There is a lot of meat to this question, so let?s tackle it one step at a time, starting with easier questions first.
Spray foam manufacturers are no longer permitted to use CFC blowing agents. HCFCs are also scheduled to be phased out. Both CFCs and HCFCs deplete the ozone layer, CFCs being the far worse offenders. The commonly used replacement in closed-cell foams is HFCs, and in open-cell foams it is water. Herein lies another trade-off: Although released HFCs do not contribute to ozone depletion, they contribute significantly more to global warming than HCFCs. However, a few HFCs are fairly low in this ?global warming potential,? and some blowing agents have moved beyond HFCs to chemicals that cause no ozone depletion or global warming. Since the vast majority of the gas is trapped in the foam, that portion isn?t adding to global warming until the end-of-life situation arises.
Closed-cell foam is a higher-density foam, often referred to as 2-pound foam. It achieves R-values of around 6.7 per inch of applied material, producing the most insulation value for a given thickness of space. In closed-cell foam, the blowing agent trapped inside of the closed cells is what provides the insulation value. Open-cell foam is a lower-density product, often referred to as half-pound foam. It achieves an R-value of around 3.5 per inch with air as the insulator. Because it uses water as a blowing agent, it doesn?t create the same density as closed-cell foams, and therefore uses about one-fourth of the material, thus reducing its overall installed price.
Some manufacturers now have "soy-based" or "bio-based" content. They may not reveal what percentage is bio-based. It is important to know that when they do say it is ?20 percent bio-based," they likely mean 20 percent of the polyoyl component of the foam is bio-based. There are typically two components of foam: the polyoyl and the isocyanate reacting agent. Since these are combined in approximately equal parts, "20 percent bio-based" really turns out to be 10 percent by volume once applied. It is a step in the right direction but is not a golden ticket to sustainability.
Both open- and closed-cell foam products will help to seal the building from unwanted air infiltration when applied correctly. This has been the huge advantage of using foam since even small voids in traditional batt insulation can have serious impacts on overall insulation value. But the cellulose field is catching up.
As all of the data is not in on the combined environmental, economic and social impacts of substituting food for fossil fuels, I don?t have a defensible opinion on either the life cycle assessment (LCA) or overall preferability of partial-soy foam to pure petroleum foam. However, I do have 3-1/2 inches of closed-cell petroleum-based foam on top of my unvented flat roof and I can defensibly say it is better to put a few barrels of fossil fuel (or even fossil fuel plus soybean oil) on my roof to eliminate consumption of many barrels of fossil fuel in power generation to cool and heat my home. Better insulation (within reason) is a positive LCA-based decision. The added benefit is that I need fewer solar PV panels to supply an equivalent percentage of my remaining home energy needs.
From an end-of-life perspective, technologies are being developed to recapture gases from foam as well as technologies to turn plastics and foam from cars into fuel. From a practical standpoint related specifically to the use of foam in homes, I find it hard to envision demolition crews cutting sprayfoam out of wall cavities to recycle it. We?ll see where the future takes us.
Dry-blown cellulose insulation, which is recycled newspaper, has had a history of substantial issues with settling and thermal failure over time, as well as with fibers getting into indoor air through air envelope leaks or duct leaks. As manufacturers explored these issues, the idea of wet-applied cellulose was born. It allows cellulose to better fill in gaps and create a thorough application that helps address settling and air infiltration. Cellulose also achieves an R-3.3 per inch. Of course, wet application implies that the insulation needs to dry before it is sealed in a wall cavity. That can be a real issue. From an LCA standpoint, cellulose has about 5 percent as much embodied energy as its foam competitors, is a recycled product, and can decompose at end of life, giving it a much smaller footprint than foam. But does that stack up against differences in insulation performance over the life of a home?
As infiltration is such a significant factor in a home's lifetime energy consumption, and since foam has a higher R value per inch than cellulose, I am inclined to believe that, although there are trade-offs, the best decision at this point in the game for walls is closed-cell foam insulation. In attics, foam has the advantage that it allows you to more effectively insulate at the roof-line (as opposed to the ceiling line), bringing your ductwork inside the thermal envelope in a sealed attic configuration.
Proper installation of any insulation product can make all the difference and is more critical than the differences between foam and wet-applied cellulose.
One final item to consider is employing a radiant barrier in your proposed system. It is especially popular in the colder regions of the Northeast, and I know with Michigan?s lake effect, it can get quite cold in Dexter (even in July nowadays). A radiant barrier addresses one issue that neither foam nor cellulose addresses well ? heat that is radiated (versus conducted or convected) through a building envelope. A simple radiant barrier, with 3/4 an inch of air space, can reduce radiant transfer of heat by as much as 98 percent with a safe bet on 90 percent when properly installed. Radiant heat is the one form of heat transfer that R-values don?t measure, and it is hard to simulate using the current hot box testing method typically used to measure conductive heat transfer, or U-value. It is worth considering since a radiant barrier, even taking up a 1-inch space in your walls, can achieve relative R-value equivalents of R-7 to R-10 ? higher than either foam or cellulose in that same inch of space.
Thanks for your interest in this topic and good luck!