Definition
What is duct sealing and insulation, and how does it work?
The duct system is a second pressure boundary running through the first one. Your building envelope — the attic plane, the walls, the rim joists, the crawl space ceiling — separates conditioned space from unconditioned space, and air sealing works on that boundary. Ducts are a sealed channel that starts inside conditioned space at the air handler, crosses the envelope into an attic or a crawl space, and comes back through it at every register. When the duct system leaks, it leaks across a boundary the envelope work never touches. On the supply side, the blower is pushing, so conditioned air you already paid for is forced out into a vented attic. On the return side, the blower is pulling, so unconditioned attic or crawl air is drawn into the airstream that gets delivered to your rooms. Both are happening every minute the system runs, and neither shows up as a draft you can feel with your hand.
Sealing and insulating are two different jobs, and they run in a fixed order. Sealing means closing the leaks: mastic brushed onto every accessible joint, every boot where the duct meets the register, the plenums at both ends of the air handler, and the takeoffs where branch runs leave a trunk line. UL-181 listed foil tape handles clean, dry, straight seams that a brush cannot reach well. Cloth-backed duct tape does not belong anywhere in the scope — its adhesive dries out and releases, and a joint sealed that way years ago is an open joint now. Insulating means wrapping the sealed duct so the air inside does not shed temperature while it crosses an unconditioned space. The order is not a preference, and the reason is the same reason air sealing outranks vapor-retarder decisions everywhere else in a retrofit: moving air is what actually carries heat and moisture across an assembly, so the leak is the thing that has to close first. A return joint open in a damp crawl space is not only losing you energy — it is transporting that crawl space's air, and whatever the air is carrying, into the stream your registers deliver. Wrap first and you have insulated a leak: the air still exits at the joint, only now the joint is buried under a layer nobody will pull back to look at. Seal first, wrap second, and the wrap is doing the job it was bought for.
Honest about limitations: this is one system, not an envelope, and it cannot fix problems that are not leaks. A duct run that was undersized when the house was built, a flex run crushed flat behind a furnace, a return path that was never cut in for a room with a door that closes — sealing does not solve any of those, and a duct-sealing scope that stops at the joints leaves them exactly where they were. Ducts buried behind finished basement ceilings or inside walls are not reachable without opening the finish, which puts access, not duct sealant, at the top of the invoice. And there is a safety item that belongs stated plainly: changing duct pressures in a house with atmospheric-draft gas appliances can affect how those appliances draft. A furnace or water heater venting up a natural-draft flue is sensitive to what the air handler is doing to pressures in the space around it, which is why draft and carbon monoxide get checked before the work and again after it — the same check a competent operator performs under depressurization during a blower door test.
We focus on retrofit projects for existing Denver homes — assessment, removal where needed, air sealing, and installation tailored to homes that are already built and lived in. New construction insulation follows a different process and is typically handled through general contractors and builders; if you're working on a new build, we can refer you to a contractor experienced with new-construction insulation scope.
According to Building Science Corporation, “water vapor diffusion, while amenable to simple analysis, is often (but definitely not always) an insignificant source of moisture in modern building envelopes.”
Qualification signals
Who needs duct sealing and insulation in Denver?
The signals that point at ducts rather than at insulation are distinctive, and most of them are ones you have already noticed. A room or a whole floor that never reaches setpoint no matter how the registers are balanced. A system that runs and runs against a bill that does not match the size of the house. Rooms that are worst at the end of the longest duct run — the far bedroom, the bonus room over the garage. Registers that barely move air with the blower on high. Visible ductwork in an attic or a crawl space with joints you can see daylight at, tape hanging off in strips, or a boot that has pulled away from the drywall. Or the structural version of the same case: your home was built before 1990, the supply runs cross a vented attic or an unconditioned crawl space, and nobody has ever been up or down there with a brush and a bucket of mastic. The strongest case of all is timing — if a crawl encapsulation, an attic project, or an equipment replacement is already being scoped, the ducts are reachable and staged right now.
