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To understand heat transfer and how insulation affects it, you have to understand the second law of thermodynamics. It’s all about nature’s tendency to normalize and balance different energy levels. Energy and heat are inseparable concepts. Atoms, the backbone ingredient of the universe, each contain a certain amount of energy at a given moment. Therefore, these atoms and the objects they make up can also be said to hold different amounts of heat.
A simple example involves two objects: your stove and a pot of water. The second law of thermodynamics explains why a hot stove eventually causes the pot and its water to heat up as well—stating that heat transfers from an object of higher temperature to that of a lower temperature. Even in the absence of what we normally think of as heat, this law is at work. A block of ice only one millionth of a degree above the freezing point will still inevitably melt.
But while this transfer of energy is inevitable, the rate of temperature change depends on conditions like the type of heat transfer—conduction, convection, or radiation—and materials the heat must travel through.
Let’s start with conduction. Bulk insulation (that is, insulation that works by how it takes up space) limits heat transferred through conduction. Great insulators (everything from down comforters to double-walled mugs) exist because fluids and gasses are less conductive than solids because they are less dense, meaning there is more space between each atom of the object.
Atoms less likely to bump into each other will take longer to transfer, and theoretically eventually equalize, their energy. Everyday examples like down comforters and double-walled mugs rely on putting something with spaced-out atoms (in this case, air) between objects to better limit heat transfer.
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.
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.