As homes continue to be built to higher energy standards, the need for supplemental dehumidification is likely to increase in hot humid climates along the Gulf Coast and in the Southeast. Stand-alone dehumidifiers are a fairly inexpensive solution to the problem. Unlike an air conditioner, a stand-alone dehumidifier continues to lower indoor humidity until the desired setpoint is reached. The downside: a dehumidifier adds heat to the house. But as long as the house has a properly sized air conditioner, this shouldn’t be a problem.

“R-value measures only conductive heat transfer.”

Of the three heat-flow mechanisms — conduction, convection, and radiation — radiation is probably least understood by the average builder. Sensing an opportunity, some marketers of radiant barriers, reflective insulations, and “ceramic coatings” take advantage of this common misconception (that R-value is a measure of conductive heat transfer alone) to promote their products. But in fact, R-values include all three heat-transfer mechanisms.

The most common method of testing a material’s R-value is ASTM C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. In this test, a technician measures the thermal resistance (resistance to heat flow) of a specimen of insulation placed between a cold plate and a hot plate.

To understand how all three heat-transfer mechanisms are involved, consider the flow of heat across a fiberglass batt. Heat wants to flow from the hot side of the fiberglass batt to the cold side. Where individual glass fibers touch each other, heat is transferred from fiber to fiber by conduction. Where fibers are separated by an air space, heat is transferred from a hot fiber to a cooler one by radiation and by conduction through the air. In ASTM C518 tests of fiberglass insulation, air movement within the fiberglass batt (that is, a convective loop) is rare, although the test captures the phenomenon when it occurs.

Since R-value measures the resistance of a material to all three heat-flow mechanisms, it remains a useful way to compare insulations and to judge the performance of insulation alternatives.

Once insulation is inserted into a wall, however, the performance of the insulation is affected by additional factors that aren’t measured by R-value testing. While R-value testing measures the effects — if any — of convective loops with a tested sample, it can’t be expected to account for air leakage through a wall caused by wind or other pressure differences acting on a defective air barrier. A leaky wall assembly insulated with fiberglass batts will not perform as well as the same wall assembly insulated with spray foam with the same R-value; but the difference in wall performance is due to the spray foam’s ability to reduce air leakage rather than to a difference in R-value between the two materials. The fact that some insulations are more porous than others does not imply that R-value tests are misleading.