Ducts running through a 130-degree attic can surrender 10 to 30 percent of your system’s cooling before the air ever reaches a register. Proper duct insulation attacks that loss directly — and in humid climates it does a second job, stopping the condensation that drips off cold, bare sheet metal onto your ceiling drywall. The work is unglamorous crawling with a utility knife and foil tape, but it is squarely DIY territory, and it pays back faster than almost any insulation project in the house. Here is what to buy, which R-value code demands, and the sealing step that must come first.
Seal Before You Wrap — Always
Insulating leaky ducts locks the leaks in place where you can never reach them again. Typical duct systems leak 20 to 30 percent of their airflow at joints, boots, and seams, so sealing comes first, insulation second. Use UL 181-rated foil tape or, better, duct mastic brushed over every joint, seam, takeoff collar, and boot connection — a $15 tub of mastic covers a typical system. Despite the name, cloth duct tape is banned for this job; its adhesive bakes out and fails in months. While everything is exposed, check that duct supports sit every 4 to 5 feet on flex duct so runs are not sagging, since kinked flex chokes airflow no matter how well you insulate it.
R-Values for Ducts: What Code Requires
Duct R-value requirements are their own table, separate from wall and attic numbers. Under the IECC, supply and return ducts in unconditioned attics must be insulated to R-8 in most climate zones; ducts in other unconditioned spaces — crawl spaces, garages, unheated basements — require R-6. Ducts running inside conditioned space technically need only R-1.9 or nothing at all, though wrapping cold supply trunks in humid basements is still smart for condensation control. Older homes commonly have bare metal trunks or thin R-4 wrap from the 1980s; upgrading those runs to R-8 is where the savings live. Diminishing returns arrive quickly past code levels, so R-8 is both the requirement and the practical ceiling for wrap products.
The Three Products That Do the Job
- Foil-faced fiberglass duct wrap: The default. Fiberglass blanket (1.5 to 3 inches thick, R-4.2 to R-8.3) bonded to a foil-scrim-kraft (FSK) vapor barrier. Sold in 4-by-25-foot and 4-by-50-foot rolls, roughly $0.60 to $1.20 per square foot. The foil facing is the condensation shield — it must face out, and every seam must be taped vapor-tight.
- Pre-insulated flex duct: For replacing runs rather than wrapping them. Modern flex comes in R-6 and R-8 with the insulation and vapor jacket factory-built. When old flex is crushed or rat-chewed, replacing it with R-8 flex ($1.50 to $2.50 per foot) beats rehabilitating it.
- Duct board and rigid wrap: Fiberglass duct board (R-4 to R-8) for plenums and rectangular trunks where blanket wrap is hard to secure neatly. Foil-faced polyiso sheets cut into wraps work for tight rectangular trunk runs.
Skip thin “bubble foil” wraps as a standalone solution — their real-world R-value is about R-1 without an air gap, far below code, whatever the packaging implies.
Condensation: The Humid-Climate Emergency
In cooling season, supply ducts run 55 to 60 degrees. Wherever humid air touches that metal below its dew point, water forms — enough, over a summer, to soak attic insulation, stain ceilings, and feed mold. Insulation stops this only if the vapor barrier is continuous: foil facing outward, seams overlapped 2 inches and sealed with foil tape, and zero gaps at hangers, takeoffs, and boots. A single fist-sized bare spot becomes a dedicated drip point. If you see rusty rings on the wrap of existing ductwork, that is condensation history — those sections need the jacket repaired, not just admired. The same physics governs cold water lines sweating in crawl spaces, which is a companion project covered in our pipe insulation guide.
How to Wrap a Duct, Step by Step
- Measure the duct circumference and add 2 inches for overlap. Cut wrap sections with a utility knife against a scrap board.
- Wrap each section around the duct with the foil facing out. Compressing fiberglass kills its R-value, so cut generously and let it stay lofted — snug, never strangled.
- Pin or staple the overlap seam, then seal it with UL 181 foil tape, rubbing the tape down hard with a plastic squeegee or the back of the knife.
- Butt adjoining sections tight and tape every circumferential joint.
- Cut and fit collars of wrap around boots, takeoffs, and hangers, taping each vapor-tight. These fiddly spots are where most DIY jobs fail — budget half your time for them.
Wear long sleeves, gloves, an N95, and eye protection; fiberglass itch is real and attic work is hot. Do the job on a cool morning.
Rectangular Trunks, Boots, and the Awkward Spots
Round branch ducts wrap easily; the fiddly geometry is where jobs stall. Rectangular trunk lines take wrap in a spiral or in wide sections folded crisply at the corners — cut the wrap so the fiberglass is not crushed at the 90-degree edges, since a corner pinched to half thickness runs at half R-value along its whole length. Supply boots (the transition boxes behind each register) sit between joists where a roll will not maneuver: cut a cross-shaped blank, cap the boot like wrapping a gift box, and tape every edge to the ceiling’s vapor plane. Where ducts pass within 3 inches of each other or hug a joist, stuff kraft-free fiberglass into the gap rather than leaving a bare channel. Plenums at the air handler deserve duct board or double-thickness wrap because they run the coldest air in the system, and leave the service panels and any dampers accessible — an HVAC tech who has to knife through your beautiful wrap to reach a filter or damper will not re-tape it kindly.
Flex Duct: Wrap It or Replace It?
Existing flex duct complicates the wrap decision. Old R-4.2 flex in good shape can be over-wrapped to reach R-8, but flex’s inner liner degrades from the inside, and 20-year-old runs are usually kinked, crushed at hangers, or chewed. The decision rule that serves well: flex older than 15 years, visibly deformed, or with a brittle outer jacket gets replaced with new R-8 flex rather than dressed up — at $1.50 to $2.50 per foot, replacement often costs less than the labor of wrapping a saggy run properly, and you fix the airflow restriction at the same time. Pull new flex tight (excess length is an airflow penalty of its own), support it every 4 feet with 1.5-inch-wide straps, and seal both collar connections with mastic before the insulation jacket goes on. Metal duct, by contrast, lasts indefinitely and is almost always worth wrapping in place.
Cost and Payback
A typical single-family system has 150 to 300 square feet of exposed duct surface in unconditioned space. DIY materials — wrap, mastic, foil tape — run $150 to $400 for R-8 coverage. Hiring it out costs $1,000 to $2,500 including sealing, more if ducts are buried or access is tight. Energy savings of 10 to 20 percent on heating and cooling are typical when previously bare or thin-wrapped ducts in an attic get sealed and brought to R-8, which puts DIY payback around two to four cooling seasons in hot climates. Utility rebates of $100 to $500 for duct sealing and insulation are common — check your provider before starting, since some rebates require a contractor’s test-in and test-out.
Quick Answers
Can I bury attic ducts in blown insulation instead? Yes, if they are sealed and R-8 wrapped first; burying adds effective R-value but demands a flawless vapor jacket in humid climates. Should return ducts be wrapped too? In unconditioned space, yes — code treats supply and return alike at R-6 to R-8. Is old wrap worth removing? Only if it is wet, moldy, or rodent-fouled; otherwise wrap over thin intact insulation to stack R-value. Seal first, hit R-8, keep the foil jacket continuous — those three rules are the entire trade secret.
