Ductwork Insulation: What Fits Your Home

What duct insulation is, and the four common ways ducts get it

Ducts get insulation in one of four common ways. Flex duct is the round, bendable kind of duct. A vapor barrier is an outer layer, usually foil or plastic, that resists water vapor passing through it. The Building America Solution Center is a guide site funded by the Department of Energy. Its guide says flex duct is a plastic inner liner on a metal coil that comes already covered by fiberglass blanket insulation and then by a foil or plastic vapor barrier. It says flex duct is typically available in R-4, R-6 and R-8.

For rigid metal duct, the usual cover is duct wrap, a fiberglass blanket with a foil-faced vapor barrier, put on the outside. Duct wrap comes in R-values from R-4 through R-8.

Duct board is made from flat sheets of bonded glass fibers with a reinforced foil laminate on the outside, and the sheets are built into ducts. Building America says duct board is inherently insulating, at R-4.3 per inch, so thicknesses of 1, 1.5 and 2 inches give R-4.3, R-6.5 and R-8.7.

The fourth way is a liner: EPA says many modern residential duct systems use sheet metal ducts lined on the inside with fiberglass duct liner. Our reading: the kind of duct changes what a good job looks like, so ask a contractor which of the four you have.

Which ducts need it: the space they run through

Building America says ducts ideally sit in conditioned space, such as a dropped ceiling, between floors, or in an unvented attic that is insulated along the roof line. EPA says ducts in all non-air-conditioned spaces, such as attics and crawl spaces, should be properly sealed and insulated, to help keep condensation from entering the system. Condensation is water that forms when moist air touches a cold surface.

Building America says a typical vented attic with a dark shingle roof can reach summer temperatures of 140°F. Our reading: that is a general federal figure, not a measurement of Tampa Bay attics. In humid climates, it says, “duct sweating” can become a significant problem if the duct's thermal and vapor barriers are not properly aligned along its whole length. The moisture side of that problem is covered in ductwork in Florida, mold in HVAC systems and house humid with the AC running.

A boot is the fitting where a duct meets a room vent. For ducts in conditioned space, Building America still suggests considering insulation on all metal duct, boots included, to minimize condensation.

The R-value numbers, and what each one belongs to

The Florida figures come from Form R402, a compliance form in Appendix RD of the Florida Building Code, Energy Conservation, 8th Edition (2023). The form is headed “Residential Building Thermal Envelope Approach,” with an “R-Value Computation Method” title. For ducts under three inches across, it lists R-6 in attics and R-4.2 in other unconditioned spaces. It tells the filer that all “installed” values must be equal to or more efficient than the required level. This page did not read the code's full text, so it cannot say whether replacement or added ducts in an existing house must meet the same R-values. Ask your building department which rule applies, and read HVAC permits in Florida on how permits work.

Other documents give other numbers, for other purposes.

• ENERGY STAR's duct sealing flyer says to insulate ducts in unconditioned areas such as attics, crawlspaces and garages with insulation of R-6 or higher.
• A Department of Energy page says the ENERGY STAR program requires a minimum of R-8 for ducts in unconditioned spaces such as attics.
• A 2017 Florida Solar Energy Center proposal describes an energy credit in the 2018 International Residential Code, a national model code, for ducts buried under attic insulation. The proposal says that credit required the duct's own insulation to be rated at least R-13 in climate zones 1A and 2A, which it ties to north and south Florida. It says that rating does not count the attic insulation burying the duct. The proposal says that credit was not then Florida code, and this page did not check whether the 2023 edition changed that.

Our reading: these numbers are not rival answers, because each belongs to a different document, and a quote that names an R-value should say which one it is meeting.

Printed R-value versus installed R-value

One insulation maker's product data sheet for its duct wrap lists installed R-values that apply when the wrap goes on following the maker's recommended procedures. For its 2.2-inch wrap, the sheet lists R-7.4 out of the package and R-6.0 installed. For its 3-inch wrap, it lists R-10.0 out of the package and R-8.3 installed. The sheet says its installed values assume 25% compression of the insulation. It also says the installed R-value is based on installation over sealed sheet metal duct, with the average installed thickness at 75% of nominal thickness. Building America gives the same warning: wrapping the insulation too tightly compresses it and reduces its R-value. Our arithmetic from the 2.2-inch row: a wrap printed R-7.4 is R-6.0 once installed. Our reading: when a quote says R-8, ask whether that is the printed value or the installed value, and ask to see the product's data sheet.

