UNDERSTANDING R-VALUE

Although the term R-Value is used by many, there is a lot of confusion about what it really means. 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. Since most of these factors are not well understood and some of them do not have a "term" to measure their effectiveness, many companies use the "R-Value" term to try to represent their product. There are some companies that will present this as an "effective R-Value", but others will not. "Effective R-Value" is intended to mean that their product will perform the same under a given condition as materials that actually have this R-Value. This does cause some confusion and misunderstanding when comparing different building materials.

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 circumstances that 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 hr test period, were 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. On a short time frame, thermal mass has the same effect as high 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.

2. Installed R-Value vs Advertised R-Value

   It should be noted that R-Value of an item is that value that was tested in a laboratory and is real, but that 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 ˝" thickness to obtain that value. When this material is installed in a 5 ˝" 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.

3. Non-Uniform Material

   Whole wall R-value is the term used to describe the average R-value for the total wall, takein 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 2x6 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.

4. Thermal Shorts

   Thermal shorts occur when there is a highly conductive material that extends from the front to the back of the wall. This "short" will then conduct or bypass a lot of heat around the insulating material. This condition exists with the web of block walls and in steel framing. Thermal shorts have the effect of reducing the insulating ability of the wall system and reducing the overall "whole wall R-value". By applying an external layer of insulating material, the effect of the thermal short can be minimized.

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.

In addition to 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

In conclusion, R-Value is an excellent and a prime gauge of the energy efficiency of a building system, but a careful review of the total system that effect R-value and the other factors need to be evaluated as well. A review of other factors that effect energy efficiency is covered in separate reports.

RELATED TERMS

k-Value

"Thermal Conductivity" – the amount of heat; in Btu’s; flowing through a one square foot piece of homogeneous material, one-inch thick, in one hour, with a temperature differential of one degree.

U-Value

Over-all heat transfer coefficient – the total thermal conductive of a complete assemble at the total thickness of the material. U values of each item can be added to determine the total U value of the assembly.

R-Value

Resistance to heat flow – the reciprocal or inverse of the U-value. R = 1/U