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?
Whole-home professional air sealing in Longmont typically runs $400 to $2,000, depending on home size and existing leakage — the fixed items (top plates, can lights, bath-fan housings, the attic hatch) hit smaller homes proportionally harder than the square-footage range suggests. Blower-door-guided air sealing, with a documented before-and-after test, adds a diagnostic fee that also unlocks the rebate paperwork for both programs on this page.
Both this property's rebate programs reduce net cost, but they are NOT the same kind of number and this page will not add them for you. Xcel's rebate is a percentage of project cost, capped at $400 for air sealing. Efficiency Works' rebate is a flat amount, tiered by measured leakage reduction. Whether a single project can claim both is not settled by anything either program has published — 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. Longmont Power & Communications, the city's own municipal electric utility, doesn't cover the whole city — verified against Colorado's official 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 International Energy Conservation Code, “the 2021 IECC (R402.1.2) sets attic insulation minimums at R-49 to R-60 for Climate Zone 5.”
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), 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, non-combined programs, and it's the one measure where the two programs' metrics genuinely differ. Xcel Energy's citywide rebate pays 30% of project cost for gas-heat customers, up to $400, requiring a documented 20% reduction in CFM50. 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 — 30% of project cost for gas-heat customers, up to $400 for air sealing, with a 1.5x multiplier when the heating fuel is Xcel gas. Requires a participating contractor and a blower door test documenting at least a 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: more than 15% reduction pays $310, more than 25% pays $460, more than 33% pays $620, and more than 50% pays $770. Efficiency Works' 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. Nothing published by either states that a single project can combine both incentive payments — confirm which applies to your address before assuming a combined figure.
According to Xcel Energy, “Xcel's insulation and air sealing rebates require a documented reduction of at least 20% in CFM50 — cubic feet per minute at 50 pascals — verified by a blower door test before and after the work.”
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 — more than 15% reduction pays $310, more than 25% pays $460, more than 33% pays $620, and more than 50% pays $770.”
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.
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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 — the model energy code for Climate Zone 5 calls for R-49 to R-60 attic insulation; ask what your contractor is quoting to. (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?
Not confirmed. Xcel'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' incentive sheet says its tiers are based on "reduction of air loss from initial testing with blower door" but does not name a specific unit. Same test equipment, possibly different measurement convention — we're not going to assume they match, and you shouldn't either when comparing the two payouts.
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. More than 15% reduction pays $310, more than 25% pays $460, more than 33% pays $620, and more than 50% pays $770. 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?
Whole-home professional air sealing typically runs $400 to $2,000 before rebates, depending on home size and existing leakage. The free in-home estimate gives exact numbers, not a range.
Is a blower door test required, or optional?
Required for the rebate on this page — both Xcel and Efficiency Works base eligibility and payment amount on a documented before-and-after blower door measurement, not a visual estimate. A contractor doing air sealing without a blower door test can't document the reduction either program requires.