Definition
What is air sealing, and how does it work?
Air sealing is the work of finding and closing the gaps, cracks, and penetrations that let conditioned indoor air leak into an attic, wall cavity, or crawl space — and let outside air leak in. In a typical pre-1990 Longmont attic, the biggest leak paths are usually top plates (where interior walls meet the ceiling), recessed can lights, bath fan housings, plumbing and wiring chases, and the attic-access hatch perimeter.
The work itself uses caulk for small gaps, low-expansion spray foam for irregular openings, and rigid foam board or mastic-sealed boxes around larger penetrations like can lights and bath fans. A blower door — a calibrated fan mounted in an exterior door — depressurizes the house to measure total leakage before and after the work, giving a documented before/after number rather than a visual guess.
Air sealing has to happen before insulation goes in, not after — insulation buried over an unsealed leak path doesn't stop the air movement, it just hides it.
Qualification signals
Who needs air sealing in Longmont?
Almost every pre-1990 Longmont home benefits from air sealing, and it's frequently skipped or done superficially in past insulation jobs — a fast top-up of loose-fill over an unsealed ceiling plane does very little for comfort or bills compared to sealing first.
The clearest signals: rooms that are drafty near outlets or baseboards, ice damming in winter (a sign of warm air leaking into the attic and melting roof snow from below), or an insulation contractor who never mentions a blower door test as part of the scope.
Cost & the cost of waiting
How much does air sealing cost in Longmont — and what does waiting cost?
This site has no single sourced cost range for whole-home professional air sealing in Longmont — price depends heavily on home size and existing leakage, and the fixed items (top plates, can lights, bath-fan housings, the attic hatch) hit smaller homes proportionally harder than a square-footage estimate would suggest. Blower-door-guided air sealing, with a documented before-and-after test, adds a diagnostic fee. That same test is what each program on this page requires before it will pay on air sealing.
Both this property's rebate programs reduce net cost. Xcel's rebate is a percentage of project cost against a per-measure cap. Efficiency Works' rebate is a flat amount, tiered by measured leakage reduction. Whether a single project can claim both is unconfirmed: Efficiency Works' own table pairs an Xcel figure with its own and totals them, but that Xcel figure doesn't match any Xcel per-measure cap we've independently verified — ask your contractor to confirm for your specific address before assuming a combined figure.
Cost figures are conservative ranges. The free in-home estimate gives exact numbers based on your home and required pre-work — not a range.
Longmont context
What's different about air sealing in Longmont?
Longmont sits at roughly 4,980 feet in IECC Climate Zone 5B, with a winter design temperature around −2°F (−10°F for boiler sizing). A leaky ceiling plane loses more conditioned air per degree of temperature difference in a Longmont winter than the same leak would in a milder climate — air sealing has an outsized effect here for exactly that reason.
The rebate landscape is genuinely two-utility here. Xcel Energy runs the gas-and-electric rebate available to every Longmont address, requiring a documented 20% reduction in CFM50 (cubic feet per minute at 50 pascals) verified by a blower door test — unless a first reading already finds the home tight enough by Xcel Energy's own measure, which skips both the sealing requirement and the second test. Longmont Power & Communications, the city's own municipal electric utility, doesn't cover the whole city — verified against the Colorado Geospatial Portal's own utility-territory data, LPC serves roughly 61% of Longmont by area. Only LPC electric customers can access the separate Efficiency Works rebate, which pays a tiered flat amount based on measured reduction in air loss from its own blower door test — its published sheet doesn't state whether that's the same CFM50 measurement Xcel uses, so this page treats them as two separate, unconfirmed-to-match metrics rather than assuming they're identical.
According to the City of Longmont Building Services, “The City of Longmont has adopted the 2024 International Codes, along with the Metro Cohort Model Code and Colorado Wildfire Resiliency Code as of July 1st, 2026.”
Process
How does the air sealing process work?
