Step by step method to calculate the wall assembly U value

n building construction, determining the overall U-value (thermal transmittance) of walls is crucial for ensuring energy efficiency and thermal comfort within a building. The U-value of a wall represents the rate of heat transfer through the wall, including conductive, convective, and radiative heat transfer. In this article, we will discuss the step-by-step method to determine the wall assembly U-value and present the results in tabular form. We will use the Opaque External Wall as an example to demonstrate the process.

Step 1: Identify the components of the wall assembly

The wall assembly consists of various layers that contribute to the U-value of the wall. It is essential to identify each component and its thickness to determine the overall U-value accurately. The components of the Opaque External Wall are:

• Exterior cladding
• Air gap
• Insulation
• Vapor barrier
• Wall framing
• Gypsum board
• Interior finish

Step 2: Determine the thermal conductivity of each component

The thermal conductivity (k-value) of each component is the measure of how easily heat passes through it. The k-value is expressed in W/m.K. The k-value of each component can be found in manufacturer specifications or reference books. For example, the k-value of insulation material can be found in ASHRAE Handbook or EN ISO 12667. The k-value of the components of the Opaque External Wall are:

• Exterior cladding: 0.15 W/m.K
• Air gap: 0.026 W/m.K
• Insulation: 0.038 W/m.K
• Vapor barrier: 0.023 W/m.K
• Wall framing: 0.14 W/m.K
• Gypsum board: 0.16 W/m.K
• Interior finish: 0.12 W/m.K

Step 3: Calculate the R-value of each component

The R-value is the inverse of the thermal conductivity and represents the thermal resistance of the component. The R-value is expressed in m2.K/W. The R-value of each component can be calculated by dividing the thickness of the component by its k-value. The R-value of the components of the Opaque External Wall are:

• Exterior cladding: 0.050/0.15 = 0.33 m2.K/W
• Air gap: 0.020/0.026 = 0.77 m2.K/W
• Insulation: 0.100/0.038 = 2.63 m2.K/W
• Vapor barrier: 0.005/0.023 = 0.22 m2.K/W
• Wall framing: 0.090/0.14 = 0.64 m2.K/W
• Gypsum board: 0.015/0.16 = 0.09 m2.K/W
• Interior finish: 0.015/0.12 = 0.13 m2.K/W

Step 4: Calculate the overall R-value

The overall R-value of the wall assembly can be calculated by adding the R-values of each component. The overall R-value of the Opaque External Wall is:

R-value = 0.33 + 0.77 + 2.63 + 0.22 + 0.64 + 0.09 + 0.13 = 4.81 m2.K/W

Step 5: Calculate the overall U-value

The overall U-value of the wall assembly is the inverse of the overall R-value. The overall U-value of the Opaque External Wall is:

U-value = 1/R-value = 1/4.81 = 0.208 W/m2.K