Definition
What is radiant barrier, and how does it work?
Radiant barrier is a reflective, foil-faced material — usually a paper, plastic film, or cardboard backing laminated with aluminum foil — installed in an attic to reflect radiant heat rather than slow it down. It's most often stapled to the underside of the roof rafters, foil side facing the airspace below, though it can also be draped over the attic floor in some installations.
This is a genuinely different category of product from the mass insulation (fiberglass, cellulose, spray foam) covered on the rest of this site. Mass insulation works by slowing conductive heat flow through its thickness and density — how much material, and how well it resists heat moving through it. Radiant barrier works by reflecting radiant heat energy across an open airspace, a different physical mechanism entirely, and it doesn't add meaningful R-value (thermal resistance) on its own.
The two aren't mutually exclusive — a radiant barrier can be added in an attic that already has adequate mass insulation on the floor — but they aren't interchangeable, and one doesn't substitute for the other.
Qualification signals
Who needs radiant barrier in Longmont?
Radiant barrier's clearest use case is a hot attic in a cooling-dominated climate, where reflecting summer radiant heat before it reaches the attic floor reduces air-conditioning load. That's a more modest benefit in Longmont's heating-dominated Climate Zone 5B than it would be somewhere with a longer, hotter cooling season.
If your attic's mass insulation is already at or near the R-49/R-60 target and you still want to address summer attic heat, radiant barrier is a reasonable add-on to discuss. If your attic insulation is under-target, that's very likely the better first dollar spent — most Longmont homes get more value from closing that gap before considering a reflective layer on top of it.
Cost & the cost of waiting
How much does radiant barrier cost in Longmont — and what does waiting cost?
Radiant barrier installation in an existing attic typically runs $1.00 to $1.80 per square foot of attic area, reflecting the labor of stapling and overlapping foil-faced sheets to the rafters (or laying it across the attic floor) compared to blowing loose-fill material.
There's no rebate figure to net against that cost. Neither Xcel's nor Efficiency Works' published measure lists itemize radiant barrier — it isn't named as excluded the way garages are, it simply isn't on either list. Price this project as a full out-of-pocket cost, and treat any rebate-adjusted comparison against mass insulation as apples to oranges.
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 radiant barrier 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) — figures that describe a heating-dominated climate, not a cooling-dominated one. The model 2021 IECC's R-49 to R-60 attic target for Climate Zone 5 is a mass-insulation figure aimed at retaining heat through a long, cold winter, not a reflective-barrier figure aimed at deflecting summer sun.
Radiant barrier's core benefit — reducing radiant heat gain into a hot attic — is more closely tied to cooling season than heating season. That doesn't make it worthless in Longmont, but it does mean the honest framing is: most Longmont attics get more value from reaching the R-49/R-60 mass-insulation target first, since neither Xcel's nor Efficiency Works' rebate programs itemize radiant barrier at all, while both do reward mass insulation work.
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 radiant barrier process work?
Attic inspection
Crew checks existing mass insulation levels, roof framing condition, and attic access before scoping a radiant barrier install.
Rafter prep
Rafters are cleared of debris and checked for adequate nailing surface where the material will be stapled.
Foil-faced material installation
Reflective foil-faced sheets are stapled to the underside of the roof rafters, foil side facing down into the attic airspace, working from one end of the attic to the other.
Overlap and seam sealing
Sheets are overlapped at the edges and seams are taped to maintain a continuous reflective surface without gaps.
Airspace and ventilation check
Crew confirms the required airspace between the foil and the roof deck is maintained, and that soffit-to-ridge ventilation isn't blocked by the install.
Final walkthrough
Crew reviews completed coverage with the homeowner — there is no rebate paperwork step on this project, since neither program itemizes radiant barrier.
Rebates & credits
What rebates apply to radiant barrier in Longmont?
Radiant barrier doesn't carry a rebate figure from either program. Neither Xcel's nor Efficiency Works' published measure lists itemize it — a different situation from garages, which Xcel excludes by name. This one is simply absent from both lists, not stated as disqualified. Mass insulation brought up to the R-49/R-60 attic target remains the measure both programs actually reward.
Radiant barrier is not itemized by either program:
- Xcel Energy — its insulation and air-sealing rebate summary doesn't name radiant barrier among its measures.
- Efficiency Works — its measure list (crawl space, basement wall, cantilever floor, exterior wall, cathedral ceiling, conditioned attic, flat-ceiling attic) doesn't name it either.
This is a different situation from garages, which Xcel excludes by name — radiant barrier is simply not itemized by either program's published measure list. Mass insulation to the R-49/R-60 attic target is the measure both programs actually pay for.
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 ENERGY STAR, “Climate Zone 5 homes need attic insulation rated R-49 to R-60 for optimal performance.”
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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 radiant barrier?
Should I do radiant barrier 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 radiant barrier 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.
Does radiant barrier qualify for a rebate?
Not itemized by either program. Neither Xcel's nor Efficiency Works' published measure lists name radiant barrier — it's simply absent from both, which is different from garages, which Xcel excludes by name outright. Either way, don't expect a rebate figure on a radiant barrier line item.
Is radiant barrier worth it in Longmont's climate?
It's a more modest benefit here than the marketing around it often suggests. Radiant barrier's main job is reducing radiant heat gain in a hot attic, which matters most in cooling-dominated climates. Longmont sits in IECC Climate Zone 5B, a heating-dominated climate with a winter design temperature around −2°F — for most under-insulated Longmont attics, reaching the R-49/R-60 mass-insulation target first delivers more benefit than adding a reflective layer.
What's the difference between radiant barrier and regular attic insulation?
Different mechanism entirely. Mass insulation (fiberglass, cellulose, spray foam) slows conductive heat flow through its thickness and density — more material generally means more resistance. Radiant barrier reflects radiant heat across an open airspace and doesn't add meaningful R-value on its own. They address different physics, and radiant barrier isn't a substitute for reaching your attic's R-49/R-60 mass-insulation target.
Can I add radiant barrier on top of my existing attic insulation?
Yes, the two aren't mutually exclusive — radiant barrier is typically installed at the rafters while mass insulation stays on the attic floor. It makes more sense as an add-on once your attic's mass insulation already meets the R-49/R-60 target than as a first move on an under-insulated attic.
How much does radiant barrier installation cost?
Typically $1.00 to $1.80 per square foot of attic area, reflecting the labor of stapling and sealing foil-faced sheets to the rafters. No rebate applies to bring that number down, since neither program itemizes this measure. The free in-home estimate gives an exact figure for your attic.