To obtain the best performance from fiberglass insulation, the Energy Star Homes program now requires most fiberglass-insulated framing cavities (including knee walls) to be enclosed by air barriers on all six sides. If builders pay attention to airtightness, fiberglass insulation can (at least in theory) meet the performance expectations that the R-value label promises. Nevertheless, in the real world, builders who use fiberglass are unlikely to reduce air leakage enough for a fiberglass-insulated wall to perform as well as a wall insulated with the same R-value of cellulose or spray-foam insulation.
“Radiant heat passes right through conventional insulation.”
The idea that conventional (mass) insulation products allow radiant heat to pass right through them — that “mass insulation is transparent to radiant heat” — is a scare tactic used by some marketers of radiant barriers. The misleading claim leads some builders to falsely conclude that radiant heat can travel like radio waves right through a deep layer of attic insulation, with the only solution being a layer of aluminum foil.
Radiant heat travels through air (for example, from an open fire to nearby skin) or a vacuum (for example, from the sun to the earth). It can’t travel through a solid material like concrete. If sunlight warms a concrete patio, the heat travels to the ground below not by radiation but by conduction; in other words, the concrete is first warmed by the sun (by radiation), and then the warm concrete gives off some of its heat to the soil below (by conduction). In this example, there is no radiant heat transfer directly from the sun to the soil.
A microscope reveals that most insulation products consist of fibers or pieces of material surrounded by air. If one side of an insulation blanket is exposed to radiant heat energy, most of the radiation ends up hitting a fiber or speck of material in the insulation layer, heating up that fiber. The warm fiber can then reradiate some of the absorbed heat to an adjacent fiber, as long as that adjacent fiber is at a lower temperature.
When radiant heat hits one side of an insulation blanket, only a tiny percentage of that radiant heat is “shine-through” radiation — that is, radiation that manages to miss all of the fibers in the insulation blanket and emerge unscathed on the other side of the blanket. “With insulations like fiberglass or cellulose, radiation can be absorbed by one piece of material and then reradiated,” explains David Yarbrough, an insulation expert and research engineer at R&D Services in Cookeville, Tenn. “There is very little shine-through radiation with any of these materials.”
The fact that heat flows through a layer of insulation, usually by a combination of two or three heat-transfer mechanisms, does not mean the insulation isn’t working. Although insulation doesn’t stop heat flow, it slows it down considerably; the more insulation, the lower the heat flow.
How much heat flows through an uninsulated ceiling into a 1,000-square-foot 32°F attic? Assuming that a 72°F house has an uninsulated drywall ceiling — that is, a ceiling assembly with an R-value of 2 — the heat flow across the uninsulated ceiling is 20,000 Btu per hour.
If insulation is added until the ceiling assembly has an R-value of 38, the heat flow is reduced by 95 percent, to 1,052 Btu per hour.
Martin Holladay is the editor of Energy Design Update.