©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Spray-foam insulation has become a weapon of choice for many builders and homeowners trying to build tight, energy efficient houses. And with its long list of attributes, that's no wonder. It fills tiny cracks and fissures in walls and roofs to form an effective air seal. The high R-values of closed-cell foam pack a lot of punch in a small space, and closed-cell versions can block the movement of moisture into wall and roof cavities. Expensive as it may be, it's at the top of its class.
But does this miracle material have a darker side? Dan Fette's question about the potential hazards of spray foam  launched an extended thread containing enough anecdotal information to give a few readers pause for thought and dissuaded others from using foam at all.
Polyurethane foam is a two-part compound mixed at the job site as it's sprayed from a high-pressure gun. Although some of its ingredients are nasty at the time of application, when it cures the foam becomes an inert material that should not off-gas any harmful chemicals. That, at least, is our common understanding and the word from manufacturers and installers.
Air Cleaning Devices 
Bathroom Vent Fans 
Typical are these words of assurance from Foam-Tech: : "Urethanes are non-toxic and only require protection for our operators during installations, but the finished product is completely safe and has no formaldehydes."
Some dissent from the field
But that sunny expectation doesn't always pan out. An anonymous poster reported developing a serious chemical sensitivity while building an "uber-green" house, which included non-toxic wood finishes and closed-cell polyurethane foam.
"I became ill after moving into the house two years ago, and had to move out," Anonymous wrote. "Any exposure to the indoor air induces neurological symptoms...I never had these sorts of problems before that I know of."
Not far behind was Marlene, who said her 66-year-old brother had experienced a "very dire situation" after IcyneneOpen-cell, low-density spray foam insulation that can be used in wall, floor, and roof assemblies. It has an R-value of about 3.6 per inch and a vapor permeability of about 10 perms at 5 inches thick. (a brand of open-cell foam) was sprayed in his house. Marlene said her brother was told he would probably have to sell his half-completed home after investing his life savings in the project.
David Posada said a strong odor persisted two months after Demilec Sealection 500 foam was sprayed into a 95-year-old house in Oregon during a retrofit. Julia, who suffers from chemical sensitivities, says her building materials consultant "won't allow her clients to use spray foam insulation of any kind."
More study is needed
William Swietlik, who identified himself as a member of the Federal Interagency Spray Polyurethane Foam Worksgroup and co-chair of the EPA's workgroup on spray foam, said that both open- and closed-cell foams are made with diisocyanates, among other ingredients, a leading cause of workplace asthma and a "well-known sensitizing toxicant to humans." He added: "Once an individual becomes sensitized to diisocyanates there may be no safe exposure level."
Swietlik said this is why occupational health authorities recommend personal protective equipment for installers, and that any unprotected workers or occupants leave the building while the foam is being sprayed and not return until "all residual vapors are ventilated and all dust particles (from shaving the finished foam) are cleaned up to safe levels."
Just what is "safe?" That's not clear. Swietlik had this to say: "The exact timing of this is not known for each specific building application as this depends on the amount of vapors and particles generated to begin with, the amount and type of ventilation, the size and configuration of the building, the foam curing factors and the installation and clean up techniques of the workers."
More reserach is needed, he said, not only on the problems posed at the time the foam is applied, but also on "whether or not there remains off-gasing from the finished foam that was applied days or months earlier that could affect sensitive, or sensitized individuals who occupy the building."
Application is the wild card
If spray foam is designed chemically to morph into an inert material after it cures, could the problem be the installation itself, an improper balance of Part A and Part B? This, at least, was the suggestion of David Posada, who raised the question after following two threads at sprayfoam.com. 
There was no unanimity on this question, but Posada added this: "Much of the published information regarding low density spray foams addresses the low- or non- toxicity of 'properly cured' or 'properly installed' foams; little seems to be said about what constitutes a improper curing or installation, what chemicals can be released, what hazards (if any) may be present, and what remedies are appropriate."
A bad mix may be only one potential problem. A poster named B. Kolodziej recounted his experience with BASF Comfort Foam during a remodel of his California home. The installer exceeded the 2-inch per-pass maximum set by the manufacturer, which causes the foam to overheat as it cures. The result is a discolored insulation layer of insufficient density that just happens to smell like rotten fish. Although Kolodziej believes the product will work as advertised if installed correctly, he now regrets he used the stuff in the first place. Further, because the foam sticks tenaciously to whatever it's applied to, removing it will be a nightmare.
Tracy Nelson summed up the dilemma this way: "Here is the simple truth of spray foam in an existing building: Mixing this material in the field can be inconsistent and as mentioned many times above, completely depends on the skill level of the installer (don't confuse the actual person who shows up at the property with the company who sells you the product and promotes themselves as experts)."
An anonymous poster said a variety of factors affect the reaction of parts A and B on application, making it "very difficult, if not impossible, to get a full and complete reaction...so A+B seldom ever equals C."
The poster included a link to an American Chemistry Council site  with information on spray foam for consumers.
Making a case for natural materials
Just what were you expecting when you chose a petrochemical over a natural insulation, like cellulose, wondered Robert Riversong?
"As I've stated more times than anyone here cares to remember, there is nothing either 'green' or healthy about petrochemical foams, or any of the 80,000 petrochemicals that never existed on earth before we created them.
"Unfortunately, most of American society is brainwashed into believing in the 'magic' of chemistry, as the advertisers and marketers have impressed on us for generations. Every product produced since the start of the petrochemical age is toxic, either to people or the environment or both."
Riversong points to studies suggesting that not enough is known of the impact on health by spray foam insulation. Moreover, he adds that the foam creates a sealed building that can't breathe, making humidity control more difficult and raising the potential for mold.
"I would suggest considering environmentally-friendly, non-toxic, fire-resistant, insect-proof, rodent-resistant, and mold-resistant cellulose, which is also significantly hygroscopic and assists with natural moisture management (as long as the thermal envelope can breath - i.e. no vapor barriers)," Riversong writes.
Where does this leave us?
The many performance benefits of spray-in polyurethane foam no doubt will continue to make it popular, particularly as the cost of energy goes up. Canadian insulation installer Dwaine doesn't dispute the problems cited elsewhere in the thread, but he also admits he is "completely in awe" of what spray foam can do.
"The blame is being laid in the wrong places," he writes. "When someone like myself sits back and reads these various comments, it is easy to see they are all for the most part human error. The biggest problem reputable companies are having right now in the foam industry is companies who are trying to make a quick buck and not follow proper manufacturer guidelines and installation instructions. I blame the governing agencies."
Dwaine thinks there are lessons to be learned from the Canadian Urethane Foam Contractors Association . Although it's not a perfect approach, it could help consumers feel confident in using foam.