For those of us who have lived, or do live, in a leaky old uninsulated shotgun house -- it's a rare 31 degrees in New Orleans today, and about 33 inside our office -- the easy answer to "which insulation" is ANY insulation. But when faced with choosing an insulation, one finds that there are many options and they are not coequal. Moreover, the differences between insulation materials aren't simply a matter of costs and qualities. They perform differently, derive from various sources, are installed differently, and are affected differently by water vapor, air infiltration, and time. Oh, and cats.
If you've come here looking for The Answer about insulation, my apologies. I'm going to relate some of my confusions, worries, and discoveries. If I find Truth, I'll let everyone know.
Here's one of the problems: one of our projects is nearly ready for insulation, so over the past few days I've had four insulation contractors over to discuss the project and give us an estimate for the work. Each has been an experienced professional; yet I've received four conflicting reports on the relative merits of fiberglass (blown or batt), cellulose (blown dry or wet), and spray foam (open cell or closed cell). Most insulators are true believers in one or the other type. (Occasionally you'll find one who will install whatever you want, no opinions offered.) So who knows best?
Let's begin with a critical point: closed cell spray foam is the most expensive option to have installed, followed by open cell spray foam, cellulose, blown fiberglass, and fiberglass batts. The reason most people are most familiar with fiberglass batts is because they're last on that list.
Next, what is insulation for? It's meant to slow heat transfer. We've all heard about R-value, which is the usual measure of a material's resistance to heat transfer. The higher the number -- R-20 vs. R-3 -- the more insulating value a material has. If I can be forgiven a gross simplification, the aforementioned insulations all give an R-value of approximately 3.4-3.8 per inch, amounting to similar insulating values in your average wall.
Here come the crucial differences, which I'll discuss as best I can in several subsequent postings:
1) there are at least three different varieties of air movement that can affect the insulation value of a material.
2) water vapor, from a muggy day outside or from your hot morning shower wants to travel through building assemblies. If vapor is moving through a wall and meets a cold surface within the wall -- say, the back side of some refreshingly air-conditioned sheetrock -- it will condense. Insulations affect and are affected by vapor in different ways.
3) quality of installation can vary. Batt insulation, in particular, has to be notched around obstacles such as electrical outlets, which affects its ability to resist heat transfer.
Other factors to consider are: loss of R-value over time; resistance to insects, pests, mold, or fire; off-gassing of chemicals used in manufacturing; sustainability of raw materials; suitability for location.
Note: I don't want to give the idea that the insulation types mentioned above are the only options. We could discuss the merits and flaws of rock-wool batts, rigid foam boards, soy-based spray foam, straw bales, recycled cotton batts, or grown-not-manufactured mushroom insulation. No kidding: http://www.ecovativedesign.com/greensulate/.
Some helpful sources:
http://www.inspectapedia.com/interiors/Insulation-Values-Table.htm
http://www.buildingscience.com/
http://www.lsuagcenter.com/en/family_home/home/la_house/publications/Building+Your+Louisiana+House+A+Homeowners+Guide+to+Shaping+the+Future+for+Louisiana+Living.htm
