Understanding Insulation R-value
Comfort in a home is typically a rule of opposites in extreme environments. If you live in a cold climate you want a warm home. If you live in a warm environment you want a cool home. One of the most effective ways to improve the energy efficiency of a home is to optimize its insulation.
The main purpose of insulation is to reduce heat transfer. Heat wants to move from warmer to colder objects. For example, if you wake up on a cold morning and step onto a cold tile floor with your bare feet, heat from your feet moves to the colder floor. When heat transfers from your feet to the floor you experience heat loss, which makes your feet feel colder. In this situation, you can choose to insulate your feet, with socks. Socks will slow down the heat lost from your feet to the tile floor, therefore your feet don’t feel as cold.
Home insulation varies in both structure and material type. Insulation can be in the form of batts, rigid foam board, or foam spray. Material types include fiberglass, cellulose, polystyrene, mineral wool, natural fibers, and more. With all the different variations of insulation there is a strong desire to standardize insulation performance.
What is R-value?
All material sold specifically for use as home insulation has something called an R-value. This value is identified by the U.S. Federal Trade Commission (FTC), and has established an “R-value rule”. The R-value rule (informally) is located under Title 16 (Commercial Practices), Subchapter D (Trade Regulation Rules), Part 460 (Labelling and Advertising of Home Insulation) of the Code of Federal Regulations. Part 460 contains 25 sections giving details of how the U.S. FTC regulates R-value, which gets complicated.
Simply put, R-value is a numerical value that represents how well a material of specific thickness can resist heat flow. Comparing materials with identical thickness, the one with higher R-value insulates better than the one with lower R-value. For example, if one inch of material A has an R-value = 10, it insulates better than one inch of material B with R-value = 5. Seems simple, the higher the R, the better it insulates, but quite a bit of debate exists regarding the accuracy of R-value with respect to insulation effectiveness.
Although R-value is the standard for rating insulation effectiveness, it is not trusted as being accurate by everyone. One of the main criticisms of R-value is that the testing environment doesn’t represent real-life conditions where insulation is used. Some of the ways in which this occurs are:
- temperature of test environment – the testing environment (median temperature of 75 degrees F) doesn’t represent a significant percentage of environments where insulation is used, and temperature changes can change R-values
- air movement – testing is done in an air-tight environment, and home insulation spaces are rarely air-tight
- moisture effects – water absorbed in insulation material can reduce R-value, where moisture is not present in the testing environment
- installation – improper installation (i.e. compacting fiberglass insulation to fit into a space that it might be too narrow for) reduces R-value of a given material
- thermal bridging – heat can escape through parts of the home structure being insulated (i.e. wooden wall studs) diminishing the overall effectiveness of an insulation of particular R-value
Finding the R-value of a given piece of insulation material (as deemed by the FTC) is a matter of measuring the heat loss between two sides of the given material, for a specific temperature difference, over a given area, over a given amount of time. This is specified by the equation below:
The equation above states that R is the temperature difference of the inner and outer boundaries of the insulation material (through its thickness), times the area of the material, times the amount of time the measurement took, divided by the amount of heat lost. Let’s revisit an earlier example (using materials A and B) to understand this equation better. If material B has an R-value = 5, let’s assume that material A (with R-value = 10) is tested at the same temperature difference, area, and the time of the measurement as material B. If this is true, then the heat loss for material B has to be double the heat loss for material A in order for its R-value (material B at R-value = 5) to be half of the other (material A at R-value = 10).
Other R-value formulas
R-value (using thermal conductivity):
R-value (using heat flux):
R-value formula complications
The two formulas above share a similarity in that one parameter in each formula is relatively straightforward to measure (the thickness in the thermal conductivity formula, the temperature difference for the heat flux formula) and the other parameter is complex (thermal conductivity and heat flux values). Even if a thermal conductivity value is available, there are complications. Thermal conductivity has a temperature dependence for a given material. In other words, a material’s thermal conductivity at some temperature is different than its thermal conductivity at a different temperature. Since this value isn’t constant for a given material, and it’s not straightforward to measure, it can be problematic to use in an R-value calculation.
Heat flux is difficult to use for a different reason. It is not something that is typically available on a list of material properties. Heat flux is typically measured with a sensor’s voltage output. Heat flux can be calculated from thermal conductivity, but not easily.
Even the initial R-value calculation shown above has its complexities. With a watch, a tape measure, and an infrared thermometer, you can find most of the unknowns, but how does one measure heat loss?
Some Things to Remember about R-value
The Code of Federal Regulations states that sellers of home insulation must list a materials R-value, which is must be tested according to the standards they have defined under Title 16/ Chapter I/ Subchapter D/ Part 460. But realize that expensive insulation, advertising a very high R-value, may not perform as expected if it installed incorrectly. Also, there may be materials (non-reflective) being used for home insulation that may not have an R-value. If so, this doesn’t necessarily invalidate the effectiveness of the material as an insulator. It just means that the material wasn’t originally designed to be home insulation.