Skip it, or put it behind something else, when any of these fit. If your ducts run entirely inside conditioned space — a common layout in homes with a finished basement and no attic runs — the leaks are leaking into your house, which is a distribution and balance problem, not an energy loss out of the envelope, and the payback argument on this page mostly does not apply to you. If your attic is still sitting at R-11 to R-19 against the R-60 minimum, the attic outranks the ducts on both area and dollars; do the attic first, and seal the ducts while the crew is up there. If the ductwork is behind a finished ceiling, you are buying drywall work to reach a few dozen linear feet, and that budget almost always buys more comfort elsewhere. If the crawl space has standing water or active seepage, that is a drainage problem first and the crawl space guide covers the moisture-first sequence. And if the real complaint is that one room is hot and the rest of the house is fine, get the system looked at before anyone seals anything — that can be a balance or a return-path problem that duct sealing will not touch.
Materials & methods
Which duct sealing and insulation approach is right for your Denver home?
Conservative cost ranges for typical Denver-metro projects. Specific quotes depend on your home, current insulation, and any required pre-work.
| Approach | What it does | Cost driver | Where it belongs | Cons | Best for |
|---|---|---|---|---|---|
| Mastic at joints, boots, plenums, takeoffs | Closes the leaks — the actual seal | Labor and access; the material itself is a small line on the invoice | Every accessible joint in the system, supply and return | Hand work joint by joint; messy; requires physical access to each connection | The default sealing method on any reachable run |
| UL-181 listed foil tape | Closes clean, straight seams a brush cannot work well | Labor; tape is inexpensive by the roll | Rigid connections and flat seams on clean, dry metal | Fails when applied over dust or at the wrong surface temperature; not a substitute for mastic at a boot | Supplementing mastic, not replacing it |
| Cloth-backed duct tape | Nothing durable — the adhesive dries out and releases | Cheap now, paid for twice when the joints get redone | Nowhere in a duct-sealing scope | It is the reason many previously "sealed" ducts leak again | Identifying and removing, not applying |
| Duct wrap over the sealed duct | Keeps the air inside from shedding temperature crossing unconditioned space | Labor plus material by linear foot of duct run | Supply and return runs crossing a vented attic or an unconditioned crawl | Does nothing about leaks, and hides them permanently if applied before sealing | After sealing, never before |
| Bringing the ducts inside conditioned space | Removes the problem instead of treating it | The largest scope here — crawl encapsulation alone costs more than any other approach on this list | Crawl encapsulation and conditioned-attic conversions | Biggest project on this list; a design decision, not an add-on | When the crawl is being encapsulated anyway, or the run is otherwise unreachable |
Cost & the cost of waiting
How much does duct sealing and insulation cost in Denver — and what does waiting cost?
Duct work is priced after somebody looks at the ducts, and any number quoted before that is a guess dressed up as an estimate. The reason is that the material is the cheap part — mastic and foil tape are minor line items — and the invoice is made almost entirely of labor and access. What drives it: how many linear feet of duct actually cross unconditioned space, how many joints and takeoffs are in that run, whether the crew is standing up in a walkable attic or working flat on their back in a low-clearance crawl, how much of the run is buried under loose-fill or behind finished ceiling, and whether the register boots can be reached from the space or have to be worked from inside the room. No primary source publishes a Denver-specific price for the diagnostic side either: a standard Denver home energy audit, or a premium audit with extended diagnostics — duct leakage testing, room-by-room pressurization, a full HVAC look — which costs more. If you want the duct system measured rather than eyeballed, that is the door to walk through.
The cost argument that matters most on this page is about order, not about the number. Duct work belongs before equipment replacement, and doing it in the other order is one of the more expensive sequencing mistakes a homeowner can make. Equipment gets sized by load calculation against the envelope it has to serve rather than by a square-footage rule of thumb, and a leaky, uninsulated duct system running through a vented attic is part of what that calculation has to carry. Seal and insulate the ducts first, and the furnace or heat pump gets sized against the house you will actually have. Do it in reverse and you have paid for capacity you no longer need, permanently, in a box you will own for as long as that equipment lasts. On the rebate side, be careful about what anyone promises you: duct sealing is not a rebate category of its own inside the Xcel Energy Insulation and Air Sealing Rebate, which pays out by check against per-measure caps. What duct work does do is ride alongside the measures that qualify — and the Whole Home Efficiency Bonus adds 25% of the rebate already paid when three qualifying measures are completed within two years of enrolling, which is exactly the configuration insulation plus air sealing plus one more measure is meant to reach.
