Determine the winter R-value and the U-factor of a masonry cavity wall that is built around 4-in-thick concrete blocks made of lightweight aggregate. The outside is finished with 4-in face brick with 1/2-in cement mortar between the bricks and concrete blocks. The inside finish consists of 1/2-in gypsum wallboard separated from the concrete block by 3/4-in-thick (1-in by 3-in nominal) vertical furring whose center- to-center distance is 16 in. Neither side of the 34-in-thick air space between the concrete block and the gypsum board is coated with any reflective film. When determining the R-value of the air space, the temperature difference across it can be taken to be 30°F with a mean air temperature of 50°F. The air space constitutes 80 percent of the heat transmission area, while the vertical furring and similar structures constitute 20 percent.

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