Your attic
How it works
How does this calculator work?
The target comes from the model 2021 IECC's Climate Zone 5 ceiling-insulation table: R-49 minimum, R-60 for a retrofit upgrade. Longmont sits in Climate Zone 5B. This is the model code's own published figure — this calculator does not claim it's the specific edition the City of Longmont currently enforces, since that hasn't been independently verified against the city's own adopted ordinance.
The gap calculation is simple subtraction against your selected current-level bucket. Because loose-fill cellulose settles 10-20% over its lifetime, a number you remember from years ago is often lower today — a measured depth check during the free in-home estimate gives you the real figure, not an estimate from memory.
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.”
According to the Building America Solution Center, “loose-fill cellulose will settle from 10 to 20 percent over time due to gravity and vibration.”
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30 seconds to fill out. Free quote, no high-pressure follow-up.
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
Frequently asked about r-value target finder
Why does this calculator only cover attics, not walls or crawl spaces?
Because that's what this site has a sourced Climate Zone 5 target for. The model 2021 IECC table this calculator cites states the attic figure directly (R-49 to R-60); a wall, crawl space, or floor target would need its own sourced figure, which this property doesn't have verified yet. Rather than guess, we're leaving those out until we do.
Is R-49 or R-60 the real target for my Longmont home?
R-49 is the model code's stated minimum for Climate Zone 5; R-60 is the recommended retrofit target, which leaves headroom because loose-fill insulation settles over its lifetime. Most contractors quote to R-60 for exactly that reason — starting higher means you're still near R-49 well into the future.
Does the calculator account for insulation settling?
Not automatically — it works from the current-level bucket you select. If you're estimating from what was installed years ago rather than a fresh measurement, keep in mind loose-fill cellulose settles 10-20% over time, so your actual current level may be lower than what was originally installed.
What if I don't know my current attic R-value at all?
Select "not sure" — the calculator will show you the target without guessing at a gap. A measured depth check during the free in-home estimate gives you the real number.
Does hitting R-49 or R-60 guarantee a rebate?
No — this calculator only addresses the R-value target, not rebate eligibility. Xcel's and Efficiency Works' rebates have their own separate requirements (a blower door test, a participating contractor, and more) beyond hitting a target R-value. See the attic insulation service page for the rebate details.
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