DUCT SEALING PROS · BUILDING SCIENCE
Why Do Ducts Leak? What Heat, Dust and Construction Do to St. George Ductwork
Why do ducts leak so much in St. George? Because attic air can top 120°F, ordinary tape fails in that heat, and ducts are built from many joints that each have to hold. A new house is no guarantee, since leaks start at construction.
Why do ducts leak in the first place?
A duct system is a network of metal and flexible pipes joined at hundreds of points: seams, collars, takeoffs, boots and the connection to the air handler. Every joint is a place where air can escape, and most are hidden above ceilings or in attics. Why do ducts leak so often? Because every one of those joints has to be sealed well, once, by someone working in a hot attic.
Over the years, the materials fail. Seams open as metal expands and contracts, flex duct gets pulled loose, and tape dries out. Small gaps become larger, and the system quietly loses more air each summer.
The short answer is that leaks are normal, not unusual. What matters is how big they are, and that is something you can measure rather than guess.
The heat is unforgiving
St. George sits around 2,800 feet in the Mojave. July highs average about 104°F, nights stay near 74°F, and the valley sees 100°F or more regularly from mid-June to mid-September, with 110°F events most summers. Humidity is usually under 20% and the valley gets only about 8 inches of rain a year. Washington County regularly sees 90°F days as early as May.
Attic air can reach 120°F or higher on a sunny summer day. Energy Vanguard, citing NREL research, notes that ducts in an unconditioned attic can add roughly 25% to the cooling load in a hot climate, even when the ducts are well sealed. Add leaks and you are pushing cold air into an oven. Our guide to attic heat and your ducts goes deeper.
Common causes of leaks in local homes
- Cloth duct tape: it heats up, cools down and lets go. Ducts sealed with it often look fine until you test them.
- Dust and gaps: leaks on the return side pull dusty attic air into the house. See desert dust and indoor air quality.
- New does not mean tight: new homes can leak too, because duct work is often finished fast and tested rarely. See new construction duct sealing.
- Damage: crushed, torn or disconnected sections that need duct repair.
- Poor connections: boots and takeoffs that were never sealed well in the first place.
Why do ducts leak in newer homes? Because the code and the schedule often do not require a test, and nobody measures until the summer bills arrive.
What to do about it
The fix starts with a measurement. A duct leakage test shows how much air your ducts lose, and sealing with mastic and UL 181B-FX foil tape stops most of it. See our overview of duct sealing methods for how each fits, and our guide to the signs of leaky ducts to check your home.
A duct system that was never tight is not a failure of your house, it is the normal state of most ductwork. The question is how much it costs you and whether sealing pays back. See is duct sealing worth it.
Our standards page explains how duct leakage is tested, so you can read the numbers yourself. If you want a second opinion on a quote that suggests replacing everything, compare it against sealing versus replacing first, because most leaky systems that are otherwise intact can be sealed.
Frequently asked questions
Why do ducts leak even in a new house?
Because duct joints are sealed by hand during construction, and few builders test the result. A new system can leak as much as an old one.
Why do ducts leak more in hot climates?
Heat accelerates the failure of tape and seals, and a 120°F attic makes every leak waste cooled air.
Can leaky ducts be fixed without replacing them?
Often yes. Sealing with mastic and foil tape, and aerosol where needed, can fix most leaks if the ducts are otherwise intact.
How do I know how much my ducts leak?
A pressure test measures it. Without one, you are guessing.
Ready to find out where your cool air is going?
Call us and tell us which rooms are giving you trouble. We will explain what a duct leakage test involves and book your duct sealing services St George crews can finish before the next heat wave.
Duct Sealing Pros
3430 S River Rd, St. George, UT 84790
(435) 281-0395