Heating Bill Before and After Attic Insulation

Natural Gas Flares

In December 2008 we insulated our home's attic to about R-50 with blown-in cellulose insulation. There was an immediate and noticeable increase in comfort and a reduction in how often the furnace ran. While I appreciate the qualitative improvement, I decided to run the numbers and find the actual quantitative difference in our natural gas heating bill.

We heat our home in the winter with a natural-gas, forced hot-air furnace and set the thermostat at 65° Fahrenheit. The only significant change to our house from winter 2008 to 2009 was the roof and attic insulation. I took the natural gas usage data from our utility bill and compared the same period in 2008 and 2009. I then visited WeatherUnderground.com and ran a History report for Heating Degree Days so I could compare heating needs for both billing periods in 2008 and 2009. I assumed that the other home natural gas uses were equivalent. My calculations are below.

Heating Bill Comparison
Before Attic Insulation:
Jan 14 - Feb 13, 2008 (31 days)
Natural Gas used - 121 Therms @ $141.72
Heating Degree Days - 1103
Therms per Heating Day - 0.1097

After Attic Insulation:
Jan 11 - Feb 10, 2009 (31 days)
Natural Gas used - 57 Therms @ $60.19
Heating Degree Days - 852
Therms per Heating Day - 0.0669

Natural Gas Use Difference: .0669 / .1097 = 60.98%
~ 39% reduction in Natural Gas Heating Bill

Wow, a 39% decrease in the total natural gas used in our home! I knew the attic insulation before was thin and had many thermal bridges and gaps, but a ~40% improvement is huge. While the price of Natural Gas goes up and down (generally up) over time, we can now expect to spend 40% less on home heating each winter from now on.

I also ran a rough break-even calculation on our attic insulation project. I totaled our natural gas bills from January through April, November and December of last year. 39% of our natural gas bill for the cold months of 2008 is approximately $230. Dividing the insulation project costs after rebate (~$1240) by the natural gas bill savings (~$230) returns 5.4 years. Our break even point is about 5½ years and the attic insulation will save 39% annually on heating bills indefinitely.

The stock market may be in the basement, but the paper in our attic has a great Return on Investment!


Heating Degree Day - the number of heating degrees in a day is defined as the difference between a reference value of 65°F (18°C) and the average outside temperature for that day.

Therm - the energy equivalent of burning 100 cubic feet (often referred to as 1 Ccf) of Natural Gas.

More Info:

Weather Underground - see 'Detailed History and Climate'

Wikipedia - Heating degree day

Wikipedia - Therm

EIA - Residential Natural Gas Prices, What Consumers Should Know

Wikipedia - Natural Gas

Cellulose Insulated Attic Makes For a Cozy Home