As energy efficiency has become an increasing concern among builders and homeowners, the attributes and performance of building materials and components are being scrutinized more closely. In order to maximize levels of efficiency by examining the details of how each individual component of a house performs on its own and as part of a dynamic system, very specific properties are measured and taken into account. This can be especially helpful when trying to select the best building materials for a given application. R-value is the measurement used when quantifying a specific material’s level of thermal resistance, which is the inverse of U-value, which measures thermal conductance. R-value is often the standard consideration when discussing the effectiveness of insulation. Inspectors may want to be familiar with the specifics of R-value ratings, especially when inspecting insulation and if they also perform energy audits.
How Does R-Value Relate to Insulation?
Heating and cooling costs account for 50% to 70% of energy used in an average U.S. home. Inadequate insulation can account for a lot of wasted energy, so it is important to be sure that insulation installed is doing its job properly. The function of insulation is to provide resistance to the flow of heat, and R-value is the measure of exactly this attribute for a given material. A higher R-value equates to higher resistance to heat flow and greater effectiveness in insulating. An insulation material’s R-value, in conjunction with how and where it is installed, will determine its overall thermal resistance and effectiveness. Adding the R-values of each layer of material contained in one building component, such as a wall or ceiling with multiple layers of insulation, will help determine the thermal resistance of the whole component. The way the insulation is installed, as well as other factors, will also affect its thermal resistance.
Important Factors to Consider When Measuring Thermal Resistance
When considering R-value as a means to determine the thermal resistance of a building component, there are other factors that must also be taken into account. While R-values are an excellent guide for comparing the attributes of different insulation products, they apply only when the insulation is properly installed. For example, if two layers of insulation are smashed into the thickness intended for one layer, the R-value does not double. Likewise, if a single layer of insulation is compressed during installation, it will not be as effective. Stuffing batt insulation sized for 5 inches into a 4-inch wall cavity will actually lower its R-value. Ensuring that insulation is correctly installed will help allow the product’s full benefits to be realized.
Also important to consider is the fact that even when installed correctly, insulation affects heat transfer through the insulation itself but not through other materials, such as glass windows and studs. If there are structural gaps in any building penetrations, even insulation with a high R-value that's installed properly cannot mitigate heat loss from air leaks. Studs and windows provide a parallel heat conduction path, and insulation between studs in a wall does not restrict heat flow through the studs. This heat flow is called thermal bridging, and the overall R-value of the wall will be different from the R-value of the insulation itself.
Calculating and Converting R-Value
The equation used to calculate R-value may be of interest to some inspectors because if the R-value is known, the equation can also be used to help calculate heat loss. The equation for determining R-value is as follows:
R-value = temperature difference x area x time ÷ heat loss
The temperature difference is expressed in degrees Fahrenheit, the area in square feet, the time in hours, and heat loss in BTUs. Since European R-value uses different units of measure (Celsius, Kelvin, meters, etc.), it may be helpful to know how to convert a European R-value into a U.S. R-value. This is done by multiplying the European value by 0.176 and dividing 1 by the result.
The FTC and DOE on R-Value
In the 1970s, the Federal Trade Commission (FTC) created a rule requiring insulation manufacturers to disclose R-values at the point of sale and in some ads. This is intended to protect purchasers from false claims made by manufacturers and to create a standard of comparison for products.
The U.S. Department of Energy (DOE) has issued recommendations for insulation R-values in new and existing homes. The recommendations are based on a comparison of the cost for installing insulation versus potential future energy savings. Their recommendations for attics, cathedral ceilings, walls and floors are generally greater than what is actually required by most current building codes.