• When adding or subtracting R values or U values always convert everything to R values, then, convert back to U value, if needed (R =1/U,  U = 1/R).

The U value of the existing insulation is .05. Therefore the existing R value is 20 (1/.05),  20 +19 = 39 new R value; 1/39 = .025 new U value

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