There are two main reasons for sealing ductwork in a home. First, and most obviously, no one wants the nice warm air moving through the ducts (which they just paid to heat) to escape before it reaches its destination and heats the different rooms in the home. Sealing the leaks maintains pressure in the system and gets all that warm air out to all the registers. If the system was designed and balanced properly, that means you get the right amount of heat in every room and everyone is comfortable.

The less-obvious reason goes back to the pressure issue. An air handler (the fan system that propels the conditioned air in a heating or cooling system) is supposed to be part of a closed loop. The air from the house (often called the "cold air return") that passes through the handler and the exchanger, where it is heated or cooled, then goes back through the supply ducts to the house. If the system leaks to the exterior at any point, there will be a pressure imbalance. For example, if the supply ducts run through an unheated crawlspace or garage, any leakage will result in conditioned air leaving the living space. So, you have lost some of the conditioned air you just paid to condition (heat or cool).

The other side of that problem is that less air is being supplied to the living space than is being drawn out of it by the air handler. That means the house is at a slight negative pressure, and that will cause outside air to be pulled-in through cracks and other leaks to replace the air that was lost from the supply ducts. The reverse is true for leaks in the return side. They will cause the house to be at a slight positive pressure, which will result in conditioned air being pushed out of the house through various leak paths.

Neither of those scenarios is desirable. The duct system, on both return and supply sides, should be sealed to prevent any leakage to or from the spaces outside the living space, so that it is a closed system with the same volume of air coming out of the house and going back in.

In this particular case, it appears the question is about ductwork that is in the midfloor between a conditioned basement and the upstairs. If that is the case, any leaks will still be within the living space of the home. While that may alter the balance of airflow between different registers, it will not necessarily result in any real losses. The caveat is that there may be some leakage in the framing at the perimeter (rim joist) area and duct leakage within the closed cavities may result in air exchange with the exterior by way of those framing leaks. On the whole, however, it would be hard to justify dismantling finished surfaces to check and/or seal whatever gaps may exist there. The bottom line is that there are probably no significant problems that need to be addressed for the situation I think is being described here.

Leaks in ducts within the living space are not losing conditioned air to the outside, or altering the pressure balance between the interior and exterior. No harm, no foul.

Steve Leathart
Building Performance Specialist
Insulation, Replacement Windows, Multifamily Retrofit
Burnham Insulation
A USI Company

Updated July 13, 2010

That's a great question because it gets to a key problem: How far should you go to improve the performance of your home? How do you prioritize home performance upgrades?

Fortunately, the highest priority improvements are often among the most straightforward. In the case of duct sealing, highest priority ducts are usually closest to the furnace (because the air pressures are highest there), and these are usually accessible. And a surprising amount of sealing can be done working from the register end. In particular, sealing duct sheet metal to the floor (or wall or ceiling) at all registers and grills is important.

In your case, my guess is that these are smaller (8" or less?) ducts distributing hot air the last 25 feet or so before it exits the registers in the floor of the second level, in which case extreme measures like removing flooring probably aren't justified.

However, it is often the case that the space between floors (like where your ducts run) is poorly insulated and very air leaky. Fixing this can be more important than sealing the ducts and may in fact be easier.

Every house is different. A home performance retrofitter can help you prioritize the specific work that your home needs to meet your goals!

Yves Vetter

In theory, no energy is saved by sealing heating ducts inside the building envelope, as no heat energy is being lost to the outside. However, this is only true in a perfectly sealed house with no structural air leakage.

In most homes there is some air leakage from the cavities between floors through gaps at the perimeter, and when ducts leak within these cavities the result is a pressurized cavity that accelerates the exfiltration of heated air. The amount of heat lost would be much less than that through the same size leak in a duct located in an exterior space, and for this reason it is generally not considered cost-effective to remove a portion of ceiling or floor to access these leaks. However, if the duct leak is large enough it can rob heat to one or more rooms being served by that duct, which can affect comfort levels and cause occupants to set the thermostat higher.

If this is a concern, a technician can measure total duct leakage, pinpoint the location of duct leaks, and through pressure diagnostics, assess the affect on building cavity pressures and building heat loss. Armed with this information, you can then make an informed decision as to whether cutting into a ceiling or floor will be worth the effort.

Scott Finley, President
Atmosphere, Inc.