Re: 2x6 Rafter Bays - Cathedral Ceilings
Here's some basic math based on some numbers provided throughout this thread.
Calculate Wall Loss Rate In BTU per hour
R11 --- 470 sq ft X 75 / 11 = 3204.54 btu/hr
R21 --- 470 sq ft X 75 / 21 = 1678.57 btu/hr ( about 48 % reduction in heat loss )
Calculate Loss Per Day
R11 --- 3204.54 X 24 = 76,908.96 btu / day
R21 --- 1678.57 X 24 = 40,285.68 btu/day
Heat Loss Cost for 90 days ( based on cost of $12 per 1 million btu of natural gas )
R11 = $83
R21 = $43
This is only for this space alone and does not reflect the other heat loss costs for the rest of the home.
Also , this only reflects the heat loss cost and does not reflect the heat gain costs for air conditioning.
Other factors --- if the square footage is based on floor space and not the actual sq ft of wall and ceiling surface area --- the square footage in the calculations would need to be adjusted higher. Which would show larger heat loss numbers. If the existing insulation is *old * as stated then it's not likely at an R11 value but at a lower R value .
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