To wrap up, it's important to think about a few things when you're dealing with R-Value and U-Value:

• R-Value should be as HIGH as we can afford. Building codes will tell us the minimum we're supposed to have
• U-Value should be as LOW as we can afford. This is a more accurate reflection of the performance of our assemblies.
• Some materials are really good at resisting the flow of heat - high R-Value and low U-Value. Insulation materials are the best example - fiberglass, mineral wool, foam, cellulose.
• Some materials are really bad at resisting the flow of heat - low R-Value and high U-Value. Most metals, solid concrete, stone are all poor insulators.
• U-Value is the inverse of R-Value.