How to Insulate Ductwork and Quickly Reduce Your Energy Bills

Before you begin, look closely at your ducts to determine what’s needed. Flexible ducts often come pre-insulated and may just need minor sealing. Rigid ducts will require precise cutting and fitting. Ducts in attics, crawl spaces, and other unconditioned areas should have an additional vapor barrier included in the insulation.

If you have internal ductwork that runs through walls, you'll want to hire an HVAC specialist to access and insulate it properly.

Before you get started, turn off the power to your HVAC unit. You should also dress appropriately. Whenever you work with fiberglass insulation, wear long sleeves, pants and socks that cover your legs, safety goggles, sturdy work gloves, and a respirator breathing mask.

Before insulating, wipe the exterior of your ducts with a clean, dry cloth so they're free from dust. This will allow the tape and mastic to adhere properly. If any of your ducts have existing insulation, assess it for wear. If it's old and damaged, you'll want to remove and replace it.

Some insulation can be removed by peeling off the tape that’s holding it to the duct. Otherwise, you'll need to cut it away with your utility knife carefully. Be extremely careful with a knife around flexible or semi-rigid ducts, as these are prone to tears. Put the old insulation into sealed garbage bags and dispose of or recycle it per your local waste collection guidelines.

Once your ducts are clean and bare, turn the system back on and visually inspect the ducts for kinks or cracks. You can also detect leaks by holding a smoke pen or lit incense stick beside the ducts. Leaking air will disturb the smoke.

Turn off the system's power again, and seal any leaks by applying mastic duct sealant with a paintbrush. Allow the sealant to dry thoroughly before proceeding to the next step.

Use your tape measure to determine the circumference of your ducts. Then, use your measuring square and utility knife to cut your insulation to that width plus an additional two inches for overlap. You'll need to cut many sections to cover the entire duct.

Wrap the insulation around the duct, overlapping and securing the seam with foil tape. The insulation should fit snugly without being compressed. Pinch the seam tightly and secure it with short pieces of foil tape. Seal the ends of each length of insulation to the duct or the next length. Repeat this process until your ducts are sealed entirely.

For lengths of ductwork that are difficult to access on all sides, use preformed insulation in a size that matches the diameter of your ducts. To install preformed insulation, you’ll need to measure and cut the insulation tubes and then remove fittings to slide them over the duct. Replace any removed fittings and use mastic to seal their joints. Seal any remaining edges with foil tape.