Polyurethane Spray Foam is a type of high R-value insulation that has many positive attributes, and is probably the best possible insulation on the market at present. I'm not too entirely sure how to go about laying all the facts out here, but I'll give it my best and begin by pointing out the benefits and the negative aspects.

Good reasons to use this type of insulation:
  • Despite the fact that it is more costly than nearly every other insulation available, this is still your best bet, especially if you've the money to do it. It will cost you more up-front, and save you a lot later on. Due to the superior quality, it will save you on heating bills and the like quite dramatically, in fact.
  • It is weather resisant, water proof and you will not get the molding and condensation that you would with typical fiber glass or other insulations.
  • It is extremely clean as far as application, that is you can have it applied between studs evenly and nicely so that you don't have the annoyance that you would with other types and it completely seals everything.
  • There are types of this insulation that can be used in furniture and the like and are obviously softer, but the type used in houses is rigid and adds a lot of strength and support to walls or wherever it is you place it. It can also be sprayed on floors and I've even seen it applied to unfinished floors, that is, dirt.
  • You only need about 3-4 inches, though you can get away with 2 sometimes, to have a pretty nice r-value. It only takes 1 inch or less to seal something off.. that's why they have little cans of it that you can use for sealing around windows.
  • It's much easier for the people applying, if you feel like being a kind and caring little human. Fibre glass is horrid to install..
Of course, as with everything, you've got your down sides:
  • You do not want to be around during the application of this stuff, though the fumes clear away quickly in a well ventilated area. It smells horrid and is the farthest thing from good for you that exists.
  • It is highly flammable when left exposed, but I guess not anymore so than any insulation. It is generally covered with paint if it's going to be in the open, though typically it's covered by plastic and drywall among other things.
  • It is a pretty high up-front cost and some people just can't afford it, even though it does save money later on.
  • I'm pretty sure this isn't even legal, but some of it contains lead. Nevermind the other slew of chemicals.. though only the applicators really have to worry about this stuff.
  • Everything in the general area should be covered because tiny specks of the spray will coat anything it reaches and it's actually not too much fun to get off. Luckily, it's pretty easy to throw drop sheets over things and tack plastic up around windows.
Enough already, how does it work?

The insulation starts in liquid form, two different liquids in fact, resin and iso, for instance. They must be heated because they are applied when quite warm.. the two liquids mix at the "gun" from which the spray is emitted. It's quite neat to watch, acutally. The spray goes out, about 10 seconds later it expands and within 20 it is hard enough to walk on, that is it can support the weight of a full grown adult. (Of course, if you have someone jumping up and down on an inch slapped between two boards or something, don't expect it to hold.. but it is VERY strong.)

The people applying this stuff have to wear masks so that they don't inhale the extremely disgusting fumes, and it's actually preferrable that they have a fresh air supply unit. It's very easy to become sensitized to the chemicals, especially if you don't wear a mask and stupidly breathe in the fumes. If you do decide to have this type of insulation in your home, for godsakes, don't stand there and watch the people spraying it unless you have a mask on! (Honestly, people do this, they stand there and watch the person spraying despite the fact that everyone else around is wearing some sort of air filtering device. It's sickening how foolish some people can be.)

So any way, when it all comes down to it.. if you've got the money to pay for quality this is probably the best insulation to go with, in my experience. (Not that I have a lot of experience.. bleh.) The only reason I know anything about this stuff is because I've worked with it, monitoring gauges and setting up the whole operation. Oh, the joys of spray-foaming! (Whatever you do, don't aspire to be one of the people that does this sort of thing.. it REALLY isn't very nice.. though I suppose it could be worse, it could always be worse.)