By Philip Schmidt, Networx

By now, you’ve probably heard something about how sealing the ductwork in your forced-air heating (and central air conditioning) system can save energy and money. Well, it’s true, no matter how old your house is. Admittedly, this rather tedious job clearly falls in the category of Unsexy Home Energy Improvements (perhaps right below weatherstripping and attic insulation). But unlike high-profile upgrades, such as  triple-pane windows and solar panels, sealing ductwork can yield some fairly big savings without an astronomical upfront investment. It’s also easy to do yourself, since it’s pretty much like icing a cake with your hands.


Why Ducts Leak

With apologies to all skilled tinners out there, one of the main reasons ducts leak is because they’re simply (and sometimes poorly) constructed. A standard duct system is made with sheet metal held together with screws, which means it’s not a whole lot different from the gutter system on the outside of your house. Sheet metal is a good material for moving air, but all the joints, fittings and seams leave many opportunities for air leaks to develop. An improperly balanced system compounds this problem by creating unnecessarily high positive and/or negative pressure in the ductwork.


Another cause of big leaks is shoddy workmanship, whether it’s the fault of the original installer, a remodeler or the homeowner. Fittings and seams that aren’t fitted or fastened properly may leak from the beginning or come loose over time. Some of the biggest causes of leaks are repairs or “seals” made with duct tape. You read that correctly. Ordinary duct tape isn’t made for ducts; it dries up and can begin falling off of ductwork within 6 months of application—maybe enough to get you through one heating season, but that’s about it.


For the record, there is “duct tape” made for ductwork, such as UL-181-approved foil tape. This is designed for direct application to metal ductwork. However, mastic is the generally preferred material for sealing most metal ducts (see How to Seal Your Ducts, below). 


How Ducts Leak

The simple answer to this question is: Air naturally finds its way out of anything that isn’t airtight. It does this because it’s constantly seeking balance in both pressure and temperature. If warm air is moving through a duct in a cold crawlspace, it’s just waiting for a chance to slip out and mix it up with the cold air outside the duct. To put it another way, air is a master of mingling, better than any socialite at a benefit dinner. Adding pressure to the system -- that’s the force in “forced-air” -- only makes the air work harder to find a way out.


Where Ducts Leak

A force-air system circulates air through the house via supply and return ducts. Supply ducts bring hot air from the furnace (or cold air from the AC system, which utilizes the furnace) to the heat registers in each room. The return ducts pull cool air from the rooms and bring it back to the furnace for reheating. Therefore, it’s important to seal all of your ducts, not only to prevent the loss of heated air but also to improve air circulation and maintain a proper balance of the system throughout the house.


Ducts can leak at any connection between two parts. Starting at the furnace, leakage can be most pronounced where the main supply duct meets the body of the furnace unit, or air handler. This is where the air pressure is the greatest. Moving down the line, leaks are common where smaller ducts branch off from the main supply duct and at bends and where straight lengths of duct are connected. At the register end, the “boot” fitting that transitions from the duct to the register grill is another likely culprit. All of these same areas are prone to leaking on the return side of the system.


If your ducts are insulated (which is a good thing), don’t let that fool you into thinking sealing isn’t required. Cary Weiner, the Clean Energy Specialist at Colorado State University Extension, warns us that “insulation alone will not stop air leaks. Insulation provides a resistance to heat flow between two spaces but does not stop air leakage. This is where sealing the ducts comes in.”


How to Seal Your Ducts