Every winter you delay a real attic-and-air-sealing upgrade on a pre-1990 Denver home, you're heating the attic through the ceiling.
According to the Air Conditioning Contractors of America, “Manual J 8th Edition is the national ANSI-recognized standard for producing HVAC equipment sizing loads for single-family detached homes, small multi-unit structures, condominiums, townhouses, and manufactured homes. A proper load calculation, performed in accordance with the Manual J 8th Edition procedure, is required by national building codes and most state and local jurisdictions.”
Cost figures are conservative ranges. The free in-home estimate gives exact numbers based on your home and required pre-work — not a range.
Denver context
What's different about duct sealing and insulation in Denver?
Denver supply runs commonly cross an attic that reaches roughly 130 degrees in summer and 20 degrees in winter, and that temperature spread is the whole case for insulating a duct here. Denver sits in IECC Climate Zone 5B (cool-dry) at roughly 5,280 feet, where winter thermal gradients across the attic plane are large. A duct in that attic is a thin metal or flex channel carrying warm supply air through a 20-degree space, above the insulation plane, outside everything the attic insulation is protecting. The insulation on your attic floor is not helping that duct. It is the boundary the duct crosses, not a blanket the duct sits under — which is the single most common misunderstanding homeowners bring to this conversation, and a fair one, because the duct is lying right there on top of the material that is supposed to be doing the insulating.
The Denver-metro eras that put ducts in unconditioned space are the same ones this site names everywhere else. Mid-century and post-war stock routed supply runs through vented attics and crawl spaces as standard practice: the 1940s and 1950s construction in Wheat Ridge, Englewood, Centennial, and Old Town Arvada, and the 1950s and 1960s original-Aurora and original-Westminster ranches. Older Denver homes with crawl spaces — Wheat Ridge, Englewood, original Aurora, and the historic Denver neighborhoods — put the return-side problem in a space that has its own air to contribute. Colorado has some of the highest residential radon levels in the country and crawl spaces are a common entry path, and late-winter snowmelt is the dominant Denver crawl-space moisture source. A return leak down there is not just an energy loss; it is a duct pulling crawl air into what the registers deliver upstairs. That is the sharpest reason duct work and crawl space work get scoped together in this metro.
One Denver rebate note worth having before you call anyone, and it is a limit rather than a selling point: nearly all of the Denver metro sits in Xcel Energy service area, and the Xcel Insulation and Air Sealing Rebate is written around insulation and air sealing as its measures — duct sealing is not one of them on its own. Rebate-qualifying work must be completed by a participating contractor, and the qualifying blower door test measures the whole house before and after, not the duct run you sealed. Both facts point the same direction: duct work belongs inside a scoped insulation and air-sealing project with a contractor who is already filing that paperwork, where it counts toward the three-measure Whole Home Efficiency Bonus configuration rather than standing on its own. Anyone telling you duct sealing has its own Xcel rebate check attached to it is telling you something the program does not say.
Attic ducts are frequently buried under loose-fill insulation, and a buried joint cannot be sealed — the material comes off the run before mastic goes on, and goes back over it after the duct is sealed and wrapped. On a small run that is hand work, not a vacuum-truck scope. Where the whole attic is coming out anyway, the sequencing gets easier and cheaper: see our insulation removal guide for what triggers a real removal scope.
According to Xcel Energy, the participating-contractor symbol marks a program that requires customers to use an Xcel Energy participating contractor to install the equipment or make the improvement.
Process
How does the duct sealing and insulation process work?
