One of the goals of COMcheck was to provide users true prescriptive envelope requirements
expressed in terms of R-values rather than overall assembly U-factors. Standard 90.1 typically provides opaque envelope requirements as assembly U-factors and then specifies calculation procedures and correction factors to be used with these assembly U-factors in calculating the required amount of
insulation. COMcheck uses the calculation procedures and correction factors found in Standard 90.1 and
precalculates required insulation R-values for common assembly types.
The key to this process is identifying typical assemblies. While the correction factors and calculation procedures are taken directly from Standard 90.1, typical assemblies are not specified. For COMcheck, a
series of assemblies used in the development of Standard 90.1-1989R were used. These assemblies are conservative in that material thicknesses were assumed to be as minimal as possible. For example, for wood-frame walls, the wall assembly is assumed to include 5/8-in. gypsum board on the inside of the cavity and stucco over the outside of the cavity. No additional R-value is assumed for exterior sheathing in the calculations. Interior and exterior air films are also included in the calculations. Similarly, a minimal roof system might be a metal deck with no ceiling and with insulation placed directly on top of the deck.
The basic assemblies described in the following sections were taken from Standard 90.1-1989R. Extensive discussion of these assemblies can be found in Code Compliance Considerations in the Development of the Building Envelope Requirements for ASHRAE/IESNA Standard 90.1-1989R (Hogan
1995). For COMcheck, the resulting U-factors were converted to a series of equations describing overall
assembly U-factors as a function of installed cavity insulation R-value, installed continuous insulation R-value, and the balance of the assembly (BOA). BOA refers to the R-value of the assembly excluding any cavity or continuous insulation. Cavity insulation is defined as insulation subject to thermal bridging from framing or furring members. Continuous insulation is defined as insulation that is continuous across framing or furring members and not subject to thermal bridging. The BOA includes all non-insulation elements of the assembly that contribute to its overall U-factor such as air films, gypsum board, sheathing, and carpets and pads (for floors). Associated with the cavity insulation is an effectiveness factor that reduces the installed R-value of the cavity insulation to an “effective” R-value for cavity insulation. In some cases, the equations used in COMcheck have a constant value for this effectiveness
factor, and in others the effectiveness factor is a function of the cavity insulation R-value. Continuous insulation is assumed to have an effectiveness factor of 1.
3.4 Roofs
3.4.1
Roof Assembly Types
3.4.1.1 COMcheck offers the following six roof types:
All-Wood Joist/Rafter/Truss. The base assembly consists of a roof truss with a 2x4 bottom chord.
The ceiling is attached directly to the bottom chord of the truss, and the attic space above is ventilated. Insulation is located directly on top of the ceiling, first filling the cavities between the wood, then continuously covering wood and cavity insulation. No credit is given for roofing materials, because they are above the ventilated space. The heat flow path through the wood members is calculated to be the same depth as the insulation. The assembly includes R-0.17 for the exterior air film, R-0.56 for 0.625-in. gypsum board, and R-0.61 for interior air film with heat flow up. U-factors are calculated for standard framing, where insulation is tapered around the perimeter with resultant decreases in thermal resistance. Table 3.3 shows the balance of assembly R-value calculation details. Area weighting factors for the parallel paths are 85% full-depth insulation, 5% half-depth insulation, and 10% framing.
Table 3.3. Balance of Assembly R-values for All-Wood Joist Roof
Description R-Value at Insulation R-Value at Joists
Outside Air Film 0.17 0.17
Wood Joists/Cavity 0 4.38
5/8-in. Gypsum Board 0.56 0.56
Inside Air Film 0.61 0.61
Total Path R-value 1.34 5.72
Total Assembly R-value = 1.0 / (0.85/1.34 + 0.05/1.34 + 0.10/5.72) = 1.45
Non-Wood Joist/Rafter/Truss. The base assembly consists of a roof supported by metal joists with
insulation between the joists. The assembly includes R-0.17 for exterior air film, R-0 for metal deck, and R-0.61 for interior air film heat flow up. The performance of the insulation/framing layer is calculated using the parallel path correction factors found in Table 8C-1 of Standard 90.1-1989.
Structural Slab. The structural slab roof consists of a 6-in. concrete slab or concrete on metal deck.
