An insulation’s R-value indicates how well it resists heat flow. Generally, the higher the R-value, the better the insulation is at doing its job. But how and where insulation is installed also affects its performance. Insulation that is compressed too tightly into a space will not give its full R-value. And a ceiling or wall’s total R-value will usually be lower than the R-value of the insulation, mainly due to thermal bridging (increased conduction) that occurs through studs or joists because of their lower thermal resistance. Oak Ridge National Laboratory offers a whole-wall R-value calculator at www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/rvalueinfo.htm.