Should You Seal Basement Ducts?

Twice this week, I've had discussions about sealing basement ducts. The first was with my colleague and fellow author for Contracting Business magazine, Adam Mufich. He mentioned that the question, "Should you seal ducts in a conditioned basement?" was getting a lot of discussion on social media, and the opinions were everywhere.

My second discussion was with Jim Bergmann, the president of measureQuick. He noticed that his ducts lit up when he shone a light through them, and he didn't like what he saw. Jim was changing this situation.

Sealing basement ducts is one of those topics on which people have strong opinions. Some think it doesn't matter, while others believe it does. Let's set aside opinions and examine the facts to determine if sealing basement ducts makes a difference.

Two Schools of Thought

One school of thought on sealing basement ducts focuses on energy savings and code compliance. Contractors and technicians in this category believe that duct sealing has little influence on energy savings, as any leakage is technically located within the conditioned space. Another reason they have this stance is that many energy codes don't specify duct leakage testing when ducts are in a conditioned space.

The other school of thought claims every duct system should be sealed as airtight as possible, regardless of location. Contractors and technicians in this category believe that any air lost through duct leakage isn't reaching the intended area. Therefore, it is impossible to ensure proper airflow delivery to every room. They also say you can't air balance a leaky duct system. Let's see if some basic airflow rules can help clarify both positions.

Three Basic Airflow Rules

Air is invisible, and you can't touch it. However, air follows some predictable rules that can help us understand its properties and how it reacts to duct leakage. By examining a few of these rules, we can begin to see whether basement duct sealing makes a difference.

The first airflow rule is that air takes the path(s) of least resistance. In a duct system, air moves more easily through larger openings closer to the fan of the indoor equipment than through points further downstream. If there are unsealed joints in the main supply trunk and take-offs, air will flow through these openings instead of going to the intended supply registers. The same principle applies to return ducts. If there is an open filter rack on the indoor equipment, it will pull air more easily than a bedroom return grille mounted farther away. This rule may sound simple, but it's easy to forget how much influence it has on duct system performance.

The second airflow rule is: one cubic foot per minute (cfm) in equals one cubic foot per minute (cfm) out. In the United States, we traditionally measure airflow in cubic feet per minute (cfm). To keep this rule simple, for every one cfm pulled into the indoor equipment's fan, there is one cfm blowing out. Let's say you have a 3-ton system operating in the cooling mode. You measure fan airflow with the Digital TrueFlow Grid from The Energy Conservatory (TEC) and determine that the fan is moving at 1,182 cfm. This measurement indicates 1,182 cfm of air being pulled into and discharged from the fan. It doesn't mean you have 2,364 cfm of total airflow. You're only working with 1,182 cfm. The fan moves air from the inlet to the outlet — it doesn't create airflow.

The third airflow rule we'll examine is that airflow is highest at the fan. Just as static pressure is highest at the fan, so is airflow. As air moves through the supply duct system and farther away from the fan, the airflow decreases as it passes through individual branch runs to rooms within a building. The same principle holds true for the return duct system, just in reverse order. As air enters the return duct system at the farthest return grille, it merges with air from other return grilles as it approaches the fan inlet. Now, let's see how these airflow rules influence sealing basement ducts.

The Influence of Airflow Rules and Duct Leakage

A complaint many of us have dealt with is an uncomfortable room, often at the farthest end of a home. During the winter, this room is very cold, and in the summer, it's too hot. Sound familiar? You can apply the airflow rules above to determine what's going on in this situation.

Since air takes the path of least resistance (Rule 1), it's easier for it to pass through an unintended duct system leak in the basement than to travel all the way to the problem room at the farthest end of a home (Rule 3). In the previous example under Rule 2, we had a 3-ton system moving 1,182 cfm. For that system to function as designed, it's essential to get as close to 100% of the 1,182 cfm from the indoor equipment to the intended delivery points (individual rooms).

There is only so much airflow the fan can circulate. If it's lost through duct leakage, you'll never have control over the air in the duct system and will come up short. It's essential to remember that to control air, you first must contain it (Rule 2). This situation is a painful truth to anyone attempting to air balance a leaky duct system. It's one that you can't balance, regardless of location. However, because our industry typically assumes airflow instead of measuring it, we often overlook this result.

Airflow, Comfort, and Energy Savings

To have comfort in individual rooms, you need the right amount of delivered Btu. Airflow is a key component of the Btu formulas, and unless you deliver the proper amount of airflow, your customers will have comfort problems.

You may wonder how room airflow delivery and unsealed basement ducts could affect energy efficiency. Uncomfortable homeowners adjust their thermostats in an attempt to achieve comfortable conditions. As they adjust the thermostat, the equipment runs longer to compensate for the reduced airflow and heat output. Even though the airflow and Btu are inside the basement, the homeowner doesn't benefit from them. The lost airflow and Btu in the basement don't help an uncomfortable bedroom upstairs.

Unsealed basement ducts are also responsible for pressure imbalances. These can lead to increased infiltration of outside air, which in turn increases equipment run time and may also cause combustion safety issues. Basement depressurization can lead to combustion appliances spewing flue gases and may draw carbon monoxide (CO) from auto exhaust into the home through an attached garage.

If You Don't Test, You're Just Guessing

System performance holds the key to determining which school of thought is correct. If you don't test your systems, you're guessing whether airflow reaches the correct places and will never observe these interactions. The conversation about basement duct leakage isn't about whether an energy code mandates the need for leakage testing; it's about whether the testing is actually necessary. It's about doing the right thing for your customers and verifying your systems truly do what you promised they would.