There are a number of acceptable calculation methods for determining U-Factors, and the appropriate method depends on the type of framing or discontinuity in the wall. Heat flow through construction assemblies with metal framing and/or sheathing is more complex and requires special consideration. The following table shows the acceptable calculation methods that can be used with each type of construction.

Applicability of Calculation Methods for Opaque Construction Assemblies

	Metal Sheathing Metal Framing	Metal Sheathing Non-Metal Framing	Non-Metal Sheathing Metal Framing	Non-Metal Sheathing Non-Metal Framing

Laboratory Tests	X	X	X	X
Series-Parallel Path (Isothermal Planes)		X		X
Two-Dimensional Models	X	X	X	X
Parallel Path Correction Factors			X
Zone Method			X
Thermal Bridges in Sheet Metal Construction	X

Laboratory Tests (8.3.2) Most construction assemblies include more than one material in the same layer. For example, a wood stud wall includes cavity areas where the insulation is located and some areas where there are solid wood framing members. The wood areas have a lower R-Value, and conduct heat more readily than the insulated areas. It is incorrect to neglect framing members when calculating the U-factor for the wall, roof, or floor assembly. The correct U-factor includes the insulation portion of the wall and the U-factors through the solid portion of the wall. Energy efficiency standards require that the U-factor of each envelope assembly be calculated taking into account framing and other thermal bridges within the construction assembly. Masonry walls have some similarity to frame walls. Many masonry walls are partially hollow, with webs connecting the inside and outside faceshells. Furthermore, some of the cores may be filled with grout. Over the face of the wall, there are significant areas that are solid and hollow, and each area has a different thermal transmittance.

Series-Parallel Path (Isothermal Planes) Method The series-parallel method provides a reasonably accurate procedure for calculating the U-Factor when one or more elements in a construction are relatively conductive. It may be used for wood framed walls and for concrete and masonry walls. Hollow masonry units are a good example of when this calculation method is appropriate. The solid webs connecting the faceshells are quite conductive compared to the air spaces in the hollow cores, and the faceshells conduct heat laterally. The heat, in effect, flows around the hollow cores.

The series-parallel method divides the construction assembly into a series of layers. For a masonry unit, the layer containing the webs and cores is treated with a parallel path calculation to arrive at an average R-Value for the layer. This is then added to the R-Values of the two faceshells, as in a series method calculation. Finally, the total R-Value is inverted to the U-Factor. This method is also referred to as the isothermal planes method, because it assumes uniform temperature across the planes separating the layers.

Parallel Path Correction Factors (PPCF)Wall Sections with Metal Studs Parallel Path Correction Factors

Size of Members	Gauge of Stud	Spacing of Framing	Cavity Insulation R-Value	Correction Factor	Effective Framing per Cavity R-Values
2 x 4	18-16	16 in. o.c.	R-11 R-13 R-15	0.50 0.46 0.43	R-5.5 R-6.0 R-6.4
2 x 4	18-16	24 in. o.c.	R-11 R-13 R-15	0.60 0.55 0.52	R-6.6 R-7.2 R-7.8
2 x 6	18-16	16 in. o.c.	R-19 R-21	0.37 0.35	R-7.1 R-7.4
2 x 6	18-16	24 in. o.c.	R-19 R-21	0.45 0.43	R-8.6 R-9.0
2 x 8	18-16	16 in. o.c.	R-25	0.31	R-7.8
2 x 8	18-16	24 in. o.c.	R-25	0.38	R-9.6
1. These factors can be applied to metal studs of this gauge or thinner

Two-Dimensional Heat Flow

Using a computer, you can model two-dimensional heat flow to more accurately predict the U-Factor of a complex construction assembly. While the series-parallel path calculation method assumes that heat flows in a straight line from the warm side of the construction to the cooler side, with two dimensional models, heat can also flow laterally in the construction, following the path of least resistance. Calculating two-dimensional heat flow involves advanced mathematics and is best performed with a computer. A model is set up by dividing the construction into a large number of small pieces, and defining the thermal resistance between each piece. The result is analyzed with electric circuit theory. The network consists of a rectangular array of nodes connected by resistance. As in the real material, the energy flow will take the line of least resistance. The computer can perform the complicated calculations necessary to solve the network, yielding the U-Factor for the unit at steady state. It can also solve the network for dynamic energy conditions. Short of performing laboratory tests, this is the most accurate method available for determining the U-Factors of concrete and masonry walls.

Zone Method For conditions for which there are no parallel path correction factors, the zone method may be used. It may be used for construction assemblies with metal framing and non-metallic sheathing, such as concrete or masonry. The use of this method is documented in the ASHRAE Handbook of Fundamentals and involves dividing the construction assembly into zones. Heat flow in the zone near the metal framing is assumed to be conducted toward the framing and the thermal resistance is smaller. The following figure illustrates the concept where the zone of higher conductivity represents an area radiating at a 45 angle from the edge of the metal framing.

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Applicability of Calculation Methods for Opaque Construction Assemblies