After having looked at hundreds of cathedral ceilings using an infrared camera in which fiberglass batts are laid over strapping, I (and many other energy professionals) have seen enough heat loss to melt the North Pole. We see why snow melts quickly on many cathedral ceilings, why people are uncomfortable and why energy bills are higher than they need to be.
The main problem is air movement through gaps created by fiberglass batts laid over strapping and the fact that air moves through the fiberglass itself with great ease. Further compromising the R-value (or insulation value) is the frequent lack of wind-baffles at the eaves, which are intended to prevent cold air from "wind-washing" the ends of the fiberglass batts. Now add a few recessed lights in the cathedral ceiling, which act as little heat pumps, forcing heated air into the ceiling and taking up space in a 9.25-inch (or less) thick cavity that would otherwise be insulated.
In fact, fiberglass batts only work to the manufacturer's rated R-value when they are installed in air-tight cavities. This has rarely been the case in the past, but now new energy code requirements call for caulking wood framing so that when fiberglass batts are used, R-values can be realized and overall building air-tightness standards can be achieved.
The obvious goal in attempting to improve the thermal performance of cathedral ceilings is to stop or slow air movement through them. There are different approaches to doing this and the approach taken depends on many factors. Access is a big one and length of the cathedral ceiling is another.
The most typical approach involves dense-packing cellulose into the full length of the cathedral ceiling. It is best if all fiberglass is pulled out first — usually from the eave or the back of a knee-wall. The densely packed cellulose virtually stops air movement through the ceiling and provides a true R-value.