Rodney Fox


One of the biggest wastes of energy I encounter when performing energy audits is from leaky ducting. From the data I’ve collected over the years, I find the average is about 10 percent. So, if your building has ductwork that runs either in your attic or crawlspace and it’s leaky, you can use this simple formula to determine how much those leaky ducts cost you during the heating season:


Annual total of 12 utility bills Example

Gas or electric, depending on how you heat your building $2,000

– Average of the three lowest monthly bills x 12 ($50 + $45 + $55) / 3 = $50

$50 x 12 = $600

= Your probable heating cost $1,400

x Industry standard factor of 1.1 $1,400 x 1.1 = $1,540

(accounts for winter variance when more consumption can be expected)

x 0.10 (10% average leaky duct factor) $1,540 x 0.10

= Annual cost of duct leakage (for heating) $ 154


Of course, your costs may vary. But wouldn’t you rather keep that money in your pocket? And what else does leaky ducting cost you? Well, for one, your health. A good forced-air system should be a closed system, where roughly the same amount of air that is blowing out of the supply registers is also being sucked back into the system through return registers. I rarely see them sized appropriately. With an unbalanced system, two things can occur:


  1. If you have leaky return ducting, which takes the air in your house and routes it back to the air handler, you’re picking up garbage air from the attic or crawl space—or both—like carbon monoxide, fiberglass, radon, mold and bacteria of all sorts and depositing this junk into your home. These contaminants can be deadly. Get headaches often? Respiratory problems?
  2. The other problem is that the furnace needs to work that much harder to condition the air again. For example, if it’s 20 degrees in your attic and there is a leak in your return ducting, what normally would have been 60–70 degree return air now gets injected with that 20-degree air, and your furnace has to work harder to heat it up. This also works in the inverse; that is, if you send cool air through leaky ducts in your attic for air-conditioning (AC) in the summer, that cool air gets injected with the 120-degree heat in the attic, making the AC work really hard to cool and dehumidify that air. If you have leaky supply ducting, you’re going to get cold rooms, inconsistent heating throughout the building, and a warm attic or crawl space—or both—venting your hard-earned money right out into the atmosphere.


In most cases, the fix is simple: flexible duct mastic over every duct joint. What’s not simple is the labor. It’s a miserable job crawling around in your crawl space or attic, trust me. But it’s absolutely necessary. If you don’t want to do it yourself, you can hire a high-school kid or someone who needs the money because, to be quite honest, it’s not rocket science applying the mastic. What it takes is a willingness to get dirty for a while. You can pick up a bucket of mastic at Home Depot for about $15. Just paint it on or smear it on with your hands, being sure to squeeze it into every crack and hole. Use this easy formula to determine how much time it will take:



1. Count all supply and return registers (grates) in your building and multiply by two.

12 x 2 = 24

2. From each supply and return register, count the number of long strides (4 ft.) it takes

to get from the register to the furnace (most metal ducting comes in 4-ft. lengths).

10 + 8 + 8 + 4 + 20 + 12 + 12 + 6 + 6 + 12 + 6 + 4 = 108

3. Multiply that number by 0.6 (many supply registers share supply ducting).

108 x 0.6 = 64.8 (65 rounded)

4. Add the total from number 1 to the total from number 3. 24 + 65 = 89


In the example above, there are 89 areas needing sealing. My average is three per hour. So, in this example, we can assume the project will take about 30 hours to complete. If you pay your neighbor’s kid $10/hour, it will cost you about $300 plus materials, which should include a good drop light and extension cord, a painter’s coverall, a bandana for your head, a headlamp, several good dust masks (or even a respirator), and a box full of nitrile gloves (because you don’t want to get that goop all over your hands). Still sound like too much? You can often skip all of the straight pipe connections because they tend to leak far less than the junctions (where one shape of metal ducting joins with another). So, if you were to seal only the 24 junction points, your cost for the neighbor’s kid’s labor would be only $80.


Using the annual leakage cost example of $154 and a cost of $350 for the work, the payback would be around two years, with an annual rate of return on investment of over 50 percent. If you use AC, the savings will increase by at least another 25 percent. That’s good business sense. With the problem fixed, you will enjoy more comfort, better health and the peace of mind of knowing you’ve reduced your carbon footprint a little more.



Rodney Fox is an energy-efficiency and weatherization wizard in the greater Santa Fe/Albuquerque area. He is committed to sustainable living and carbon reduction on a massive scale, one building at a time. 505.216.6119,,