Cellulose vs Fiberglass vs Spray Foam: Best Attic Insulation for Michigan Winters

Every attic insulation quote in Macomb County eventually lands on the same three-way choice: blown cellulose, blown or batt fiberglass, or spray foam. Most of what gets written about them compares price per square foot and stops there, as if a material's only job is filling a joist bay. It has a second job that matters just as much through a Michigan winter: managing the air and moisture moving through that joist bay, because that is what actually decides whether an attic runs cold and dry or warm and wet. This guide stays on the material science, how each option handles air movement, moisture, settling, and its own R-value in genuinely cold temperatures, so you can read a bid sheet with more context than the sheet itself gives you. Installation methods and pricing are their own conversation, and we keep the cost differences here qualitative on purpose.

01 / THE SHORT ANSWERThree materials, one winter job

All three materials slow heat loss through the attic floor or roofline. Where they diverge is in how much they also resist air movement and moisture, and that divergence is exactly what a Michigan winter tests. Loose-fill cellulose is dense enough to meaningfully slow air movement through the fill itself, though it is not a true air barrier at gaps and penetrations. Loose-fill or batt fiberglass is lighter and more open, which lets more air pass through it, especially in deep cold when convective currents inside the material pick up. Closed-cell spray foam is the outlier: it is applied wet, expands, and cures into a continuous air barrier as well as an insulator, which is why it costs more per square foot installed than either loose-fill option.

MATERIAL	AIR MOVEMENT	MOISTURE BEHAVIOR	TYPICAL USE
Blown cellulose	Dense fill slows air within the material; gaps still leak	Absorbent; can hold moisture if wetted, dries slower	Attic floor, open vented attics
Fiberglass (blown or batt)	More open to air movement, worse in deep cold	Does not absorb moisture itself; airflow can carry humidity through it	Attic floor and rafter bays
Closed-cell spray foam	Forms a continuous air barrier where applied	Also acts as a vapor retarder at sufficient thickness	Roofline, cathedral ceilings, unvented assemblies

None of the three is universally best. The right material depends on where it is going, whether the attic is vented or unvented, and what problem you are actually trying to solve. Our attic insulation R-value guide covers the depth targets zone 5 recommends regardless of which material you choose; this page is about how each one gets there.

02 / AIR MOVEMENTHow each material handles moving air

Heat leaves an attic two ways: by conduction straight through the material, and by convection, warm air actually moving through or around it. Insulation R-values are tested for conduction. Air movement is the part a lab number does not capture, and it is the part that decides how an attic actually performs on a windy, single-digit night.

Blown cellulose

Cellulose is dense, cellulose fiber treated for fire and pest resistance, and installed at a higher pound-per-cubic-foot density than fiberglass at the same R-value. That density is what slows air movement within the fill itself: a gust that would ripple through a loose fiberglass blanket has to work harder to move through the tighter mat of cellulose fibers. It is not an air barrier, and it does nothing for the bypasses at the hatch, the recessed lights, or the top plates covered in our air sealing versus insulation guide. But across the open field of the attic floor, away from those bypasses, cellulose resists wind-driven air movement noticeably better than an equivalent depth of fiberglass.

Fiberglass, blown or batt

Fiberglass insulates by trapping still air in a web of fine glass fibers, and it does that job well as long as the air inside it actually stays still. The lighter, more open structure that makes fiberglass easy to install also makes it easier for air to enter and move through, particularly at the surface exposed to attic wind-washing near the eaves, a phenomenon covered on its own in a separate guide. In a still, draft-free attic fiberglass performs close to its rated value. In a windy or poorly baffled attic, moving air within the material itself can undercut the number on the bag, which is one reason fiberglass batts in older attics often show cold streaks where a lab test would predict none.

Closed-cell spray foam

Spray foam is applied as a liquid that expands and cures in place, filling every irregular gap in the framing and bonding to the substrate as it goes. Once cured, closed-cell foam is functionally airtight at any thickness that matters, which is the entire reason it costs more to install than a loose-fill material: you are buying an air barrier and an insulator in one application, not just an insulator. That air-seal quality is also why foam is the material of choice for the unvented, conditioned-attic assemblies covered in our unvented hot roof guide, where there is no ventilation path to fall back on if air leaks.

