Leaky, Disconnected and Crushed Ducts:
Where the Heat Goes Before It Reaches the Room

What goes wrong with ducts

Most duct findings in Russ’s reports fall into a handful of patterns. Here’s how they sort out, roughly from most common to least:

• Not insulated or not sealed. The big one. Bare metal ducts in a crawlspace, open joints, torn insulation wrap. This showed up in 58 of those reports.
• Unsealed holes where ducts leave the furnace closet. Common in newer houses with the furnace in a hall closet. The ducts go up through the ceiling into the attic, and the gap around them is only partly foamed or left open.
• Lying on the ground. Flex duct that fell off its straps, or was never hung, resting on the vapor barrier or the dirt.
• Disconnected. A run that pulled off its fitting, sometimes with insulation blowing out at a floor register upstairs.
• Crushed or kinked. Someone crawled over it, stored something on it, or bent it too tight around a joist.
• Chewed. Rodents tear flex duct, and a hole in a supply duct is a hole like any other.

Some houses had several at once. One Lake Oswego crawlspace had flex ducts on the ground, crushed in several places and not securely connected, and the clue upstairs was duct insulation that had blown out through a floor register. One Canby basement had ducts held together with loose duct tape and silicone caulk. In a Troutdale house, a long section of duct had dropped off its hangers and was lying on the crawlspace floor.

Why a leak under the house costs more than it looks

ENERGY STAR estimates that in a typical house, about 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poorly connected ducts. Where that air goes is the problem. A duct leaking inside the heated part of the house still warms the house. A duct leaking into a vented crawlspace or an attic is heating the outdoors.

Supply and return leaks do different damage. Supply ducts push air out, so a leak there dumps warm air under the floor and the room at the end of the run stays cold. Return ducts pull air back to the furnace, so a gap in a return under the house sucks in crawlspace air: damp, dusty, and sometimes smelly. The post on what a musty smell in the house can mean covers that side of it.

Bare ducts have a smaller cost of their own. A bare metal duct in a cold crawlspace gives up heat through its walls the whole length of the run. In summer, with air conditioning, a cold uninsulated duct in a humid crawlspace can sweat. Rust on duct seams in a crawlspace, which Russ notes fairly often, usually points to moisture on the duct at some point.

Duct tape, foam and the wrong fix

Duct tape is named for ducts and fails on them. In the late 1990s, Lawrence Berkeley National Laboratory ran aging tests on duct sealants. The cloth-backed, rubber-adhesive tape most people call duct tape failed faster than every other sealant they tested, and it was the only type that failed consistently. Mastic, foil tapes and aerosol sealants held up. ENERGY STAR’s advice to homeowners is to use mastic or metal tape and never duct tape.

So when Russ sees gray cloth tape peeling off a duct joint, he writes it up, even when it still looks stuck. Silicone caulk at a duct joint goes in the same note. A proper joint is fastened mechanically (sheet metal screws on metal duct, a clamp or strap on flex) and then sealed with mastic or listed foil tape.

The furnace closet in newer houses has its own version. Installers often run a bead of expanding foam around the ducts where they go up into the attic and leave gaps in it. Russ’s reports on these note open gaps around the duct and recommend sealing with an approved fire-rated sealant, since the opening connects the living space to the attic.

What a home inspection can and can’t see

Russ goes into the crawlspace and attic when they’re accessible and safe, and he looks at the ducts he can see: connections, supports, insulation, tape, gaps, rust, damage. Upstairs, he runs the system and checks for airflow at the registers. A register with nothing coming out gets traced to the crawlspace when the duct can be followed.

The thermal camera helps in finished ceilings. In a Vancouver house, a large cold patch on the living room ceiling pointed to a leaking duct above it, and the report sent it to an HVAC contractor to open up and seal. The camera shows temperature, not air movement, so a pattern like that is a strong clue and not proof. The thermal imaging post explains what the camera can and can’t show.

Some ducts can’t be seen at all. Runs inside walls, between floors, or buried under attic insulation are out of view. Low crawlspaces, deep storage and blocked access cut off more. When part of the duct system couldn’t be looked at, the report says so. For a number on how much the whole system leaks, an HVAC or energy contractor can run a duct leakage test, which pressurizes the ducts and measures the air that escapes. (Some newer houses have an ENERGY STAR testing label in the mechanical closet from exactly that kind of test.)

Older houses add one more check. Some Portland-area ducts from the 1930s through the 1970s have wrap or joint tape that may contain asbestos, and the post on asbestos ductwork covers what that looks like and why it gets tested before anyone tears it out.

Damage the ducts did to the house

Ducts get installed after the framing, and sometimes the installer cuts the framing to make room. In one Lake Oswego crawlspace, a floor joist had been cut through to fit a register and its duct. That’s a structural finding as much as an HVAC one, and the post on notched and cut floor joists goes into how those get evaluated.

Abandoned ducts cause a different problem. Old runs left in a crawlspace after a furnace change trap moisture against the vapor barrier, give rodents a place to nest, and block the view of the framing. One Portland report found loose fiberglass, plastic sheeting and disconnected flex duct piled on the vapor barrier, and recommended clearing it out.

For the rest of the heating and cooling system, the guide to what to look at in the HVAC system before you make an offer covers the furnace, heat pump and air conditioner. To set up an inspection, you can book online.