Baseline blower door test
A calibrated fan mounted in an exterior door depressurizes the house, and a manometer measures the home's existing air leakage rate — the "before" number both rebate programs require.
Leak-path identification
With the house depressurized, the crew walks the attic and living space feeling for air movement at top plates, can lights, bath fan housings, and other suspect penetrations — the pressure differential makes leaks easier to find than at normal pressure.
Sealing the leak paths
Caulk for small gaps, low-expansion spray foam for irregular openings, and rigid foam or mastic-sealed boxes around larger penetrations like can lights and bath fans.
Attic-hatch and kneewall sealing
The attic-access hatch perimeter and any kneewalls (common in 1.5-story homes) get weatherstripped or foam-sealed — frequently the single largest leak path in a home that otherwise looks well-sealed.
Final blower door test
A second blower door test measures the after number, documenting the percentage reduction in leakage — the figure both Xcel and Efficiency Works use to determine rebate eligibility and, for Efficiency Works, which tier applies.
Rebate paperwork
Crew completes whichever program's forms apply to the address — Xcel's application (requiring the documented 20% CFM50 reduction, unless the baseline reading already found the home tight enough by Xcel Energy's own measure), and a separate Efficiency Works application if the home is LPC electric. Each program's paperwork is filed separately.
Rebates & credits
What rebates apply to air sealing in Longmont?
Air sealing in Longmont draws on two separate programs, and it's the one measure where the two programs' metrics genuinely differ. Xcel Energy's citywide rebate pays a percentage of project cost for gas-heat customers (ask your contractor for the current rate), requiring a documented 20% reduction in CFM50 — unless a first reading already finds the home tight enough by Xcel's own measure, in which case no sealing and no second reading are needed. Longmont Power & Communications electric customers may separately qualify for an Efficiency Works rebate, paid as a tiered flat amount based on measured reduction in air loss from its own blower door test — a metric Efficiency Works' own sheet does not name, so this page does not assume it's the same measurement as Xcel's CFM50.
Xcel Energy — applies regardless of electric utility:
- Xcel Energy Insulation and Air Sealing Rebate — requires a participating contractor and, unless a first blower door test already finds the home tight enough by Xcel Energy's own measure, a documented 20% reduction in CFM50 — cubic feet per minute at 50 pascals.
Efficiency Works — Longmont Power & Communications electric customers only:
- Efficiency Works Air Sealing Incentive — a tiered flat payment based on measured reduction of air loss from an initial blower door test, in four tiers: more than 15% reduction, more than 25%, more than 33%, and more than 50%. Efficiency Works publishes the amount paid at each tier on its own rebates page. Its own incentive sheet does not name the specific unit this reduction is measured in — this page does not assume it matches Xcel's CFM50 metric above. Requires an Efficiency Works assessment first and applies only to residential electric customers of Longmont Power & Communications.
These are separate programs from separate sources, measuring leakage reduction in ways that are not confirmed to be equivalent. Efficiency Works' own table pairs an Xcel incentive payment with its own and totals them, but that Xcel figure doesn't match any Xcel per-measure cap we've independently verified — confirm which applies to your address before assuming a combined figure.
According to Xcel Energy, its Insulation and Air Sealing product write-up requires a pre-improvement blower door test in CFM 50 — cubic feet per minute at 50 pascals — on any project that has not already had a recent home energy audit that included one; unless that test finds the home already tight enough by Xcel Energy's own measure, air sealing is required and a post-improvement test must show at least a 20% reduction in air leakage before attic or wall insulation can qualify for a rebate, and the insulation measures carry R-value conditions of their own beyond the blower-door result.
According to Efficiency Works (Platte River Power Authority program, for Longmont Power & Communications electric customers), Efficiency Works pays a tiered flat incentive for air sealing based on measured reduction of air loss from a blower door test, with four tiers set at more than 15%, more than 25%, more than 33%, and more than 50% reduction, and the amount paid at each tier published on its own incentive sheet.
