Fiberglass batts are the most common insulation materials used in construction, and the vast majority of the time they are not installed properly, leading to very inefficient construction. This inefficiency is not a problem with the product; rather it lies entirely with the installation. In order for it to work properly, it has to fit snugly, without any compressions or gaps, and be tight against the sheathing or drywall to provide a good air barrier. When any of these don’t happen, the insulation doesn’t perform like it’s supposed to. And most of the time, most of these things don’t happen, and the insulation isn’t doing much good.
These problems are not confined to fiberglass. Batts made of lambs wool, mineral wool, cotton, and any other material are subject to the same installation problems. Now before we go blaming the people who install the stuff, we need to acknowledge that it is very difficult to install properly. Batts are designed to fit between walls studs, floor joists, and ceiling joists. They are available in a wide range of thicknesses and widths that fit standard construction spacing dimensions like 16” or 24”. If every stud cavity was the exact dimension and had no obstructions like wires, pipes or ducts, batts would be relatively easy to install. The problem is that once you put in all the windows, doors, interior walls, wiring, plumbing and HVAC, most homes and few, if any, standard size, empty stud cavities for the insulation to fit into. So practically every single batt has to be trimmed to fit the space it goes into, and requires cutting around each receptacle, pipe, and wire. This is very tough work to do, and since most insulation installers get paid based on how much they install, and they don’t get paid much for this, they blow through their projects very quickly, stuffing and jamming insulation anywhere it will fit. And without good quality control, the insulation goes in poorly, is covered up with drywall, and no one ever thinks about it again, until the homeowners start freezing their buns off in the winter because the insulation was installed wrong.
There are alternatives to batt insulation, including blown in cellulose and fiberglass, spray foam, and foam boards. They generally work better than batts, but they are all subject to installation problems. Are you sensing a theme here? Yes, installation matters a lot. You can take just about any good material and do a bad job installing it. Blow in and spray in products generally do a better job of filling gaps around obstructions and you don’t have to worry about trimming it to fit a special sized opening. You do still have to make sure that they are installed properly without any gaps, and they are in contact with air barriers.
The only insulation that doesn’t require a separate air barrier is spray foam, which is an air barrier itself. But let's get back to batts. Building and energy codes require that insulation be installed according to manufacturer’s instructions, which require tight fits, no compression, and trimming closely around all obstructions. The problem is that the work is almost never done according to these instructions and building departments don’t enforce the code, so most people just go on doing a poor job of installation and homeowners continue to suffer the consequences.