Trace the system and find the boundary crossings
Crew walks the attic, the crawl space, and the basement to map where the supply trunk and branches run, where the returns are, and exactly where the duct crosses out of conditioned space. Accessible run is separated from run that is buried under loose-fill or hidden behind finished ceiling, because those two numbers price very differently.
Combustion safety check before anything changes
Gas appliances venting up atmospheric-draft flues are checked for proper draft and carbon monoxide production before duct pressures are altered. This is the same check a competent operator runs under depressurization during a blower door test, and it happens first here for the same reason: the appliance has to be known-good before the house around it is changed.
Measure the leakage rather than eyeball it
Where the scope justifies it, duct leakage testing puts a number on the system instead of an adjective. This is the diagnostic that sits inside a premium energy audit alongside room-by-room pressurization, and it is what separates a duct system that needs sealing from a duct system that needs balancing.
Seal the joints, boots, plenums, and takeoffs
Mastic is brushed onto every accessible connection — branch takeoffs off the trunk, the seams of the supply and return plenums at the air handler, and every boot where a duct meets a register. UL-181 listed foil tape handles clean straight seams on dry metal. Cloth-backed duct tape found on existing joints comes off; it is not re-used and it is not sealed over.
Seal the boot-to-drywall connection
The gap where a register boot meets the ceiling or floor finish is a direct opening between the duct and the attic or crawl space, and it is the connection most often skipped because it is worked from inside the room rather than from the duct side. It gets sealed as part of the scope, not left for the homeowner.
Insulate the sealed run
Duct wrap goes on after the sealing is finished, covering supply and return runs where they cross unconditioned space, with the facing sealed at every seam so the wrap is continuous rather than segmented. Nothing gets wrapped before it has been sealed.
Verify, re-check combustion, and document
Sealed and wrapped runs are photographed for the project file before insulation goes back over anything, because none of it is inspectable afterward. Draft and carbon monoxide are re-checked now that duct pressures have changed. Photos and any blower door readings go into the paperwork the participating contractor submits with the larger insulation and air-sealing scope.
Rebates & credits
What rebates apply to duct sealing and insulation in Denver?
Be precise about what duct sealing is and is not inside the 2026 Xcel rebate stack, because this is a place homeowners get oversold. Duct sealing is not a rebate category of its own in the Xcel Energy Insulation and Air Sealing Rebate — that rebate pays out against per-measure caps, by check after application, and its named measures are insulation and air sealing. What duct work does is get scoped alongside the measures that do qualify, on the same project, with the same crew and the same paperwork. Where that matters is the Xcel Whole Home Efficiency (WHE) Bonus, which adds 25% of the rebate already paid when three qualifying measures are completed within two years of enrolling, with entry through a blower door or infrared home energy audit or a Home Energy Squad Plus visit — insulation, air sealing, and one more measure is the configuration that gets there. Rebate-qualifying work must be completed by an Xcel participating contractor, with a blower door test before and after documenting a reduction of at least 20% in CFM50, and that test measures the whole house rather than the duct run. The $600 Insulation + Air Sealing Combo Bonus applies when qualifying insulation and air sealing precede a qualifying space-heating heat pump installed and invoiced by December 31, 2026 — worth knowing here, because duct work belongs before that heat pump goes in, not after. The income-qualified Xcel IQ Program is available through four eligibility pathways, one of them geographic with no income paperwork. Confirm what your specific scope files under with the participating contractor before work starts, not after.
- Xcel Energy Insulation and Air Sealing Rebate — standard utility rebate for Xcel heating customers, paid by check after application. Work must be completed by a participating contractor, with a blower door test before and after documenting a reduction of at least 20% in CFM50.
- Xcel Whole Home Efficiency (WHE) Bonus — adds 25% of the rebate already paid when three qualifying measures are completed within two years of enrolling. Entry requires a blower door or infrared home energy audit or a Home Energy Squad Plus visit.
- Xcel $600 Insulation + Air Sealing Combo Bonus — $600 stacked bonus when rebate-qualifying air sealing and insulation are installed before a qualifying space-heating heat pump, within two years of each other by invoice dates. The heat pump must be installed and invoiced by December 31, 2026.
