Estimate the R-values for a 5 cm-thick fiberglass board and a 2.5 cm-thick polyurethane foam layer. Then compare their respective conductivity-times-density products if the density for fiberglass is 50 kg/m3 and the density of polyurethane is 30 kg/m3 . Use the units
GIVEN
- 5 cm thick fiberglass board, density = 50 kg/m3
- 2.5 cm thick polyurethane, density = 30 kg/m3
FIND
(a) R-values for both
(b) Conductivity-times-density products for both
ASSUMPTIONS
(a) One-dimensional, steady conduction
Ranges of conductivity for both of these materials are Using mean values we find:
fiberglass board k = 0.04 W/(m K)
polyurethane foam k = 0.025 W/(m K)
For the 2 in. fiberglass we have
t = 0.05 m
k = 0.04 W/(m K)
From section 1.6 the R-value is given by
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