The assembly includes R-0.17 for exterior air film, R-0.33 for built-up roofing, R-0.13 for concrete slab on metal deck, and R-0.61 for interior air film heat flow up.
Metal Roof without Thermal Blocks and Metal Roof with Thermal Blocks. The base assembly
consists of a roof where the insulation is draped over metal purlins and compressed where the metal structural members are attached to the metal purlins. R-values for additional continuous insulation may be added to the base assembly. Two cases of screw-down metal building roofs are considered in COMcheck. One case involves the use of a 1 in. x 3 in. foam thermal block (other than compressed
insulation) between the purlin and metal roof members (NAIMA 1998). The other case is identical but without the thermal block material at the purlins. The base assembly R-value for uninsulated roofs is 0.78, representing the interior and exterior air film coefficients. Balance of assembly U-factors and framing factors is used as coefficients of a linear regression equation developed to represent the assembly U-factors of standard insulation R-values for metal building roof assemblies, as listed in Table 3.4.
Table 3.4. Metal Building Roof (MBR) Assembly U-Factors for Standard Insulation Thicknesses
Insulation
R-Value MBR with Thermal Block Assembly U-Factor MBR without Thermal Block Assembly U-Factor
R-10 0.104 0.138
R-11 0.098 0.134
R-13 0.088 0.122
R-19 0.07 0.101
Other. COMcheck allows the user to define a roof assembly by specifying its overall effective U-
factor. This option permits the user to accurately describe the performance of any roof assembly not adequately covered by the predefined roof types.
3.4.2
Roof Area
Proposed Area. With COMcheck Version 2.0, skylight areas are subtracted from roof gross areas as
input by the user to determine opaque roof areas, which is then used in the loads calculations.
Budget Area. The roof area (AreaRF) used in calculating budget loads is the sum of the opaque roof
area and skylight area in excess of 3% of total roof area. The first 3% of skylight area is ignored in calculating both loads for the proposed design and budget design loads (see Skylights for additional
information).
3.4.3
Roof U-Factor
Proposed U-Factor. The U-factor for opaque roof assemblies is determined using the following
equation:
URF = 1/(RRF-BOA + RRF-CVI × AFRF + RRF-CNI) (3.1) where URF = opaque roof assembly U-factor input to the trade-off engine
RRF-BOA = R-value for the balance of the assembly from Table 3.5 for roof type RRF-CVI = R-value for the cavity insulation as input by the user
AFRF = parallel path adjustment factor from Table 3.5 for roof type RRF-CNI = R-value for continuous insulation input by the user.
Table 3.5. Roof U-Factor Calculation Coefficients
Roof Type RRF-BOA AFRF
All-Wood Joist/Rafter/Truss 1.45 0.9
Non-Wood Joist/Rafter/Truss 0.78 1 - 0.00738 * RRF-CVI
Structural Slab 1.24 N/A
Metal Roof without Thermal Blocks If RRF-CVI = 0, 0.78
If RRF-CVI > 0, 4.25 If RRF-CVI = 0, AFRF = N/A If RRF-CVI > 0, 0.298251 Metal Roof with Thermal Blocks If RRF-CVI = 0, 0.78
If RRF-CVI > 0, 4.55 If RRF-CVI = 0, AFRF = N/A If RRF-CVI > 0, 0.514723 Other User input is overall assembly U-factor.
Budget U-Factor. The U-factor for roof assemblies (URF) is based on Equation 8-7 in
3.4.4
Roof Loads
The cooling and heating loads for roofs (CLRF and HLRF) are calculated usingEquations 3.2 and 3.3. CLRF = AreaRF × CCoefRF × URF × CDD50 (3.2) HLRF = AreaRF × HCoefRF × URF × HDD65 (3.3) where CLRF = cooling load for roof
HLRF = heating load for roof AreaRF = roof area in square feet
CCoefRF = 7.393 (from Standard 90.1-1989R, Appendix C, Table C.6-11: 6.04E-04 × 12.24 × 1000 = 7.393)
HCoefRF = 13.874 (from Standard 90.1-1989R, Appendix C, Table C.6-11: 2.28E-04 × 0.608488 × 100000 = 13.874)
URF = U-factor of roof assembly
CDD50 = cooling degree-days base 50°F for the site. HDD65 = heating degree-days base 65°F for the site.