Appendix E

Engineering Calculations for Table 3-2

Table 3-2 presents data describing two characteristics of structural materials—combustible mass and energy content—that relate to how the homes built from those materials would burn and their potential emissions if they were destroyed by a WUI fire. Although the committee searched for this type of data in the existing literature, no data of this type are currently available. The values in the table were therefore calculated by the committee to demonstrate how combustible mass and energy content can be derived from published information on the quantities of different typical materials found in residential construction. The table provides values for two example single-family homes for which construction material quantities were publicly available (EPA, 2016; Messerschmidt, 2021).

The committee wishes to emphasize that multiple assumptions are involved in completing the engineering calculations that resulted in Table 3-2. The data in that table are intended only as an illustrative example, and an uncertainty analysis was therefore not performed. For the reader who may be interested in replicating, improving upon, or extending these calculations to expand the scientific community’s understanding of fire loading in WUI fires, the committee’s assumptions and calculations are detailed in this appendix. The first section describes combustible mass calculations, and the second section describes energy content calculations.

CALCULATING COMBUSTIBLE MASS

Combustible mass must be obtained from the source or calculated before energy content can be determined. Messerschmidt provided material quantities directly on the basis of combustible mass (Messerschmidt, 2021), and no engineering calculations were needed to provide those values in Table 3-2.

The US Environmental Protection Agency (EPA) provided quantities of each material in terms of weight, area, or length, which can be found in Table 1-1 of the reference (EPA, 2016). The committee applied conversion factors to convert the quantities from source units into mass equivalents (Table E-1). To convert area and length values to a mass basis, the committee made informed assumptions about the specific material composition, when not specified, and sought out sources for the relevant density information.

CALCULATING ENERGY CONTENT

The committee identified the best available sources for net calorific value. When possible, energy content data were derived from the Society of Fire Protection Engineers (SFPE) Handbook of Fire Protection Engineering (SFPE, 2016) or from Xie et al. (2019) to conform with standard fire loading calculation methodology. In all

TABLE E-1 Conversion Factors Used in Calculating Mass-Basis Quantities for EPA Example Home

NOTE: OSB = oriented strand board.

^a If the source did not provide a specific material composition for a given construction material, the composition assumed is given in parentheses in the first column.

^b The linear density of lumber was assumed to be 0.95 kg/board ft, based on the density of white pine (eastern and western averaged) adjusted to a moisture content of 12 percent. Source: https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf.

^c A ½ inch assumption was made for all OSB to match Messerschmidt (2021). Area density (kg/ft²) for ½ inch OSB was obtained from https://roofonline.com/weights-measures/weight-of-plywood-and-osb/.

^d The conversion factor approach used for most materials was not appropriate for glass fiber insulation because this material is composed of only a fraction of combustible adhesives in addition to the noncombustible glass fibers. For this material, the equivalent combustible mass was obtained by multiplying the replacement mass provided in the source (EPA, 2016) by 10 percent, the combustible fraction according to Messerschmidt (2021) and https://pharosproject.net/common-products/2080060#contents-panel.

^e EPA provided replacement mass values for wood shingle siding and roofing asphalt shingles (Appendix A1 in EPA, 2016), but the area densities (kg/ft²) that resulted were too high to be reasonable. Instead, the committee obtained area densities for each of these materials from https://www.engineeringtoolbox.com/roofing-materials-weight-d_1498.html. The percentage of combustible material of a shingle was obtained from https://greet.es.anl.gov/greet_building, Building Life-Cycle Analysis with the GREET Building Module: Methodology, Data, and Case Studies, as 20 percent asphalt by mass.

^f Area density (kg/ft²) of gypsum board was obtained from https://www.certainteed.com/drywall/products/regular-drywall/.

^g Not a combustible material.

^h The window materials were obtained from Building Industry Reporting and Design for Sustainability (BIRDS) New Residential Database Technical Manual (NIST Technical Note 1878, https://nvlpubs.nist.gov/nistpubs/TechnicalNotes/NIST.TN.1878.pdf) for a 1 m² vinyl casement window composed of 40 percent vinyl and 10 percent other materials.

SOURCE: EPA, 2016.

TABLE E-2 Net Calorific Values Used to Calculate Energy Content

^a For these materials, the reference provided ranges, from which mean values were calculated (as the center of the range).

other cases, net calorific values were sourced from measurement data (Phyllis, n.d.) or from widely adopted reference data (United Nations standard net calorific values [UNSD, n.d.]). The applicability of standardized data for representing in-use construction materials requires validation.

The combustible mass values taken from Messerschmidt (2021) and calculated from EPA (2016) were then multiplied by the mean net calorific values in Table E-2 to obtain the energy content data shown in Table 3-2.

REFERENCES

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