Understanding

R-Value

There are many conditions that determine the effectiveness of any building material to provide comfort and energy efficient performance. R-Value is only one of these factors or measurement tools.

R-Value is the measure of resistance to heat flow through the defined material. The higher the R-Value the less heat will transfer through the wall, making the system more energy efficient.

R-Value is determined by testing or by the addition of tested components. When “effective” R-value is used, it represents that at a given condition or circumstance, the system performs the same as a product with the “real” R-value.

It is important to understand the conditions at which the “effective” R-value was determined, and see if your application is the same. For example, a masonry wall may have a high “effective” R-value for a 5 hour test period, but have a very low R-value during a 24 hour test period. Where “real” R-value products will have the same R-value during both test periods.

There are several items to consider when evaluating R-Value and it’s effect on your project

1. Effective R-Value

An example would be masonry products. Masonry product have a very low R-Value but have a high thermal mass. Foam type products have a high R-value but low thermal mass. There is not a standard measurement or number for the energy effectiveness of thermal mass. While R-value resists the flow of heat, thermal mass delays the transfer of heat but does not reduce it. During a short period, thermal mass has the same “effective” R-value, so it is sometimes advertised with this R-value rating. It is not a real R-value but, under those conditions, an “effective” R-value. Please review other technical bulletins for more details and other related items.

1. Installed R-Value vs Advertised R-Value

It should be noted that R-Value of an item is the value that was tested in a laboratory and is real, but the item may not perform at the same R-value when installed. Most insulating materials obtain their insulating values from trapped air spaces, often the higher the density of air pockets, the higher the R-value. If these air spaces are compressed, a lower R-value will result. For example, batt type insulation may be rated at R-19 in its free state, but requires a 6 ½” thickness to obtain that value. When this material is installed in a 5 ½” frame cavity, the batt is compressed and the R-value is less than R-19. Likewise, putting two batts in the space for one does not increase insulating value; it might even reduce the R-value. Review the Technical Bulletin on Air Infiltration in Insulation for more related information.

1. Nonuniform Material

Whole wall R-value is the term used to describe the average R-value for the total wall taking into consideration variations and non-uniformity’s in the insulating material. Often, the R-value of the insulating material is advertised without stating the effect of the total system.

An example would be installing an R-19 batt insulation in a 2×6 frame wall. The resultant “whole wall R-value” is the average of the R-19 insulation and the R-5 stud. Tests have shown that the actual “whole wall R-value” of an R-19 wall system to be R-13.7, much less than the R-19 advertised. This applies to all non uniform wall systems.

The R-value of building materials may be effected or altered by the following items.

• Non uniform material
• Thermal shorts
• Material compression
• Air infiltration in the insulation
• Humidity
• Temperature swing or range of environment
• Effects of time and aging.

Along with R-Value, several other factors need to be considered to determine the overall energy efficiency of a building structure.

• Thermal Mass
• Air infiltration in the building
• Radiated heat gains
• Internal heat gains
• Latent and Sensible heat transfer