What characteristic does R-value primarily measure?

R-value is a measure that quantifies the thermal resistance of insulation materials. Specifically, it indicates how effective the insulation is at resisting heat flow; the higher the R-value, the better the insulation's ability to slow down heat transfer. This is critical in residential construction, as effective insulation helps maintain comfortable indoor temperatures and can lead to energy savings by reducing heating and cooling costs.

In contrast, the other choices do not relate to the measurement of insulation material in the context of heat flow. Weight, color, and soundproofing are not indicative of thermal resistance, making them irrelevant in this scenario. Therefore, the focus on insulation effectiveness as defined by R-value firmly establishes why this characteristic is the correct answer.