The sheet is one maker's and was printed in September 2010, so this page does not claim that other products, or the maker's current ones, list the same values.

The vapor barrier, seams and fittings: what a finished job is checked against

The Department of Energy's weatherization installer job aids for ducts give the desired outcome as reduced heat transfer through the duct system and minimized condensation on it. The aids are written for installers, so this page reports what they check and gives no steps. A takeoff is the fitting where a branch duct leaves the main duct. The job aids' checklists and steps include these points.

• The insulation has an attached and continuous vapor barrier.
• All insulation seams are sealed.
• The insulation is mechanically fastened and sealed, with no exposed ducts.
• On flex duct, the metal fittings, including boots, elbows and takeoffs, are insulated separately with duct wrap of the minimum acceptable R-value and a vapor retarder.
• The insulation is not compressed.
• The ducts were prepared and sealed first.
• The metal-duct line on the checklist reads minimum R-8, with a note that an approved variance request can replace that figure.

A vapor retarder is a layer that slows the passage of water vapor. The insulation maker's data sheet says that, where a vapor retarder is required, all tears, punctures and other penetrations of the facing should be sealed to give a vapor-tight system. Our reading: that list works as a set of things to ask for in writing, not as a how-to.

Buried ducts and the limits of an R-value

The 2017 Florida Solar Energy Center proposal says minor duct condensation has been noted in research on partially covered or buried ducts with R-6 insulation. It says minor condensation was also found on a buried R-8 duct in a 22-day lab test set up to represent South Carolina summer conditions. It says the condensation in those cases was not reported to be severe enough to drip onto materials below. It also says there has not been adequate published research on the long-term performance of deeply buried ducts in hot and humid climates. This is a plan for research, not a report of results. Our reading: an R-value alone does not settle whether a duct sweats, because the outer cover and where the duct sits matter too.

Heating ducts and furnace ducts

ENERGY STAR's flyer says that in houses with forced-air heating and cooling systems, ducts distribute conditioned air throughout the house. It says to wrap ducts that run through unconditioned space with duct insulation, to keep them from getting hot in the summer or cold in the winter. Our reading: the same ducts carry cooled air in summer and heated air in winter, so one insulation question covers both. What a furnace is and how a forced-air system works are in forced-air heating. Work near a furnace, a flue or a gas line is combustion work for licensed professionals, and this page gives no steps for it.

What insulation does not fix

Insulation slows heat moving through the duct wall, but it does not close a gap. ENERGY STAR's flyer ends with a step to have a professional check that the system is balanced after sealing and insulating the ducts. A Duct Retrofit Decision Guide for heat pump installers, hosted on a Department of Energy site, says gaps in duct insulation allow heat transfer to the unconditioned space around the duct. It adds that gaps may signify a larger issue, because insulation may be blown off by air leaks in the ducting. Building America says that before insulation is added to flex duct, the runs should be checked for excessive coils or loops and for a duct wall collapsed by kinks or sharp bends. Our reading: insulation does not fix leaks, a crushed duct, an undersized return or an airflow problem, so those come first or alongside. See duct sealing vs duct repair and return air vent too small for those problems.

If insulation is wet or moldy, see mold in HVAC systems and ductwork in Florida. EPA's duct cleaning guidance says to check first that a system has no asbestos-containing materials, such as insulation and register boots. It says asbestos-containing materials should not be disturbed or removed except by specially trained and equipped contractors. Our reading: that caution applies before anyone cuts into old duct insulation, so this page gives no removal steps.

Questions to put in writing before you hire

Put these questions to any contractor who proposes duct insulation, and ask for the answers in writing.

1. Where do my ducts run: an attic, a crawl space, a garage or inside the conditioned space, and what kind of duct is it: flex, wrapped metal, duct board or lined?
2. What R-value is the insulation, is that the printed value or the installed value, and can I see the product's data sheet?
3. Which minimum R-value does my building department apply to this job, and does the work need a permit?
4. Does the insulation have a continuous vapor barrier, and how will seams, tears and punctures in it be sealed?
5. Will boots, elbows and takeoffs be insulated too, and with what?
6. Will the ducts be sealed first, and will leakage be tested afterward?
7. Is any existing insulation wet, moldy or compressed, and could it contain asbestos before anyone disturbs it?
8. Will airflow be measured after the work, and who will give me the written results?

Written answers also give you something to compare between quotes.