As this example illustrates, it's important to seal the envelope completely. One of spray foam's biggest selling points is its air-sealing ability, but it can't seal places where it's not sprayed. One of the nice things about using spray foam in new construction is that you can do a Blower Door test before the drywall goes in. Even better, you can test for leaks with a fog machine.
Thank you, Allison. We have a split system unit (actually 5) with the heat being propane. The foam guy said that it needed ventilation, so completely sealing the attic wouldn't be a good idea. It sounds like you are saying that the systems need to be vented out of the attic or replaced with some type of closed system. But I shouldn't bother spraying if the plan is to leave the soffet vents open. Is that correct?

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Spray foam insulation helps lower utility costs because it blocks heat and cold from the outdoors. Recent studies by the U.S. Department of Energy state that around 40 percent of all energy is lost through poor insulation. Spray foam insulation is able to insulate homes at least 50 percent more than traditional insulation products. Spray foam insulation can also provide protection against moisture, which otherwise leads to the unwanted development of mold and mildew. And it can also be used for noise reduction as well. In fact, spray foam is good for homes and buildings in which outdoor noise can be a distraction.
Never spray foam on a surface with a moisture level of more than 20 percent—the foam won’t stick. If the surface you’re spraying hasn’t been exposed to water for a few weeks, it should be OK to spray. But if you’re patching a wall that recently sustained water damage, or was just assembled with new, wet lumber, you may want to test the moisture level before you spray. You can get a moisture meter here.
I'm in the process of redoing my basement and read through the posts. I took out one inch styrofoam covered by 1/4 " beadboard and installed 2X4s and plan on putting in fiberglass batts between them(r-13) with 1/2 in drywall over top of that. Do you think I will see a significat increase in heat control? Should I have done more?... it's a little after the fact, but still curious to know if I will see a big difference based on your experience.
Johns Manville Corbond® Open-cell Appendix X Spray Foam provides an R-Value of R-3.7 per inch, and meets AC 377 NFPA 286 Appendix X requirements for application without an ignition barrier in attics and crawl spaces. This low-density, nonstructural insulation allows for high yield while prodiving excellent heat, air and sound control.   Recommended for use in residential, commercial and industrial applications. 
Closed-cell (aka two-pound foam) is denser than open-cell at about 2 pounds per cubic foot. Its R-Value is between 6-6.5 per inch. As a result, this kind of foam is much more expensive than its counterpart. The reason closed-cell doesn't need a vapor retarder is because it already has one. It's permeance is 0.8 perm, which means it can handle cold climates without the use of an additional board or drywall. Closed-cell uses hydroflurocarbons (HFCs) as part of its makeup. However, this material has been known to have a high global warming effect. If you want a green insulation solution, this is not the material to use. A way to avoid this and still use closed-cell is by installing it alongside fiberglass batts.
Some states, municipalities and utilities have programs. Check with your coatings manufacturer for further information. As a membership benefit, RCMA provides maintains a database of current information on rebates and tax credits for installing reflective roofs. RCMA’s customized search tool can be used to find the most up-to-date listings of reflective roofing financial incentives available.
The process by which heat energy in the form of light (usually IR unless the substrate is hot enough to glow in the visible range) is emitted more strongly by warm surfaces and absorbed by other materials especially those of low IR reflectivity (think matte black finish). Radiant heat transfer does not require a medium. Foam insulation materials, such as spray foam insulation, are opaque to thermal radiation, like most solid materials.
Global warming potential (GWP), as defined by the EPA, is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2).  Spray foam insulation products that use water as the blowing agent - typically open-cell foam however Icynene’s ProSeal Eco is a 100 percent water blown closed-cell spray foam – have a global warming potential of 1, the lowest possible number.  This is because water in the mixture reacts during the application process to release carbon dioxide and heat. The GWP of the blowing agent is that of carbon dioxide. Carbon dioxide has a GWP of 1.

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Before filling the wall cavity, spray the perimeter with a 1/2-in. layer, which will expand to a 1-in. layer. These layers are called “lifts.” This will ensure that the foam penetrates any voids where the studs meet the sheathing. This also reduces the risk of bulges in the sheathing. If foam is sprayed into a large gap between the stud and the sheathing and is immediately covered by a thick layer, the pressure of the expanding foam can actually push out any sheathing that isn’t securely fastened. Wait a few minutes after picture-framing the cavity before you go back and lay down the first layer in the center of the wall.
Sealection 500 is a spray-applied adhesive, which once applied, expands in place using water-blown technology, without CFCs, HCFCs, or ozone-depleting chemicals. Unlike traditional fiberglass insulation, Sealection 500 will maintain its performance; time and temperature have no effect on the spray foam insulation's R-value of 3.81 at 1 inch. Sealection 500 never deteriorates or decomposes. The product retains its shape and half-pound density over time, and never settles or compacts.