Installations present a number of considerations and recommendations in order to provide a safe environment for application and to avoid overexposure through inhalation. Appropriate ventilation and protective gear is required when applying insulating foam sealant. Installers should wear protective gear, often referred to as “personal protective equipment” (PPE), such as protective clothing, gloves, and eye protection. Direct skin contact should be avoided, and recommendations by the product manufacturer should be followed.

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We are turning our roof into an unvented roof assembly by raising the roof and blowing in SPF. We are planning to leave the existing vapor barrier down but remove the fiberglass batting and then adding 6" of SPF in all the cavities, to completely seal and insulate the house. Should we have any concerns about doing it "upside down" and not spraying the foam directly to the underside/sheathing of the roof?

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This is more common with closed cell foam, but it happens with open cell foam, too. Since closed cell foam has a higher R-value per inch, installers generally spray 2" in walls and 3" in rooflines to meet the energy code requirements of R-13 and R-19, respectively. (I'm not going to dive into the energy code here, but these numbers apply to many climate zones, the latter being allowed under the UA tradeoffs rule. See the Energy Nerd's blog on this topic if you want to argue.)

Steve: If your spray foam installer leaves the vents open, he will be committing the 2nd of the 4 problems I described above. You will most likely have comfort and efficiency problems. You may well have condensation problems. You will be spending a lot of money on a product that likely won't perform as it should. Don't let him leave the vents open. If the installer you've chosen doesn't understand this, you may want to choose someone else.
Allison--would be very interested in your take on the GHG issues of foams, which have received lots of smart commentary recently. To highlight a few: architect Jesse Thompson's comments on Tom's Good vs Bad post on Energy Circle, the very thorough piece by Alex Wilson on Green Building Advisor and Michael Anschel's cautionary diatribe on Remodeling. 

How Much Does Spray Foam Insulation Cost

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.
c) Make sure that the over expanded access is properly removed from the walls. We had a junior cutter taking too much off by bending his saw into the cavity. Look closely at the inside wall corners, and ceiling/wall corners to make sure that the foam is properly removed flush with the studs. If it has not been cut back flush it may lead to uneven drywall finishes .

How Much Spray Foam Garage