (as in HVAC and insulation), sometimes it doesn't (How do you compute the payback period for granite countertops vs. Corian vs. Formica?). Our Wall-Mart culture has produced a mindset in the building industry (again IMHO) that supports a 3 year payback. When doing our house (our last house, I'll be leaving horizontally) I was happy with paybacks in the 8-10 year range (yeah, that's _so_ un-American of me :laughing
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I point this out because discussions with foam installers will inevitably involve concepts of leakage and payback. They have had the discussion many times and are masters, you will be having in for the 1st time.
When they crew was done, I went through the house and spent about 3 days and 15 cans of Great Stuff (closed cell foam) and hit every small hole and void I could find. The crew also foamed around the windows with a low expansion foam, and caulked where 2x6's made headers/jacks for windows and doors. When the guy came to do the energy star test (the design build group paid for it), he did the door blower test (the "how tight is the house" test) three times because the numbers were far lower than he calculated. He did the calculations knowing it was a foam house.
The house holds smells more than our old fiberglass house. We've not put in an air exchanger yet, time will tell. We do have a problem with a black airborne mold on the toilet but fiberglass houses in the area have that too. My next house project is UV filters in the ducts (not the HVAC units). The other big win is the humidity levels in the winter are great. We do not need to run a humidifier like we did in our old fiberglas house.
In summary, I would go for the foam if you can swing it. It's one of the few things you can't go back and change easily. Yes, it bucks the trend of modern housing which tends to put the money only where you can see it but this stuff is great. If energy cost continue to go up, it gets better.
Sorry for the very long post. Part of that is I'm too wordy, part of that is I want to fully answer the OPs question
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Pete