- Xcel IQ Program — income-qualified, four eligibility pathways (one is geographic, with no income paperwork).
- Power Ahead Colorado (DRCOG) — Denver-region incentive program. Published policy funds heat pump and heat pump water heater incentives; insulation and air sealing are recommended preparation steps, not incentivized measures. Not yet live.
For current Xcel rebate amounts and program rules, see the Xcel Energy insulation and air-sealing rebates program page. For Colorado-program status (including HEAR closure and Power Ahead Colorado launch), see the Colorado Energy Office Home Energy Rebate page. Eligibility may depend on income, program funding levels, and qualifying product specifications.
Service area
Where do you provide duct sealing and insulation services in the Denver metro?
We connect homeowners with local insulation pros throughout Denver and the surrounding Front Range.
- Lakewood
- Arvada
- Aurora
- Wheat Ridge
- Centennial
- Englewood
- Littleton
- Westminster
- Thornton
- Park Hill
- Washington Park
- Highland
- Golden
- Broomfield
- Northglenn
Related insulation services
Get a quote
Tell Us About Your Home
We've Got It. Here's What Happens Next.
We've got your info and are routing it to a local pro. When they call, here's what to look for in the quote you receive: (1) R-value target — the 2021 IECC sets a ceiling minimum of R-60 in Climate Zone 5 (Table R402.1.3); R-49 qualifies only where uncompressed insulation extends full-height over the top plate at the eaves. (2) Air sealing scope — insulation and air sealing work together; sealing the leaks first is what lets the insulation do its job. If a quote skips both, get another quote. The federal IRA Section 25C tax credit ended December 31, 2025, and Colorado's HEAR single-family program is now closed for both Region 1 (the Front Range) and Region 2. Xcel Energy programs remain the primary rebate stack for Denver-area insulation projects in 2026.
Frequently asked
What do Denver homeowners ask about duct sealing and insulation?
Should I do duct sealing and insulation if my home was built after 2010?
Probably not, and the reason here is specific to ductwork rather than to insulation. Post-2010 homes were built under duct-tightness requirements and had their duct systems tested before anyone signed off on the house, so a retrofit duct-sealing scope on a newer home usually finds much less to recover than the same scope finds in a 1955 ranch with supply runs stapled through a vented attic. What makes the case for testing a newer home is a symptom you can name out loud: a room that never reaches setpoint no matter how the registers get balanced, a system that runs more or less constantly against a bill that does not match the house, or a visible disconnect in a run you can actually get to and point at. Absent a symptom like that, the money buys more somewhere else in the house, and we will say so.
Do you handle new construction insulation in Denver?
We focus on retrofit insulation for existing homes. New construction insulation typically goes through your general contractor or builder, and the process is different — pricing structures, code compliance steps, and project timing all work differently for new builds. If you're working on a new construction project and need an insulation contractor, we can refer you to a partner with new-construction experience. Send us your project details through the form below and note that it's new construction in the message.
What is the difference between sealing my ducts and insulating them?
They are two jobs and they run in one order. Sealing closes the leaks — mastic at every joint, boot, plenum, and takeoff, plus UL-181 listed foil tape on clean straight seams. Insulating wraps the sealed duct so the air inside does not shed temperature while it crosses a vented attic or a crawl space. Sealing comes first, always. Wrap a leaky duct and the air still exits at the joint; you have just buried the joint under a layer nobody is going to pull back to look at. Seal first, wrap second, and both are doing the work you paid for.
Is duct tape not what you seal ducts with?
No, and the name is one of the more expensive misnomers in the trade. Cloth-backed duct tape has an adhesive that dries out, shrinks, and releases from the metal, which is exactly why joints somebody taped years ago are hanging open today with the tape curled off in strips. Ducts get sealed with mastic — a thick sealant brushed onto the joint that stays flexible and stays put — and with UL-181 listed foil tape on clean, dry, straight seams. If a quote says "tape the joints" and does not say mastic, ask which tape and ask what happens to it in a 130-degree attic.
Where do ducts actually leak?