For current Xcel rebate amounts and program rules, see the Xcel Energy insulation and air-sealing rebates page. For Efficiency Works eligibility and current incentive values, see Efficiency Works home rebates. Eligibility may depend on which electric utility serves your address, program funding levels, and qualifying product specifications.
Get a quote
Tell Us About Your Home
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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 — ask what R-value your contractor is quoting to, in writing, and confirm the figure your permit is reviewed against with the City of Longmont's building department. (2) Air sealing scope — insulation alone does nothing if air leaks aren't sealed first, and Xcel requires a blower door test to rebate it. (3) Rebate handling — ask which programs your specific address qualifies for. Xcel and Efficiency Works are separate programs with separate paperwork, and not every address qualifies for both. (The federal IRA Section 25C tax credit ended December 31, 2025 — no federal insulation credit applies to 2026 work. Xcel Energy's insulation and air-sealing rebate remains active for its gas and electric customers. Longmont Power & Communications electric customers may separately qualify for an Efficiency Works rebate — check your bill to see which utility serves your address. Boulder County EnergySmart pays a third, income-qualified-only rebate as of January 1, 2026.) (4) Removal scope — older homes often need old insulation removed before new install. If a quote skips all four, get another quote.
Frequently asked
What do Longmont homeowners ask about air sealing?
Should I do air sealing if my home was built after 2010?
Probably not yet. Post-2010 homes were built to recent code — most attics started well above the pre-1990 baseline. If your bills are normal and comfort is fine, hold the money. The 10-15 year mark is when settled batts and unsealed penetrations start showing up; that's when air sealing pays back on a newer home.
Do you handle new construction insulation?
We focus on retrofit insulation for existing homes. New construction insulation typically goes through your general contractor or builder, and pricing, code compliance, and timing all work differently for new builds. If you're working on new construction and need an insulation contractor, note that in the message field below and we'll refer you to a partner with new-construction experience.
Do Xcel and Efficiency Works measure air sealing the same way?
Confirmed, and the answer changed. Xcel Energy's rebate document states its metric explicitly: CFM50, cubic feet per minute of leakage at 50 pascals of pressure, verified by a blower door test. Efficiency Works' public incentive sheet names no unit at all, which is why this page used to say the two might not match — but its contractor-facing Homes Program Guide does name one, and it is the same one: each air-sealing tier is set as a minimum shell leakage reduction measured in CFM50. Same test, same unit. What differs between the two programs is the reduction each one requires, so compare the thresholds rather than assuming the measurements are incomparable.
What does Efficiency Works actually pay for air sealing?
For Longmont Power & Communications electric customers only: a tiered flat amount based on measured leakage reduction, in four tiers — more than 15% reduction, more than 25%, more than 33%, and more than 50%. Efficiency Works publishes the amount paid at each tier on its own rebates page; this site doesn't republish the figures, because they change and we can't monitor them for you. What matters for planning is the shape: it's a step function, not a percentage of cost — the payment doesn't change based on your project's price, only on the measured result.
Can I get air sealing done without full insulation?
Yes, air sealing is its own scope and can be done on its own. It's also often worth doing before insulation even if you're not ready for the full insulation project yet — sealing first means any insulation added later performs as intended instead of sitting over uncontrolled air leaks.
How much does air sealing cost for a typical Longmont home?
This site doesn't publish a cost range for whole-home professional air sealing — price depends heavily on home size and existing leakage. The free in-home estimate gives an exact number, not a range.
Is a blower door test required, or optional?
Required for the rebate on this page — both Xcel and Efficiency Works set eligibility on a measured leakage reduction rather than a visual estimate, and a blower door is what measures it. Xcel's product write-up asks for a reading before the work and, unless that first reading already finds the home tight enough by Xcel Energy's own measure, another after it showing at least a 20% reduction — and that reduction gates the attic and wall insulation rebates too, not the air sealing one alone. An air-sealing scope written without a blower door leaves that reduction undocumented, and both programs require it documented.