Hi Phil,
It'll be difficult to answer your question without making sure you understand exactly what insulation does, and how it works. Hopefully, in the process, it'll help people make a more informed decision on what will work best for them.
Having worked in the residential energy field for years, I can confidently say that all cyanurates will break down over time. Never be sold on what the sales rep tells you, their job, after all, is to sell a product. That's not the important issue, however. All insulations have major drawbacks that the consumer is unaware of.
Residential energy, 101:
The concept of residential energy is always based on heat loss. Warm air is always drawn to colder air. I cant tell you how many homes I've visited homes where the owner complained about drafts, and always blamed the windows, because people see the windows as the opening in a sealed box. This is true in only a very small percentage of situations. Often, drafts are caused by gaps in the thermal barrier, or insulation. If it's cold on one side of a wall, warm air will be drawn to it. Since we know that warm air rises, it travels up from the heat source and is drawn to the colder wall, creating a circular pattern of airflow in the room. If there is even a small gap in the insulation of a wall, the warm air will travel faster to reach the cold air, and create a draft that you can actually feel, running across your feet as you watch TV. I know this is irrelevant to your question, but humor me, and I'll get there.
Fiberglass insulation is the insulation of choice, however, if the wall that the insulation is in is not completely sealed, fiberglass acts only as an air filter. This is why we always tape and mud our sheet rock joints, but what about the outside? Do they always glue or caulk the sheathing to seal the wall from air movement? what about the gap between the sheets, if it's mounted horizontally? A simple 1/8" gap in the sheathing across a wall can reduce fiberglass's resistance value by as much as 38%. For outside walls, the best choice would be dense-packed cellulose, as it seals all the gaps as it's packed into the walls. Not to mention that it's completely fireproof in this state. (We've tested and tested this, trust me on this one) Plus, the denser it's packed, the less the chance of settling and getting gaps at the top of the wall, common with fiberglass. The drawback of cellulose? Moisture. If moisture gets into your wall, the cellulose will hold the moisture, and the wall will rot.
I bet you wont find a sales rep for fiberglass or cellulose that even knows of these problems. both of these products do their intended jobs well, under the right circumstances. Now, about cyanurates.
Spray foam insulation is comprised of a chemical called polyisocyanurate, the same stuff you can buy in sheets to put up behind your vinyl siding. There are 2 types, closed cell and open cell. The stuff in sheets is always closed cell, which is why when you cut it, it's smooth, with no air pockets. Spraying the foam means it has to be mixed, and in the mixing process air is introduced, resulting in open "cells" or air bubbles in the mix. Take a can of Great Stuff from Lowe's, spray a bit of it and let it dry, then cut it, you'll see hundreds of small air bubbles throughout it. These are voids in the thermal barrier. while it will seal air well, it wont have a very good resistance.
Another problem with sprayed insulation is that it shrinks. Yes, while it's wet, it expands like crazy, but, as with any chemical reaction such as paint, chemicals are released into the atmoshpere in the curing process through evaporation. This causes shrinkage during curing. Remember those 1/8" gaps I mentioned before? Most contractors would consider an 1/8" gap acceptable, but if that gap runs alongside your studs due to shrinkage, you could multiply the gap x the number of studs....... you get the picture.
The average lifespan for cyanurates is about 10 years. Find a house with vinyl siding that's 10 years old, and pull off the bottom run of siding, and cut out a small piece of the foam backer. Squeeze it in your hand, and it instantly turns to powder. You can imagine how distraught you'd be if you found out that your house doesnt heat well after 10 years, especially if you knew that you had to remove all that stuff from the walls to get it right again.
As far as their sales pitch that it's much much better, well, using the above information as a guide, it has it's advantages. It's moisture resistant, seals gaps in airflow situations, and it does provide a
comparative R value. It's biggest downfall? Longetivity.
I hope this helps a bit, my fingers are tired, and I need a beer.
junk in ten years
i know of a shop that foamed then painted the roof 7 years ago 