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There are three main variables that determine how insulation works: heat transfer, R-value, and location. To maintain comfort in your space, the heat gained in summer and the heat lost during winter must be replaced by your HVAC system. By properly insulating a space, heat resistance occurs which decreases the heat flow process and allows your space to remain a comfortable, even temperature.
In the same way we measure the level of energy (heat) in an object by determining its temperature, we can measure the ability of a material to resist temperature changes by its R-value. The term comes from Thermal Resistance. Commercial insulating materials like cellulose, fiberglass, and spray foam are tested and assigned an R-value rating that says how well they limit heat from moving through them. The higher the R-value, the better the material insulates. More technically, in order to rate materials “apples to apples”, R-value is the measure of thermal resistance per inch. A one-inch cube of a material rated R-7 will be twice as effective at insulating as a one-inch cube of a material rated R-3.5. Multiple units of an R-value rated material can add up. A six-inch thick layer of R-4 material, for instance, would be rated R-24.
R-value is important, but doubling the R-value of a building’s insulation won’t double its resistance to losing or gaining heat. There are complicating factors including doors, windows, studs, and air leakage. Temperature change (and moisture with it, but that’s another topic to cover) through conduction in materials of studs, windows, doors, and other building elements or through air movement still happens apart from your insulation’s R-value rating. R-value is only one component of overall performance. Later on, we’ll discuss the dramatic difference sealing can make in a building’s energy efficiency.
Insulation’s ability to resist heat flow is dependent on where and how it’s installed. For instance, compressed insulation loses some of its projected R-value. R-value can also be a cumulative measurement, like weight. The R-value of an attic, wall, or crawlspace is different from the insulation itself the same way the weight of fifty people is different from the weight of each one but still measured with the same units (pounds). The insulation has a base R-value while an assembly of insulation also has an R-value that’s based on the density and thickness of the insulation in a given area.