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R-values indicate the insulating effectiveness of building materials. Like resistance in electrical circuits, R-values increase linearly as the thickness of the material increases—doubling the batting thickness will yield double the insulation R-value.
What is an R-Value?
R-Value is a measure of an insulation material’s ability to resist heat flow. The higher the R-Value, the more effective the insulation. R-values are additive, meaning that when a layer of one material is added to another, the combined materials have an R-Value equal to the sum of the individual R-Values of the different layers.
R Values for different insulating products can vary significantly depending on how they are installed. A manufacturer’s R-Value for a product is usually only valid when the insulation is properly installed in its intended application. For example, squashing two layers of fiberglass batting together does not increase the R-Value of the combination. This is because a layer of insulation only resists thermal energy flow through conduction, and it’s the friction between adjacent insulating materials that produces heat.
Additionally, a manufacturer’s R-Value may be based on a laboratory test conducted at a specific temperature. This can lead to different results if the test is conducted at a different temperature, which is why manufacturers should always provide R-Values for their products using the same testing method.
It’s also important to remember that R-Values only apply to a material or assembly that’s not enclosed by other materials. The effectiveness of a wall that is insulated with spray foam or cellulose, for example, can be greatly reduced by the presence of studs and windows in the structure.
What is the U-Factor?
The U-factor measures how well a window resists heat flow through the glazing and the frame. U-factors are calculated using a number of factors including thermal breaks, low-emissivity coatings, and spacers. In general, the lower the U-factor, the more energy efficient the window.
The NFRC determines guidelines for U-factor ratings and other energy performance criteria for windows and doors. Having an understanding of these criteria can help inspectors and homeowners understand what the NFRC is looking for in their systems.
Like R-value, U-factors vary based on the thickness of the insulation. Generally, thicker insulation offers better performance than thinner materials. However, this isn’t always the case; for example, cotton batting or glass wool offer higher R-values when compressed than they do when they are not.
Another factor in determining the U-factor is the direction of heat flow through an assembly. In general, thermal conductance is reduced if the material is facing away from the exterior wall of a home. This helps reduce the amount of heat that is transferred through the walls and into the attic or basement.
The U-factor can also be improved by the use of argon or krypton gases between the panes of a double or triple glazed window. This is because these gases are heavier than air and thus offer more resistance to the transfer of heat. This can drastically improve the overall system U-factor of the window and make it more energy efficient.
What is the V-Factor?
In order to understand how the R-value of insulation works, it helps to start with a basic understanding of thermal conduction. Conduction is the rate of heat transfer through a material at a given temperature differential (DT). The inverse of thermal conductivity is resistance and this is what is measured and reported to determine the R-value of an insulation product.
R-values are normalised to a thickness of 1 m (3 ft 3 in) and then sorted by the lowest value. This is done because different materials have differing properties and performance at varying thicknesses. A material with a higher R-value typically has greater insulating capacity.
The R-value of a material increases in a linear fashion as the thickness of the material is increased, just like the resistance of an electrical circuit. However, this is not always the case for insulating products that are compressible such as cotton or glass wool, where the R-value decreases when the material is compressed.
While the R-value is an effective metric to describe the ability of an insulation material to restrict the flow of thermal energy, it only relates to the thermal resistance of the material through conduction. There are also other pathways for thermal energy to pass such as through radiation and through gaps between components or structural elements. These other mechanisms are not addressed by the R-value metric and must be accounted for in building design.
What is the C-Factor?
The C factor is a method of comparing the thermal resistance of a material or assembly by dividing its R-value by its thermal conductivity. For example, a steel door with polyurethane insulation has an R-value of up to 11 based on the ASTM test method but only up to 3.8 if you use the LCTR.
In simple terms, the higher the R-value, the more insulation you get for your money. However, there are many other factors that affect how well a particular insulation works. For example, the type of material, its density and thickness all contribute to how well it resists conductive heat flow. The type of building and the way it is assembled also play a role.
Another important point to understand is that the R-value only applies to a single layer of insulation. It does not account for the fact that heat moves in and out of a building in multiple ways, including convection and radiation. In addition, if there are gaps in a wall structure or between studs, the R-value will be less than expected because the gap will provide a direct path for heat to move through the wall.
Increasing the thickness of an insulation material will increase its R-value. This is because the thermal resistance increases in a linear manner as the material gets thicker. However, the same is not true for compressible materials such as cotton batting and glass wool.