Is R-value alone insufficient for evaluating a wall's performance?
Yes. All the insulation in the world will have little effect if it is installed in such a manner that air can leak through. Even in an otherwise well insulated buildings, many small leaks can allow large volumes of air to infiltrate the structure. Wind increases infiltration rates as well as convective heat loss (wind chill effect).
Moisture condensation is another area of concern with fibrous insulations. In the past few years there has been growing concern about the effects water vapor has on wall systems in tightly constructed buildings.
The theoretical basis for this concern is as follows: vaporized water generated by daily living activities penetrates the interior sheathing (usually drywall) and enters the wall cavity. As the vapor contacts the cold outside wall, it can condense into actual water droplets which create the potential for mold or mildew, blistering paint, degraded insulation and even structural damage due to rotten studs.
The degree to which these concerns are realized depends upon the tightness of the structure. Moisture condensation in older, more loosely built structures has not been a large concern because water vapor could readily escape along with the heated air. In a tighter, energy conscious building those escape routes are more likely to be blocked, creating the potential for problems.
Weather or not there is damage to structural materials depends upon their composition, but there is another potential problem. The condensed water can saturate the insulating material, thereby increasing the conductivity (reducing its R-value) and causing considerable heat loss. Even relatively small amounts of moisture in fiberglass and cellulose insulations are known to significantly reduce their insulation capabilities.
In an attempt to prevent this, vapor barriers are installed. These barriers can be created with many different materials, so long as they restrict (though not eliminate) the water vapor that passes through.
So, in order to evaluate the insulating performance of a given wall system, it is important to consider the tightness of construction, the R-value of the materials, the quality of the installation, and the wall's susceptibility to moisture condensation.