Heating and cooling your home sucks more money out of your bank account than any other type of energy consumption (unless you live in a high performance home, in which case the proportions are different but the total is smaller). The building envelope, which comprises insulation and an air barrier, controls the flow of heat between inside and out. A good building envelope is the first step to keeping heating and cooling costs low. (Well, OK, good design is the first step. You're right.)
Not all types of insulation are equal. Insulation is rated by R-value, but just putting R-13 insulation in your walls is not a guarantee that it's performing at R-13. That's why RESNET came up with a method for grading the installation quality of insulation, with grade I being the best and grade III being the worst.
Fiberglass batts often result in a grade III because the contractor fails to take the time and effort to install them properly. Batts can achieve better grades when installed carefully, i.e., the batts are cut and adapted to the correct dimensions. Building inspectors usually don't have the time to look at installation quality, though. They look only to see that a home has insulation in the places that need to be insulated and that it's the correct R-value.
Homes that are going for certification in a program like ENERGY STAR for new homes often have better installation, but only after the insulation contractor trains their guys to do it well. Even then, though, they typically get only to grade II. I think I've seen only two grade I installations with fiberglass batts. (One is the last photo below.)
Here then are a few photos to show you some typical problems that cause fiberglass batts to perform poorly.
This photo shows ceiling joist cavities insulated with fiberglass batts that were cut too long, leading to compression. They're also not going to make good contact with the air barrier, the ceiling drywall.
The extra wood in this wall section not only is a thermal bridge but also creates irregularly sized cavities that require more care in cutting the batts to fit without compression. Insulation contractors rarely take the time make sure every piece is cut properly.
Irregular framing details are difficult to insulate with fiberglass batts.
Fiberglass batts not cut to the proper length are just ugly. The result is compression and incompletely filled cavites, which lead to more heat flow. These R-15 rated batts will most certainly not perform at R-15.
Oops! Someone cut this one short.
That's OK. They made up for with these extra long batts. (No, it's really not OK.)
And of course, we can't talk about the installation quality of fiberglass batts without mentioning electrical junction boxes. Those places where you plug your stuff in or turn lights on are often a thermal liability, as you see here. If you can see straight through to the exterior sheathing, a lot of heat will flow through there.
The photos above all come from a house that was going for ENERGY STAR and EarthCraft House certification but what you see is more typical of a regular house. (I don't know if they ever got certification.)
I promised you above that I'd show you what fiberglass batts look like when installed well, so here it is.
This installation was in a modular home, and they took the time to do it right. A few months earlier when I first visited their plant, they were doing grade III. The plant engineer told me on this second visit that they'd usually put their worst guys on the insulation crew, but after they realized the importance of it, they started putting some of their best guys there.
Back to the main point, in my experience and that of a great number of HERS raters and others who have training in building science, fiberglass batts are almost never installed well. Grade I is certainly possible, but it's rare. These problems have caused the Green Curmudgeon (aka Carl Seville) to suggest that maybe fiberglass batts should be outlawed. I don't think I'd go that far, but I do suggest that you understand the problems and know that you're probably not getting the rated R-value if you use them.
The installation problem is also well documented by others in the field. In 2009, Martin Holladay wrote an article on fiberglass batt installation in his Musings of an Energy Nerd blog. He cited research done by the California Energy Commission in which not a single home of the 30 in the study had the fiberglass batts installed correctly. Rob Yagid, an editor at Fine Homebuilding magazine, wrote an article detailing the same kinds of fiberglass batt installation problems that I showed above and cited Oak Ridge National Lab's research on the diminished R-value of poorly installed fiberglass batts. This is not an isolated or rare problem.
A note on batt manufacturers. In case you're wondering why the photos have the gaudy blue strips, the manufacturer of the batts in that house asked me to remove their name from them. Since the point of this article is about installation and not differences in batts from one manufacturer to another, I consented to their request. I've seen these same problems with batts from all the fiberglass insulation manufacturers, so it doesn't really matter if the name is blocked out.