Which Type of Insulation Is Best for the Attic?

Colorado attics lose heat fast. Between the altitude, the temperature swings, and roofs built for a Front Range climate, an under-insulated attic is one of the quickest ways to watch a heating bill climb every winter.

About 9 out of 10 American homes are under-insulated, and if yours was built before your neighborhood’s current energy code, the odds are good that it is.

The fix starts with knowing what “enough” actually looks like. Denver’s residential energy code sets a minimum of R-60 for ceiling and attic insulation, and mountain communities in colder climates can require even more. If your insulation sits level with or below your floor joists, you’re almost certainly under that mark, no matter what material is up there.

Once you know you need more, the real question is which material. The main attic insulation types are fiberglass, cellulose, spray foam, and mineral wool. Each performs differently, costs differently, and suits different attic spaces.

Contact REenergizeCO to schedule a home energy audit and find out exactly what your attic needs before the cold arrives.

What Type of Insulation Does Your Attic Actually Need?

No single insulation type is best for every attic. The right choice depends on four factors: your home’s current energy-efficiency needs, the condition of your existing insulation, how much more insulation you need to add, and your budget. Start with those four, and the material question gets easier to answer.

The first number to know is R-60. Denver’s residential energy code sets R-60 as the minimum ceiling and attic R-value for new construction and major renovations. An attic below that mark means warm air escapes in winter, cold air bleeds out in summer, and your HVAC system runs longer than it should.

Two quick self-checks tell you whether you’re likely under that mark.

1. First, look at your attic floor joists. If your insulation sits level with or below those joists, you almost certainly need more.
2. Second, notice whether your heating or cooling system runs longer than it used to, or whether some rooms never quite reach the right temperature.

Both point to air leaks and insulation gaps working against you.

Once you’ve confirmed you need more, the R-value per inch of each material tells you how many inches it takes to reach R-60. Here’s a quick reference across all four types:

• Fiberglass batts: R-2.9 to R-3.8 per inch (high-density batts reach R-4.3)
• Blown-in fiberglass: R-2.2 to R-2.7 per inch
• Cellulose insulation (blown-in or dense-pack): R-2.9 to R-3.4 per inch
• Mineral wool: approximately R-3.7 per inch
• Closed-cell spray foam: R-7 to R-9 per inch

These figures come from DOE insulation guidance. R-value depends on material type, thickness, and density, so a professional measurement matters more than a visual estimate.

One more standard worth knowing: RESNET Grade 1 is the top installation-quality rating and requires uniform fill with no gaps, voids, or compression. Even the right material at the right depth underperforms if you don’t install it properly.

Figure out how far under Denver’s R-60 minimum your attic space sits, then match the right insulation material to your budget and attic access.

Fiberglass Insulation Cost, R-Value, and Trade-Offs

Fiberglass is the most widely used attic insulation material in the country, and for most Denver homeowners doing a straightforward top-off, it’s the starting point for any comparison.

It comes in three forms. Fiberglass batts and rolls are pre-cut panels that fit between standard joist spacing. Loose-fill fiberglass is blown in, filling around obstructions and settling into irregular attic spaces without cutting, making it a strong choice when adding insulation over existing material.

Advantages:

• Cost-effective upfront compared to spray foam and mineral wool.
• Excellent fire and moisture resistance; resists mold and pests.
• Standard-density batts run R-2.9 to R-3.8 per inch; high-density batts reach R-3.7 to R-4.3 per inch, per DOE insulation data.
• Blown-in loose-fill insulation conforms to the existing attic floor without demolition.
• Does not settle significantly when installed properly.

Disadvantages:

• Blown-in fiberglass runs only R-2.2 to R-2.7 per inch, the lowest R-value per inch of the four materials here. Reaching R-60 requires more depth.
• Many blowing machines over-fluff fiberglass during installation, reducing density and actual R-value if the installer doesn’t monitor it.
• Fiberglass does not air-seal on its own. Address air leaks separately before adding more insulation.

Installation quality is the deciding factor. RESNET Grade 1 requires fiberglass batts to fully fill each cavity side to side and top to bottom, cut to fit around wiring and piping, with faced batts stapled neatly without buckling. A compressed batt or one with gaps around a wire loses a measurable portion of its rated R-value.

Fiberglass is the budget-friendly baseline, but it has the lowest R-value per inch of the four. You’ll need more depth to hit R-60, and a proper Grade 1 install matters as much as the material itself.

Cellulose Insulation Cost, R-Value, and Trade-Offs

Cellulose is the other common blown-in option for attic floors. It edges out fiberglass on R-value per inch and carries a sustainability credential that matters to many Front Range homeowners.

Cellulose contains at least 70% recycled paper content and is treated with fire retardant before installation. Blown-in cellulose is a loose-fill insulation option common across Colorado homes built in the last few decades, and it fills under-insulated attic spaces efficiently.

Advantages:

• Blown-in cellulose runs R-2.9 to R-3.4 per inch; dense-pack runs R-3.0 to R-3.4 per inch, a meaningful step up from blown-in fiberglass.
• Made from at least 70% recycled paper, making it one of the more energy-efficient insulation options for homeowners focused on sustainability.
• Inexpensive, with material costs comparable to fiberglass.
• Fills irregular attic spaces and around obstructions well when blown in properly.

