Cellulose vs. Fiberglass in a Colorado Attic: What Actually Matters at R-60

Here is the fastest way to lose R-value on an attic job: blow cellulose on top of existing fiberglass.

Cellulose is denser. Put it over loose fiberglass and it presses that fiberglass down, and compressed fiberglass insulates worse than fiberglass at its intended depth. You end up paying for added inches while quietly giving back some of what was already up there. The homeowner sees a deeper attic. The assembly performs worse than the math on the invoice says it should.

That single mechanic drives most of how we decide between the two materials in Boulder County, and it is almost never the thing homeowners find when they search “cellulose vs fiberglass.” So let’s start there and work outward.

We install both. The existing insulation usually decides which one.

We run cellulose and we run blown fiberglass. Some homeowners have a strong preference and that is fine, both are legitimate products that will hit code and last decades.

But on a top-off, where we are adding inches to an attic that already has insulation, we match like for like. Fiberglass over fiberglass. Cellulose over cellulose. The reason is not brand loyalty or what is on the truck that morning. It is that matching materials is the only way to know what R-value you actually end up with. Mix densities and you are guessing, because you no longer know what the bottom layer is doing under the weight of the top one.

When a homeowner wants to switch materials, that is a real conversation, and the honest answer is that switching means removing what is there first.

The Boulder County code numbers, and why your address decides

City of Boulder is R-60. Boulder County is R-49. If you are domiciled inside city limits, the city number governs, it trumps the county.

The broader trend is one direction. Colorado is moving to R-60 across the board, and a good chunk of the state is already there. If you are insulating in 2026 and your jurisdiction still allows R-49, going to R-60 is the decision that ages well. You are not going to look back and wish you had less.

What R-60 means in inches:

• Loose-fill fiberglass: roughly 20 inches. It should not settle meaningfully. Depth at install is depth in ten years.
• Cellulose: about 19.7 inches at install. It will settle one to two inches over time. That is expected and designed for.

Those two lines look almost identical and they are not. Which brings us to the objection everybody arrives with.

Cellulose settles. That is not the problem. Search cellulose insulation and you will find people warning you it settles, as though that were a defect. It settles because it is engineered to, and the manufacturer publishes exactly how much.

Every cellulose product has a spec sheet with two depths: installed depth and settled depth. The installed number is deeper. The settled number is where it lands, and the R-value is rated at the settled state. Installed correctly, you get your R-60 after settling, not before.

The actual risk is a contractor who installs to the settled depth. Blow it in at 19.7 inches and you finish at R-60. Blow it in at the settled depth instead and it settles from there, and now you are under code with no way to tell by looking.

So the question to ask any contractor bidding cellulose is simple, and you should ask it in writing: what depth are you installing to, and can I see the manufacturer’s coverage chart showing the settled depth and rated R-value? A contractor who installs cellulose regularly will have that table on hand in about fifteen seconds. If the answer is vague, that tells you something.

Where cellulose has a real edge

Pests. Cellulose is treated with borates, and it holds up better against rodents and insects than loose fiberglass does. In attics where we have seen activity, or in older housing stock where we expect it, that matters.

Sound. Cellulose is denser, so it dampens sound better. Being straight with you: in an attic specifically, the practical benefit is debatable. The assembly, drywall, and framing do most of the work, and the STC difference from insulation alone is smaller than marketing suggests. If sound is your main goal, an attic top-off is not the lever.

On the environmental question, both materials are largely recycled at this point. Cellulose is recycled newsprint treated for fire and pest resistance. Fiberglass is spun from sand and recycled glass cullet, think bottle, window and automotive glass, which runs somewhere in the 20 to 50 percent range of raw input depending on the manufacturer. Neither is the obvious green choice over the other, whatever the brochures say.

Air sealing comes first, and here is the honest version

We seal top plates, wire and pipe penetrations, around can lights, HVAC punctures, bathroom exhaust vents, and open chases.

The reason it goes first is simple physics: insulation slows heat moving through a surface. It does nothing about air moving through a hole. Blow 20 inches over an unsealed lid and warm indoor air keeps leaking straight past it, carrying moisture into a cold attic. You paid for R-60 and you are not getting R-60 performance, because the assembly is still leaking.

Air sealing is cheap. Call it $0.15 to $0.25 per square foot. Against the cost of the insulation itself, it barely registers. The impact is disproportionate to the price, which makes it the single best-value line item on most attic estimates.

Now the part most contractors will not tell you.

You cannot fully air seal an attic that still has insulation in it. The only way to seal 100 percent of the penetrations is to work off a blank deck, which means removing the existing insulation first. If a contractor guarantees you complete air sealing as part of a top-off, over insulation they are not removing, they are telling you something that is not true. They cannot see what they have not exposed.

When we sell air sealing as part of a top-off, we describe it as a best-effort pass, and the honest ceiling is around 50 percent of the leakage addressed. We would rather say that on the estimate than have you find out later. If complete air sealing is what you want, the scope is removal and replace, and we will price it that way.

What we actually find in Boulder County attics

Existing depths cluster into two groups, and it usually maps to whether anyone has touched the attic since the house was built.

Four to six inches. Older homes where insulation was never addressed. This is well under any current code, and often less than half of R-49. These attics are also where we most often find the conditions that force a removal rather than a top-off.

Ten to twelve inches, sometimes up to fifteen. Newer construction, built to a more recent code cycle. Still short of R-60, but a straightforward top-off gets there.

If your attic is in the first group, the good news is that the improvement is dramatic. Going from R-15 to R-60 is a different experience of your house in January than going from R-40 to R-60. The people who notice the biggest change are almost always the ones who started with the least.

When it has to come out

Not every attic is a candidate for a top-off. What sends a job to full removal:

• Existing depth is very low. If there is only a few inches of degraded material, removing it and starting clean is often the better value than burying it.
• Compaction. Insulation that has been crushed, walked on, or stored over is no longer performing at its rated R-value, and adding to it does not fix the layer underneath.
• Uneven or lumpy coverage. You cannot reliably hit a target R-value across an attic when the base layer varies wildly. Depth becomes unverifiable.
• Contamination. Rodent or pest activity. This one is not negotiable, you do not insulate over it.
• Moisture damage. Wet or previously wet insulation means there is a water problem, and that gets solved before anything gets covered up.

Two other conditions warrant their own conversation: vermiculite in older attics, which carries asbestos risk and requires specific handling, and heavily settled cellulose from decades ago that is no longer at spec.

If any of these describe your attic, the question stops being cellulose versus fiberglass and becomes removal versus top-off. That is the bigger decision and it should be made first.

Floor temperatures swinging. Floors that read cold in ways that don’t match the thermostat. Usually a sign the insulation between you and the crawl space has failed or fallen.

If you’ve got the musty smell plus cold floors plus allergy complaints that don’t line up with the season, the crawl space is where to look.

What to ask on your next estimate

What R-value are we targeting, and does my address fall under city or county code?

What is the existing depth and material, and are you matching it or removing it?

If cellulose: what is the installed depth, and can I see the settled-depth spec sheet?

Is air sealing on this estimate, what specifically is being sealed, and is it a full or best-effort pass?

If it is a top-off, what would push this job to removal instead?

Any contractor worth hiring can answer all five without hedging.