How to Find Leaks in Ductwork

HVAC ductwork in attic.

The crawlspace under our house seems abnormally cool in the summer and warm in the winter. My first thought is that the ductwork leaks, but a friend said it may be just radiating warm or cold air depending on the time of the year. What are your thoughts? -Scott

Hi Scott,

If your crawlspace is enclosed, it will stay cooler in the summer and warmer in the winter than the outside air due to the more constant ground temperature, but it should not be excessively different as you describe. It’s possible that the ducts are poorly sealed, allowing air to escape, or that they were inadequately insulated.

To check, go under the house when the HVAC system is running and feel along the ducts for air leakage. Also, look for discoloration of the insulation or rusting of the ducts that could indicate a leak. Pay particular attention to the joints in pipes and where runs of ducts meet in case a pipe has become disconnected. Repair any leaks using duct mastic or metallic duct tape rather than standard cloth tape. You can also have a HVAC professional run a pressurized test of the system to check for leaks.

If your ducts are inadequately insulated, you may see condensation forming on the outside of the ductwork during the summer when humidity is high and the air conditioner is running. Poor insulation can not only increase your utility bill in winter and summer by up to 30%, but it may cause excess moisture in the crawlspace which can led to mold. According to the U.S. Dept. of Energy, ducts in unheated spaces should be insulated to an R-value of R-4 to R-8 in warmer parts of the country, and an R-6 to R-11 in colder climates.

Another possibility is that your floor may need insulating. If so, install batts of fiberglass insulation between the joists in the crawlspace with the paper vapor barrier facing up toward the living area and hold it in place with support wires. For more on insulating under a house, see our episode on the Scariest Utility Bill.



  1. I have been shopping for coating for my exterior heating ducts, the sheet metal ducts are installed on an Armstrong foam roof. I need a coating that will protect sheet metal from weather and seal connection to duct entrance to building. I have read specs on many sealers none say they are exactly for purpose. thank you


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