A home inspector needs to understand R-value ratings if they are inspecting insulation or performing energy audits. R-values are a measurement of thermal resistance, the ability to resist heat flow.
Increasing the thickness of fiberglass batting increases its R-value. However, there are other factors that affect the insulating effectiveness of a wall.
What is an R-Value?
R-Value is the measure of how well a specific type of insulation or construction material slows down the conductive flow of heat, which helps to reduce energy bills and improve building occupant comfort. Knowing how to select insulation with the proper R-Value ratings can help you design buildings that are efficient and comfortable for all seasons.
While R-Values are an important indicator for comparing insulation materials, they can be misleading when used in isolation. Manufacturer R-Values apply only to properly installed insulation products and do not account for thermal bridging. For example, squashing two layers of batting together in the thickness intended for one layer does not double the R-Value; it simply increases the amount of air spaces between studs that can cause heat loss through these gaps. Windows also provide a parallel heat conduction path that is not included in the R-Value of a wall, and even well-installed insulation cannot mitigate this if there are air leaks around window frames or drywall.
While insulating a home is not typically at the top of many homeowners’ to-do lists, it can have dramatic effects on energy costs and is often required by local code. By adding insulation with the right R-Value and sealing air leaks, you can save 10% or more on your annual utility bills. Choosing insulation with a higher effective R-Value is key, as is selecting a product that will not lose its R-Value over time. LP NovaCore sheathing with XPS foam, for instance, offers better long-term R-Value retention than other foam insulations.
Types of Materials with R-Values
R-value is the measure of thermal resistance, and it is used to compare insulation products. It can be influenced by the material’s thermal conductivity and thickness, but also by other factors such as air infiltration and radiation. The R-value of a material is determined by testing it in controlled laboratory conditions. It is important to understand that R-values are only true for properly installed insulation materials in a wall or building. If the R-value of a material decreases during installation, it will not be as effective as originally advertised in its performance.
For example, fiberglass batting loses some of its insulating power when it is compressed. Adding more batting will increase its thermal resistance, but it will not necessarily double the original R-value; it will be closer to the new R-value specified by the manufacturer. This is why some manufacturers have specific instructions on how to install their product for maximum efficiency.
Wood logs are another type of insulation with a wide range of R-values. Many wood log walls are insulated with R-4 fiberglass, while others have a much higher R-value with blown or closed cell spray foam insulation. It is important to explore the R-values of different insulating materials before making your selection. It is also important to consider that a high R-value in a log wall will not be as effective as a lower R-value in a SIP or other energy efficient construction system.
Effective R-Values
A good R-value is the key to making your building energy efficient. The higher the R-value, the better the insulation will perform and the more money you will save on your heating and cooling costs. However, determining which insulation is the best for your climate zone and building type requires more than just knowledge of R-values. You must also understand how the R-values are calculated, especially if you are using an existing product with a new label.
The R-values used to rate insulation are based on laboratory testing of the material. The value measures the amount of heat that will flow through a square foot of insulation one inch thick at two different temperatures. The value is calculated by dividing the thermal conductivity of the insulation material by its thickness.
When you are purchasing and installing insulation in a home, the actual R-value that will be achieved will depend on how the insulation is installed as well as the attachment method. For instance, fasteners passing through the insulation will create thermal bridges and significantly reduce the R-value of the insulation. This is why it is important to consider the effectiveness of insulation in your overall wall assembly and how you will attach it to the studs and other structural components of your home.
The Federal Trade Commission requires clear labels on insulation that indicate the true R-value of a product over time, as it settles and ages. This allows builders and homeowners to compare products and be confident they are getting the best possible R-value for their budgets and climate zone.
Nominal R-Values
Nominal R-Values are the manufacturer’s advertised value of a specific insulation product and are typically higher than effective R-Values. The latter are determined through testing or using specific software to measure the thermal resistance of a complete construction assembly, accounting for factors like air gaps between insulation panels and their overall condition (e.g., age and moisture).
Energy codes have evolved over time, requiring higher R-Values to reduce heat transfer through materials. This helps achieve lower energy bills for building owners and occupants throughout the year. But there are other factors that play a role in the performance of buildings. One is thermal bridging, which refers to the amount of energy that is lost through the edges of insulation materials.
This can be reduced by reducing the thickness of insulation and using materials with lower thermal conductivity, such as air or argon within closed-pore foam. This can significantly improve R-Values in some cases.
To show how these factors affect R-Values, Oak Ridge National Laboratory conducted a study to compare the nominal R-Value and effective R-Value of common wall designs with an insulated core made from EPS foam and OSB laminates. They found that the effective R-Value of these types of walls varies significantly depending on climate zone, wall construction techniques and insulation thickness. The results of this study are helpful in understanding how energy efficiency is achieved through the use of different building materials and construction methods.