What is U-Value in roofing insulation?
This post is part of four-part series exploring energy efficiency values in insulation, such as K-Value, C-Value, R-Value, and U-Value.
How is U-Value measured? What’s the difference between U-Value and K-Value, C-Value, or even R-Value?
This series of articles concludes with examining the purpose and metrics of U-Value. Unlike R-Value, K-Value, and C-Value, which are more defined by construction and energy-efficiency, U-Value is more defined by engineering parameters. We can dive into the scientific calculations of U-Value and how it functions in relation to engineering qualifiers. The more effective way of communicating how U-Value is measured is through a simple illustration.
Let’s take a basic residential wall built by 2″x4″ studs that are vertically spaced the standard 16″ apart from center to center. The interior may have a thick plastic vapor barrier between the studs and a typical 3/8″-thick gypsum board. We can ignore the energy value of the doors and windows as well as the K-Value of the plastic film vapor barrier for the sake of simplicity. The exterior material and any additional insulation placed between the outside face of the wall and the exterior material can be calculated into the U-Value. And this is where it gets complicated, but don’t worry, this math can be easier than you might expect.
The U-Value is the total calculated value of all the building materials in that given wall:
- K-Value of the gypsum boards, wall studs, interior and exterior insulation, exterior material, and any additional siding;
- And, the C-Value of the interior vapor barrier and any additional vapor barrier, if relevant.
What’s U-Value?
U-Value measures thermal transmittance as defined by ASTM International:
“Transmittance, thermal, n: the heat transmission in unit time through unit area of a material construction and the boundary air films, induced by unit temperature difference between the environments on each side.” (ASTM International C168)
The collective energy barrier provided by that single wall in our illustration is what constitutes the U-Value. In truth, entire volumes of books have been created based solely on the calculation, nuances, and variances of U-Value in energy efficiency. Without being too simplistic, we see the U-Value as the collective energy capabilities of an entire construction entity, such as a wall, a building, or even a commercial roof.
If you’d like to learn more about how a highly efficient commercial roof can transform your energy costs, call our R&A Contracting team today.