Disadvantages:

• Accidental wetting or high humidity can cause cellulose to absorb large quantities of water, reducing thermal resistance, draining the fire retardant, and creating a real risk of mold damage.
• Keep it away from chimneys and metal flues; it can be a fire hazard near those heat sources if not installed correctly.
• Loses some R-value over its lifespan as it settles.

Installation method matters. Blown cellulose is installed by covering the open area with netting, cutting a slit for the hose, and filling until the cavity is fully covered with no voids.

One important pre-check: wet or damp existing insulation and moldy or rotted attic framing are signs to address before adding new cellulose. Adding blown-in insulation over a moisture problem makes it worse.

Cellulose edges out fiberglass on R-value per inch, but it carries a real moisture-absorption risk if it ever gets wet. A dry, well-vented attic matters as much as the material choice.

Spray Foam and Mineral Wool Cost, R-Value, and Trade-Offs

Spray foam is the highest-performance option on this list, and also the most expensive. Mineral wool is the fourth material an audit might call for, particularly when fire resistance or sound dampening is a priority alongside thermal performance.

Closed-Cell and Open-Cell Spray Foam

Foam insulation starts as a liquid and expands on contact, hardening into a rigid or semi-rigid layer that fills every gap. Spray foam fills spaces around obstructions and provides both air sealing and insulation in one application, a real advantage in an attic with complex framing or significant air leakage.

Closed-cell spray foam runs R-7 to R-9 per inch. Closed-cell spray foam resists moisture, acts as an air barrier, and also functions as a vapor barrier, eliminating separate air-sealing work in most applications. It hits R-60 in fewer inches than any other material here.

Open-cell spray foam runs R-3.5 to R-3.8 per inch. Open-cell foam is an effective air barrier but not a vapor retarder. Both types cost significantly more than fiberglass or cellulose and require professional installation. Closed-cell spray foam is also the better choice for cathedral ceilings and roof decks with limited depth.

Spray foam advantages:

• Highest R-value per inch of any common attic insulation material (closed-cell).
• Handles air sealing and insulation in one application, reducing air infiltration through the attic envelope.
• Closed-cell foam resists moisture and acts as a vapor barrier.
• Excellent long-term durability with minimal settling.

Spray foam disadvantages:

• Higher upfront cost than fiberglass, cellulose, or mineral wool.
• Potential for over-insulation if a professional doesn’t size it correctly.

For a full cost breakdown, REenergizeCO’s guide on spray foam insulation pricing covers what to expect per square foot across different attic configurations.

Mineral Wool

Mineral wool insulation has an R-value of about R-3.7 per inch and is available as batts, loose-fill, or sprayed-on. It’s non-combustible, making it a strong choice when fire resistance is a priority. It also provides excellent sound insulation, useful in attic spaces above living areas.

Mineral wool is listed among ENERGY STAR-certified attic-eligible insulation materials. It absorbs water and loses R-value if compressed, so installation quality and attic moisture control matter here just as they do with cellulose.

Closed-cell spray foam hits R-60 in the fewest inches and air-seals as it insulates. Expect to pay more and to bring in a professional for the work.

How REenergizeCO Helps You Pick and Install the Right Insulation

The self-check from the first section tells you whether you’re likely under-insulated. A REenergizeCO home energy audit tells you exactly how far under you are, what’s causing it, and which material fits your attic and your budget.

The audit starts with a thickness assessment across the attic floor joists, walls, and baffles. Thermal imaging goes further, showing where heat actually escapes through gaps and thin spots a visual check misses.

The audit looks for two concrete signals: visible floor joists with little insulation above them, and ice dams forming along the roofline. A warm, poorly insulated, and poorly air-sealed attic lets roof snow melt and refreeze at the eaves, which is both a comfort problem and a potential roof-damage issue.

Before adding new insulation, address moisture first. Wet or damp insulation and moldy or rotted attic rafters or floor joists are signs to bring in a professional before proceeding. Adding new material over a moisture problem traps it. The audit catches this before it becomes a more expensive fix.

From there, REenergizeCO walks you through three options depending on what the audit finds: retrofit with blown-in fiberglass or cellulose to top off existing insulation, replace old insulation that’s damaged or settled below useful levels, or invest in closed-cell spray foam insulation for the highest performance and air sealing in one step.

Skipping this step has real consequences. An under-insulated attic drives up energy bills, forces your HVAC system to run longer, and creates the temperature inconsistencies that make a Denver home uncomfortable from October through March. Proper insulation also reduces the load on air ducts, which means your equipment doesn’t have to compensate for heat escaping through the ceiling.

Attic Insulation Isn’t One-Size-Fits-All

Fiberglass is the accessible, budget-friendly baseline. Cellulose edges it out on R-value per inch and brings a recycled-content advantage. Closed-cell spray foam delivers the highest thermal performance and handles air sealing in the same step. Mineral wool earns its place when fire resistance or sound control matters.

All four share the same starting point: know your current R-value, compare it to Denver’s R-60 minimum, and address any moisture issues before adding new material. Installation quality matters as much as material choice.

Denver’s first cold snap typically arrives in early to mid-October. If your attic is under-insulated, September is the practical window to schedule an audit, select a material, and complete the work before heating season starts.

Proper attic insulation also lays the foundation for larger energy-efficiency upgrades: a well-sealed, well-insulated attic means any new HVAC equipment or heat pump is sized correctly from the start, rather than compensating for heat loss the insulation should have stopped.

Contact REenergizeCO to schedule a home energy audit and find out exactly what your attic needs before the cold arrives.