When heat energy is transferred between two materials, an equation exists to describe the conductivity of the materials. One of the terms in this equation is the heat transfer coefficient, which is a value that describes how well or poorly a material conducts heat. The heat transfer coefficient is dependent on the fluids on either side of a wall or heat exchanger, as well as the surface areas of the connections. This value is measured in watts per meter squared-Kelvin. Heat transfer depends on the fluid velocities as well as the type of heat being transferred.

Determine the individual heat transfer coefficients of the fluids on either side of the wall. This fluid is air in the case of most building applications, including the heat transfer through windows. Some common values are listed on engineeringtoolbox.com.

Measure the surface area of each side of the wall in square meters. If the surface areas are different, label them as A1 and A2.

Measure the thickness of the wall in square meters.

Look up the thermal conductivity of the wall material. A list of values for common materials can be found on ndt-ed.org.

Substitute these values into the equation (1/U_A) = (1/h1_A1) + (x/k_A) + (1/h2_A2) where A is the surface area of the wall, h1and h2 are the thermal conductivity coefficients of the fluids on either side of the wall and x is the thickness of the wall. Solve the equation for U, the overall heat transfer coefficient. For example, the heat transfer coefficient of a window 2 square meters in area and 1 centimetre thick with air at 20 W/m^2_K on one side and water at 500 W/m^2_K on the other would be calculated as (1/U_2) = (1/20_2) + (0.01/0.96_2) + (1/500_2), solving for U gives 960,000 W/m^2K.