What it actually is
Cellulose insulation is recycled paper fiber — around 85% post-consumer newsprint — ground down and treated with borate mineral salts. The borates do two jobs: they give the material a Class I fire rating, and they make it unappealing to insects and rodents.
It arrives compressed in bales, goes through a machine that fluffs and meters it, and gets blown through a hose into your attic. Nothing about it is exotic. It has been installed in American homes for decades.
The numbers that matter
About R-3.1 per inch, settled. Manufacturer sheets quote a more optimistic 3.2 to 3.8, but 3.1 is what we design to because it is what we actually measure once the material has settled. To hit the R-49 that Climate Zone 5 calls for, you are looking at 16 inches of settled material — less if you are topping up over existing insulation that is still doing its job.
Density is the real argument. Cellulose installs at roughly 1.5 to 1.6 pounds per cubic foot on an attic floor, noticeably denser than loose-fill fiberglass. That density is why it fills tight around wiring, junction boxes and irregular framing rather than bridging over them and leaving a void. Voids are where attics lose their rated performance.
Where it beats fiberglass, honestly
The advantage that matters in Michigan shows up on the coldest nights. When the temperature difference across a loose, low-density insulation layer gets extreme, air can begin to move within the insulation itself — warm air rising through it, cold air sinking, a slow convective loop that carries heat past the material rather than through it. Low-density loose-fill fiberglass is more susceptible to this than denser cellulose.
The practical translation: two attics rated at the same R-value can perform differently at 5°F. The denser one holds up better. In a climate where the coldest week of the year is exactly when you care most, that is not an academic distinction.
Cellulose also does a better job around obstructions. An attic floor is not a clean rectangle — it has wires, boxes, bracing, and framing that was nailed together by somebody in 1954 who was not thinking about your energy bill. A dense blown material conforms to that. Batts have to be cut around it, and usually are not cut well.
Where fiberglass is the right call
We are not material partisans, and it would be easy to write this page as though cellulose wins everywhere. It does not.
- Open new-construction walls. Batts in clean, accessible stud bays are fast, cheap and perfectly effective. See new construction work.
- Weight-sensitive ceilings. Cellulose is heavier. On an old plaster ceiling in questionable condition, that occasionally matters and needs a look before we load it up.
- Where a homeowner has a strong preference. It is your house. If you want fiberglass and it will perform adequately for your situation, we will install fiberglass and not make it weird.
What good installation looks like
The material is roughly half the outcome. The rest is the crew.
Settled depth, not initial depth. Cellulose fluffs up coming out of the hose and settles over the following weeks. Installing to 15 inches on day one and calling it R-49 is how you end up with R-42 by spring. We install past the target so the settled result is what you paid for.
Depth markers. We staple ruler cards to the trusses. You can go up with a flashlight in two years and verify what is there. Very few installers do this, and it is the cheapest accountability there is.
Baffles at the eaves. Blowing deep insulation toward the edge of an attic without baffles buries the soffit vents. The attic stops breathing, moisture builds under the roof deck, and you have traded a heating problem for a rot problem.
Air sealing first. Covered in detail on the attic insulation page, and it is the step that separates a job that works from a job that looks finished.
The environmental note, without the sermon
Cellulose is made from paper that would otherwise be waste, and it takes substantially less energy to manufacture than fiberglass or foam. If that matters to you, it is a genuine point in its favor. If it does not, the performance argument above stands on its own — we mention it because it is true, not because it is the reason to buy.