R-value (insulation) contain no trapped CFCs or HFCs (e.g., half-pound low density foams). On certain brands, twenty-year tests have shown no shrinkage or reduction in insulating value. This has led to controversy as how to rate the insulation of these products. Many manufacturers will rate the R-value at the time of manufacture; critics argue that a more fair assessment would be its settled value. The foam industry adopted the LTTR (Long-Term Thermal Resistance) method, which rates the R-value based on a 15-year weighted average. However, the LTTR effectively provides only an eight-year aged R-value, short in the scale of a building that
Construction of a complex null tetrad to the horizon r=0, formula_74 where formula_75 refers to its on-horizon value. The very coordinates used in the adapted tetrad above are actually the Gaussian null coordinates employed in studying near-horizon geometry and mechanics of black holes. Construction of a complex null tetrad Calculations in the Newman–Penrose (NP) formalism of general relativity normally begin with the construction of a complex null tetrad formula_1, where formula_2 is a pair of "real" null vectors and formula_3 is a pair of "complex" null vectors. These tetrad vectors respect the following normalization and metric conditions assuming the spacetime signature formula_4 Only after the tetrad