Ductwork sealing and replacement

Your heater may be doing a fine job of warming the attic. ENERGY STAR estimates that in a typical house, about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts. In a lot of Bay Area houses those ducts run through a vented attic or a crawl space under a raised floor, which is outside the house in every way that counts. Air that leaks there was paid for and never arrived.

Ducts are also the part of the system nobody looks at. They sit under insulation or behind a crawl-space hatch, quietly coming apart at the seams for thirty years.

Signs the ducts are the problem

ENERGY STAR's list is a good starting point, and it matches what turns up in Bay Area attics:

• Rooms that are hard to heat or cool compared with the rest of the house.
• Stuffy rooms, and high bills that don't match how the house feels.
• Ducts running through an attic, crawl space or garage.
• Flexible duct that is tangled, kinked or crushed.

Up close, the usual suspects are boots that have pulled away from the ceiling, flex duct flattened under old insulation or a careless foot, cloth duct tape that dried out and let go, and outer jackets torn open by rodents. A disconnected run can blow straight into the attic for years before anyone notices the room at the end of it never warms up.

Seal, repair or replace

The market range for each of these is in the price section below. ENERGY STAR's sealing advice is simple: mastic or metal tape, never duct tape, and insulate every duct you can reach in an attic, crawl space or garage.

What California's 2025 Energy Code requires

This is the part a lot of duct quotes skip. The 2025 Energy Code took effect January 1, 2026, and the Energy Commission's compliance manual lays out the duct rules for existing houses.

25 feet or more of new or replacement duct. The installer seals the system, and an ECC rater (the new name for a HERS rater) tests it. The system has to leak no more than 10 percent of its airflow in total, or no more than 7 percent to the outside. If neither target can be reached, every accessible leak is sealed and the rater confirms it with a visual inspection and a smoke test. The only exception is a duct system that contains asbestos.

An entirely new duct system (at least 75 percent new material) has a tighter target: no more than 5 percent leakage. If the air handler and ducts sit in a vented attic, the attic itself then falls under the altered-ceiling rules, which can mean R-49 insulation and air sealing on top of the ducts. That pairs naturally with blown-in insulation and air sealing.

Garages. If the air handler, coils, plenum or altered ducts are in a garage, any length of new duct triggers sealing to 6 percent leakage or less, or a full visual and smoke-test check of every accessible leak in the garage.

Replacing equipment. Replacing the air handler, the outdoor unit of a split system, a coil or a packaged unit triggers duct sealing and an ECC test to the same 10 or 7 percent targets. The exceptions are ducts already tested by a rater, systems with less than 40 feet of duct, and systems containing asbestos.

Duct insulation. New ducts in unconditioned space need R-6 or R-8, depending on climate zone. The 2025 code raised several zones to R-8, and in the Bay Area that covers three of our four zones:

The zone assignments come from the Energy Commission's ZIP code table. Zone lines don't always follow city limits, so an address near an edge is worth checking.

The ECC rater is an independent third party, and the test and the permit are both real costs. A duct replacement quote with no testing line and no permit line is missing something.

Asbestos in old duct systems

Older houses can have asbestos in duct wrap, joint tape and furnace insulation. EPA lists pipe, furnace and boiler insulation among the materials to watch for, and its advice is to leave undamaged material alone and have removal done by people trained to handle asbestos. The Energy Code also exempts duct systems containing asbestos from its sealing and testing rules. If we see duct wrap or tape that might contain asbestos, we stop and refer you to an asbestos consultant for testing before anything is cut or sealed.

Ducts and attic insulation are one job

Ducts lying on an attic floor get in the way of everything else. Insulation blown over a leaky duct hides the leak. A duct replaced after new insulation goes in means digging through the new insulation. And rodent damage usually means the ducts and the insulation both need attention. So the order matters:

• Inspect the ducts and the attic together.
• Seal or replace the ducts, and get them tested where the code requires it.
• Air seal the ceiling.
• Then insulate, over and around the finished ducts.

If old insulation is coming out anyway, after rodents or for any other reason, that is the cheapest moment to fix the ducts. See insulation removal. In houses on raised foundations, the same logic applies under the floor; see crawl space insulation.

What this page doesn't cover

The furnace, heat pump or air conditioner itself. This page is about the ducts that carry the air. If the equipment is being replaced, the duct testing rules above still apply, so it is worth having the ducts looked at before the new unit goes in.

Getting it quoted

Useful things to have: photos of the ducts in the attic and crawl space, the age of the system, which rooms run hot or cold, and whether the equipment is being replaced soon. You can book a free in-home inspection or send photos and a short video for an estimate.

The written quote says whether the work is sealing, repair or replacement, how many feet of duct, the duct R-value, and whether ECC testing and the permit are included. Nothing is due before you sign. The estimator gives a first range, and the cost page shows where the numbers come from.