In which condition would R-value be a more important measure than U-value?

R-value is a measure of thermal resistance, indicating how well a material resists heat flow. It is particularly significant in the context of insulation materials because higher R-values denote greater insulating effectiveness. Therefore, when comparing different insulation materials, the R-value provides a clear understanding of each material's ability to limit heat transfer, which directly impacts energy efficiency in buildings.

In contrast, U-value measures the rate of heat transfer through a building element and is more relevant when assessing the overall performance of walls, windows, or roofs rather than individual insulation materials. While the other choices relate to energy efficiency, they don't specifically focus on the insulation characteristics of materials where the R-value is paramount.