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Is this calculator designed to account for the inherent R-value in my ceiling/roof or should I add that into the "existing insulation" number? In my case I'm considering adding insulation in the "slanted" ceiling of my finished attic - not cheap to do. The inherent R-value of the slated ceiling and roof is approximately 3.5 (including interior air layer, drywall, 4 inches of airspace, plywood roof sheathing, asphalitic shingles, and external air layer). If I assume "no insulation" the calculator assumes R=1, and with my project I'd add R-11 for a total of R12 and the calculator says my project would pay for itself in 6.2 years, which I'd consider to be worth doing. However, if I assume that since my ceiling/roof has an inherent R-value of 3.5 (4 was the closest I could input for existing insulation, so I input 4) and I'd add R-11, then my project would pay for itself in 26.8 years, which would make it not worth doing in my mind. Please clarify as this makes a HUGE difference. Thanks!
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