At the connections, almost always, and rarely down the middle of a run. The specific places: branch takeoffs where a run leaves the main trunk, the seams of the supply and return plenums at the air handler, every joint where two sections of duct meet, and the boot where the duct connects to a register — including the gap between that boot and the drywall, which is a direct opening into the attic or crawl and is the connection most often skipped because it gets worked from inside the room. Flex duct adds its own: connections at the collar that were never banded properly, and long runs that have been crushed or kinked behind something.
How do I know if my ducts are the problem and not my insulation?
The tells are different. Insulation problems show up as whole-house or whole-floor temperature drift and a bill that climbs with the weather. Duct problems show up as distribution failures: one room or one end of the house that never reaches setpoint no matter how the registers get balanced, the worst rooms sitting at the end of the longest run, registers that barely move air with the blower on high, and a system that runs more or less constantly. Visible evidence helps more than either — go look at the ducts in the attic or crawl space. Daylight at a joint, tape hanging off, or a boot pulled away from the drywall settles the question without any instrument.
My ducts are already wrapped. Do I still need them sealed?
Quite possibly, and the wrap is the reason nobody has checked. Insulation on a duct does nothing about air leaving through a joint — it slows heat moving through the duct wall, which is a different mechanism entirely. A duct that was wrapped without being sealed first is a leaking duct with a blanket over the leak, and from below it looks finished. The honest answer is that the wrap has to come back at the connections to find out. That is real labor and it belongs on the quote as its own line, which is also why sealing before wrapping is so much cheaper than doing it in the other order.
Is this the same thing as air sealing my house?
No, and this is the distinction worth holding onto. Air sealing works on the building envelope — the attic plane, the rim joists, the top plates, the doors and windows, the penetrations through the boundary between conditioned and unconditioned space. Ducts are a separate pressure boundary that runs through that one: a sealed channel crossing the envelope, pushing air out on the supply side and pulling air in on the return side, entirely independent of how tight the attic plane is. You can have a well-sealed envelope and a duct system dumping a share of every heating cycle into a vented attic. The air sealing guide covers the envelope scope; this page covers the ducts, and a serious retrofit usually wants both.
Can sealing my ducts affect my furnace or water heater?
It can, and that is why the check comes before and after rather than never. Changing duct pressures changes the pressures in the space around a gas appliance, and an appliance venting up an atmospheric-draft flue — an older furnace, many water heaters, some fireplaces — is sensitive to that. Draft and carbon monoxide get checked before duct pressures are altered and again once the work is done. It is the same combustion-safety check a competent operator performs under depressurization during a blower door test, for the same reason. A crew that seals a duct system without ever looking at the flue is telling you something about the rest of the work.
Should I seal the ducts before or after I replace the furnace?
Before, and this is the sequencing question worth getting right. Equipment gets sized by load calculation against the envelope it has to serve rather than by a square-footage rule of thumb, and a leaky, uninsulated duct system crossing a vented attic is part of what that calculation has to carry. Seal and insulate first and the new equipment gets sized against the house you will actually have. Do it in reverse and you have bought capacity you no longer need, in a box you will own for as long as that equipment lasts. There is a rebate reason too: the Xcel $600 Insulation + Air Sealing Combo Bonus applies when qualifying insulation and air sealing precede a qualifying space-heating heat pump, installed and invoiced by December 31, 2026.
Does duct sealing qualify for Xcel rebates?
Not as a measure of its own, and you should be skeptical of anyone who says otherwise. The Xcel Energy Insulation and Air Sealing Rebate pays out by check against per-measure caps, and its named measures are insulation and air sealing — duct sealing is not one of them standing alone. Where duct work counts is in the company it keeps: the Whole Home Efficiency Bonus adds 25% of the rebate already paid when three qualifying measures are completed within two years of enrolling, and duct sealing is commonly scoped on the same project as the insulation and air sealing that do qualify. Rebate-qualifying work has to be completed by a participating contractor, and the blower door test that goes with it measures the whole house before and after, not the duct run. Ask your contractor to confirm what your specific scope files under before the work starts.