Today we understand that to insulate the space between the studs with fiberglass is not the same as insulating an entire wall. We must consider the wall as a system. The lumber creates thermal bridging and air infiltration within the wall, around windows and doors and at connections between the wall, the ceiling and the floor. Now we know that it is important to insulate the wall and not just the area between the framing members.

Consider though, fiberglass insulation performs well in controlled laboratory tests, how does it perform when actually installed in a home? Fiberglass is susceptible to air infiltration. When air enters a wall through a crack in the siding or near a window opening, it flows through the fiberglass, significantly reducing its ability to resist thermal flow. Moisture, also, causes fiberglass to significantly lose its insulating ability. R-value doesn't account for these problems.

In an effort to create more comprehensive standards, researchers at the Building Technology Center at the Department of Energy's Oak Ridge National Laboratory in Tennessee are proposing a system for rating "Whole Wall R-value," which is the insulating value of a whole wall system.