When your ductwork needs replacing, you will usually hear three options: spiral duct, flexible duct, and rectangular rigid metal duct. They are genuinely different — different shapes, different costs, different strengths — and the right choice depends on your home, your budget, and what you are trying to fix. Here is the honest version we walk every customer through.
Spiral duct: the efficiency pick
Spiral duct is round galvanized pipe with a continuous spiral seam. Round is the most efficient shape for moving air — lower friction, less turbulence, quieter. A long run has far fewer joints than rectangular duct, which means fewer places to leak, and the joints it does have seal up tight. You see it in commercial buildings and in homes where ductwork is exposed, because it looks good bare.
The catch is space and cost. Round pipe needs more clearance in height than a flat rectangular run, so very tight attics can rule it out. And it is the most expensive of the three to fabricate and install.
Rectangular rigid metal: the space-saver
This is the metal duct most people picture — rectangular runs with folded seams every four to five feet. A rectangle fits neatly against joists, inside soffits, and through tight attics, which is why it has been the standard for decades.
The trade-off is physics: more joints per foot means more potential leak points, and corners create turbulence that makes the blower work harder for the same airflow. It performs well when designed and sealed properly — most of the rigid duct problems we see are bad design or bad sealing, not bad material.
Flexible duct: the practical workhorse
Flexible duct is a wire coil wrapped in insulation and a vapor jacket, and it deserves a fair hearing, because most of what goes wrong with flex is installation, not the product itself. Properly installed flex — pulled taut, supported every few feet, no sags, no kinks, sized to ACCA Manual D — moves air exactly as designed. It is quieter than bare metal, it navigates framing and tight attics where rigid pipe cannot go, and it costs less, which is why it is the most common ductwork in American homes.
The honest downside: flex *punishes* sloppy installation. A run left sagging between supports, pinched around a corner, or crushed under stored boxes can lose a large share of its airflow. That is an installer problem, not a flex problem — and it is why we install it to the manufacturer’s spec, not by feel.
How we quote it
For most homes we recommend a flex duct system: flex supply branches and a properly sized flex return, all designed to Manual D and installed taut and supported. It is the best value — quiet, efficient when installed right, and sized to give your system the air it was built for. Where a customer wants the longest lifespan and the tightest sealing, we also quote a metal trunk (spiral or rigid, sized to the system) with short flex branches — metal costs more up front and earns it back over decades.
Whichever option you choose, the sizing is what makes it work. Ductwork that was never designed for your equipment is the real reason rooms do not heat or cool right — and new ducts sized right fix that regardless of the material.
If your ductwork is on the table, call K&A Mechanical at (757) 630-2069 and we will walk your attic with you and show you exactly what we are proposing and why.
