Signs Your Ductwork Is Leaking Conditioned Air

Start with the parts you can reach safely. A flashlight and ten minutes tell you a lot.

• Look at exposed ducts in the garage, closet or crawlspace for gaps, sagging runs and crushed sections.
• Look for old duct tape that has dried and let go. ENERGY STAR warns never to use duct tape on ducts, since it does not last.
• Feel for air at joints with the blower running.
• Check that the return is not undersized or blocked by furniture. ENERGY STAR notes that many systems have return ducts that are too small.
• Open all the registers, then compare airflow room by room.

Keep safe footing in an attic. Step only on framing, and stay out when it is hot. Skip the attic altogether if you are unsure, since a foot through a ceiling costs more than any duct repair.

A proper job has three parts: find the leaks, seal them, and prove the result. ENERGY STAR's guidance for hiring a contractor covers each one.

Inspection. The contractor should look at the full system, including attic and crawlspace runs. We inspect ducts before we quote the repair, so you pay for the problem you have.

Sealing. Leaks and connections should be sealed with mastic, metal tape or an aerosol-based sealant. Mastic is a thick paste that hardens over joints. Runs in unconditioned space should be insulated to R-6 or higher.

Verification. ENERGY STAR says the contractor should evaluate airflow after repairs are finished and check the supply and return balance.

Safety is part of the verification. Sealing a duct system changes pressures in the house, so gas or oil appliances should be checked to confirm they are not backdrafting, which means exhaust flowing back into the home. ENERGY STAR calls for a combustion safety test after the ducts are sealed.

Testing. Numbers beat a hand held up to a seam. PNNL's Building America guide describes the duct leakage test: seal the registers, connect a calibrated fan and hold the ducts at 25 pascals. The airflow needed to hold that pressure is the leakage figure, written CFM25. A total leakage test counts every leak, while a leakage-to-outdoors test adds a blower door so it counts only air leaving the house. The same guide lists 4 CFM25 per 100 square feet of floor as the limit in the 2012 through 2021 IECC. That target is meant for new ducts and is no pass mark for a 1980s house, but it gives a before-and-after result some scale.

Duct work has trade-offs, and the right answer depends on how much of the system anyone can reach.

• Hand sealing or aerosol. Mastic is cheap and lasting, but it only goes where a hand can reach. Aerosol sealant, an option ENERGY STAR lists, reaches hidden joints from inside, and it will not close a disconnected run or fix a crushed one.
• Partial sealing. Flat-roof chases and slab runs often stay out of reach. Sealing the accessible half is still useful, so expect a partial result. ENERGY STAR's figure of up to 20 percent is a ceiling and no forecast for your house.
• Testing or skipping it. A leakage test adds cost to a small repair, and a reconnected run may not need one. On a whole-system sealing job, it is the best proof you got what you paid for.
• Tighter ducts can expose other problems. Closing return leaks can reveal a return that was always too small, and the blower may get louder. Ask about that fix before the work starts.
• Seal or replace. Sealing costs less and leaves walls alone. Replacement fixes a bad layout, at the price of access work in ceilings and closets.