Having recently discovered some serious carpenter ant damage in one corner of the house, I had some big gaps to fill -- temporarily. (We were going away for 2 weeks and I wanted to make sure no critters moved in.) Fortunately, I love expanding, insulating foam. It's a character defect, I know, but there is something about foam expanding to fill any accessible void and hardening into a styrofoam-like substance that just tickles my fancy. (I've also used blobs of it, sprayed metallic gold, for creating the illusion of palatial brick-a-brack on a theatrical set.) Ooooh!

Now (serious note) I am a blood cancer survivor, and so am highly suspicious of any substance that our species did not evolve with.  Although it's hard to imagine life without expanding foam, apparently this deprivation is one of the hardships that made the Neanderthals what they are today. Hence my suspicion. And what with us folks in the northern hemisphere entering the fall and winter seasons and concerned about insulation, we are undoubtedly going to turn to this stuff on occasion to fill dark voids in our pathetic, sunlight-deprived lives.

The foam I'm most accustomed to using is the kind that expands rather noticeably and gets truly hard, not the kind that fluffs up a bit and stays kind of spongy. The former is a polyurethane material, and the latter is latex based. My choice in polyurethane spray foam has been Dow's Great Stuff, for the scientific reason that it's what is on the shelf around here.

The biggest risks from this sort of material appears to be the normal risks that arise from being stupid with pressurized cans of chemicals during application: catching fire, being asphyxiated, or getting body parts glued together. Read the can. I don't care if you need your reading glasses and a magnifying glass. The can will tell you to ventilate, wear gloves, and make sure no ignition sources are within reach of the fumes. Needless to say, working with polyurethane foam in, let us say, a closed closet, nude, by the light of a candle, would be highly inadvisable from any number of standpoints as well as being darned hard to explain to the rescue squad. The can will also advise you against being truly stupid in a variety of others ways that will suggest to you that penetrating the can with a nail, keeping it in your car in the hot sun, or eating it like spray cheese on a Ritz are bad, if bold, experiments best left to safety professionals and idiots on TV with amazing health insurance policies.

Some of the hazard comes from flammable propellants found in many other aerosols and fuels, such as isobutane and propane. Some of these are heavier than air, and so can possibly pool in low places; for instance, I have heard of one guy who blew up his pickup containing a leaking propane tank when he threw his tools into the back and struck a spark. The bed was serving as a pool of propane. A basement is generally a low place. Furnaces in basements are widely suspected to be sources of ignition.

What about other, more subtle risks? I found the full product safety sheets on the Dow website, for instance for their Gap and Crack Filler. From what I read on the web, the people at most long-term risk from polyurethane foam are the ones involved in its manufacturing; this EPA report (scroll down to polyether polyols production) gives some idea.

Oh, yes. Wear gloves, for Pete's sake. I have finally learned that there is NO way to keep from getting the stuff on your hands otherwise, and it does not come off with any household solvent. Generally, I find it wears off after many days of picking at it.

There's a nice one-page overview of the pros and cons of polyurethane foam and various other insulation types at the Mesothelmiona and Asbestos Awareness Center. See our Green Web Links.