// CODE SNIPPET

A9.4.6.2 Double-Layer Roof

2021 Oregon Energy Efficiency Specialty Code (OEESC) > Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations > A9 Determination of Alternate Assembly U-Factors, C-Factors, F-Factors, or Heat Capacities > A9.4 Calculation Procedures and Assumptions > A9.4.6 Metal Building U-factor Equations > A9.4.6.2 Double-Layer Roof

Go To Full Code Chapter

The U-factor of metal building roofs that are insulated with double layers of fiberglass insulation (see Figure A9.4.6.2-1) shall be calculated using the procedure outlined in this section. The procedure assumes the insulation is compressed over the purlin and there may be a thermal spacer block present.

There are six steps in the calculation process:

Step 1—Characterize the thermal conductivity of the fiberglass.

Step 2—Determine the U-factor for the insulation in the cavity.

Step 3—Determine the U-factor over the structural framing member.

Step 4—Area weight the U-factors calculated in Steps 2 and 3.

Step 5—Determine the U-factor from the finite element analysis results.

Step 6—Determine the U-factor for any continuous insulation if present.

Step 1: The thermal conductivity of the fiberglass batt insulation is represented by a thermal curve of the form in Equation A9.4-12:

where
k	=	thermal conductivity, Btu/h•ft²•°F
ρ	=	density, lb/ft³
A	=	0.014917
B	=	0.0004377
C	=	0.0056897

Step 2: Assume the double-layer fiberglass batt forms a parabolic profile defined by Equation A9.4-13:

The presence of two layers of fiberglass adds complexity because each layer has distinct reference properties (see Table A9.4.6.1). As the double layers are compressed, the thickness of each layer needs to be determined by considering that each layer achieves the same compressive force. Instead of having a closed-form analytical solution that predicts the U-factor for the cavity, the double-layer system requires that the parabolic profile be numerically integrated. The compression of the double-layer system is presented in Figure A9.4.6.2-2.

The thickness of the second layer (Y₂) is described by Equation A9.4-14:

where
Y_c	=	compressed thickness of the double layers, ft
ρ_o₁	=	reference density of first layer, lb/ft³
ρ_o₂	=	reference density of second layer, lb/ft³
W₁	=	reference weight of first layer, lb/ft²
W₂	=	reference weight of second layer, lb/ft²

The solutions to Equation A9.4-14 are Equation A9.4-15a and A9.4-15b:

Select the smaller value of Y_2,a and Y_2,b as Y₂. Y₁ shall be calculated as the difference between Y_c and Y₂. Next, the R-values for the two compressed layers of insulation shall be calculated and converted to a U-factor. This process shall be repeated along the entire profile and the results numerically integrated using maximum 0.04167 ft increments.

It is important to note that Equation A9.4-14 does not apply when the two layers of insulation are the same material. In this case, each compressed layer has the same thickness, which simplifies the U-factor calculations. The numerical integration still needs to be completed to determine the U_co.

Step 3: Determine the U-factor over the structural framing member. The variable Y_o represents the thickness of the thermal spacer block and the thickness of the compressed insulation. The density of the compressed insulation is determined by Equation A9.4-16:

where
ρ_c	=	density of the compressed insulation over the framing member, lb/ft³
t_c	=	thickness of the compressed insulation over the framing member, ft

The thermal resistance of the compressed insulation is determined by Equation A9.4-17:

Determine the overall framing U-factor (U_fo) at the structural framing member, including the air film resistances, using Equation A9.4-18:

where
U_fo	=	U-factor over the structural framing member, Btu/h•ft²•°F
R_TB	=	R-value of the thermal spacer block, h•ft²•°F/Btu
R_c	=	R-value of the compressed insulation, h•ft²•°F/Btu

Step 4: Determine the overall area-weighted U-factor for the entire system using Equation A9.4-19:

where
U_es	=	area-weighted U-factor for the entire system, Btu/h•ft²•°F

Step 5: Calculate the adjusted overall U-factor (U_adj) using Equation A9.4-20:

where
U_adj	=	adjusted overall U-factor represented by correlation with the finite element modeling, Btu/h•ft²•°F

Step 6: If there is any continuous insulation present, calculate the overall U-factor using Equation A9.4-21:

Figure A9.4.6.2-1 Geometry of double layers of fiberglass batts.

where
X	=	distance from edge of purlin or girt, ft
Y	=	distance from edge of roof panel or wall panel, ft
L	=	length from edge of purlin or girt to centerline of cavity, ft
w_f	=	width of purlin or girt flange, ft
Y_o	=	distance between purlin or girt and the roof panel or wall panel, ft
Y_m	=	distance from edge of roof panel or wall panel at the cavity centerline, ft

Figure A9.4.6.2-2 Compression of double layers of fiberglass insulation.

Related Code Sections

A9.4.6.2 Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations, Double-Layer Roof

The U-factor of metal building roofs that are insulated with double layers of fiberglass insulation (see Figure A9.4.6.2-1) shall be calculated using ...

A2.3.2.2 Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations, Double Layer

is for unfaced insulation installed above the first layer and parallel to the purlins and then compressed when the metal roof panels are attached. A ...

2021 Oregon Energy Efficiency Specialty Code (OEESC) > Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations > A2 Roofs > A2.3 Metal Building Roofs > A2.3.2 Rated R-Value of Insulation > A2.3.2.2 Double Layer

A3.2.2.4 Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations, Double-Layer

or adhered to the insulation, is installed inside of the girts to form a continuous layer. A thermal spacer block or thermal break strip between the girts ...

2021 Oregon Energy Efficiency Specialty Code (OEESC) > Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations > A3 Above-Grade Walls > A3.2 Metal Building Walls > A3.2.2 Rated R-Value of Insulation for Metal Building Walls > A3.2.2.4 Double-Layer

719.4 Fire and Smoke Protection Features, Double Reinforcement

Plaster protection more than 1 inch (25 mm) in thickness shall be reinforced with an additional layer of approved lath embedded not less than 3/4 ...

2019 Oregon Structural Specialty Code (OSSC) > 7 Fire and Smoke Protection Features > 719 Fire-Resistance Requirements for Plaster > 719.4 Double Reinforcement

A9.4 Normative Appendix A Rated R-Value of Insulation and Assembly U-factor, C-Factor, and F-Factor Determinations, Calculation Procedures and Assumptions

the assembly U-factor. For double-layer metal building roof systems, the calculation procedure outlined in Section A9.4.6.2 shall be used to calculate ...