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.
Insulation Works Through Heat Transfer
- Heat and energy are the same thing
- Energy moves from hotter (higher energy) objects to colder (lower energy) objects
- Heat transfer via conduction happens much faster in objects with tightly packed atoms, solids, than those with spread-out atoms, gasses.
- Solid objects with pockets of air, like a down comforter or double-walled mug, can keep heat in or out of where we want it.
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. 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.
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.
Determining The Best Location
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.