03 / MOISTUREMoisture and frost behavior in a cold attic

Michigan attics deal with two separate moisture questions: does the material itself hold water if it gets wet, and does the material let humid air pass through it to condense somewhere colder. The two questions produce different rankings.

Cellulose is genuinely absorbent. It will take on moisture from a roof leak, a plumbing stack, or chronic high humidity, and it holds that moisture longer than fiberglass does, which can slow drying and, in a persistent leak, contribute to matting and settling. Fiberglass does not absorb water the way cellulose does, but that is not the same as being moisture-proof: humid air still moves through an open fiberglass fill and can carry moisture vapor to the cold roof deck above it, where it condenses or frosts regardless of what the insulation itself does with water. That transport mechanism, air-carried moisture reaching a cold surface, is the same physics behind the sequencing argument in our air sealing versus insulation guide, and it happens above both cellulose and fiberglass alike when the air path underneath is not sealed.

Closed-cell spray foam is different again. At sufficient thickness it acts as its own vapor retarder as well as an air barrier, which is why it changes the moisture conversation entirely rather than just slowing it down. That is also why foam demands more care in the design: seal the wrong side of an assembly, or apply it too thin, and you can trap moisture rather than exclude it. Our old attic insulation guide covers what decades of past moisture events look like in older Macomb County homes, most of it tracing back to matted, water-affected cellulose or fiberglass rather than the material choice itself being wrong from day one.

04 / COLD-TEMPERATURE DRIFTWhat deep cold does to rated R-value

The R-value on an insulation bag is measured at a standard laboratory temperature, and it assumes the material is doing nothing but conducting heat through still air trapped in its structure. Real winter nights do not hold to that assumption. As outdoor temperatures fall into the single digits and below, the air trapped inside loose fiberglass fill can begin to move on its own, through small convective currents driven by the temperature difference between the warm side of the insulation and the cold side just inches away. That internal air movement carries heat past the fibers instead of being stopped by them, and the practical effect is a measurable drop in real-world performance compared to the tested number, worse than the same drop would be on a mild fall night. Cellulose's higher density resists this internal convective drift more effectively than fiberglass does, which is one reason building science researchers have documented cold-climate performance gaps between the two loose-fill materials specifically at deep cold temperatures.

Closed-cell spray foam behaves differently again, because it is a rigid, closed-cell structure rather than a loose fill of fibers with air moving between them. There is no internal air path for convection to exploit, so its performance is far more stable across the same cold-weather swing that erodes loose fiberglass, one more reason it is the material of choice where there is no ventilation path to fall back on.

The number on the bag is a laboratory condition. A January night at zero degrees with wind against the eaves is a field condition, and the two materials that look identical on a spec sheet do not always agree once the temperature actually drops.

None of this means fiberglass is a poor choice in every attic. In a well-sealed, well-baffled attic floor with minimal wind exposure at the eaves, the practical gap between fiberglass and cellulose performance narrows considerably, because the convective drift that widens the gap needs moving air to exploit in the first place. It does mean that an attic already fighting wind-washing at the eaves or poor baffling is a worse candidate for fiberglass specifically, independent of anything to do with R-value depth.

05 / SETTLING AND COST LOGICSettling, longevity, and where foam earns its price

Loose-fill materials both lose some loft over time, though not identically. Cellulose settles somewhat more than fiberglass by design and by installation practice, which is why a proper cellulose installation accounts for settled depth rather than fluffed depth from day one, and why an attic insulated decades ago at what was then a correct depth can be meaningfully under today's zone 5 target purely from settling, moisture history, and compaction, a pattern our older-home insulation guide walks through directly. Fiberglass batts, properly installed, hold their loft better over time than loose fill of either type, though a batt that was compressed or cut short during installation never delivers its full rated value in the first place, regardless of what happens afterward.

Spray foam does not settle in the way loose fill does. Once cured, it holds its thickness and its bond to the substrate for the life of the roof assembly barring physical damage or a leak that compromises the substrate underneath it. That permanence, combined with the air-seal quality covered above, is the core of why foam costs meaningfully more per square foot installed than either loose-fill option: you are buying a one-time, largely maintenance-free assembly rather than a material that may need a top-up blow decades later.

We keep this comparison to material behavior on purpose and leave exact installed pricing out of it, because attic insulation is priced by square footage, depth, and access, and a fair number needs someone measuring your specific attic. What we can say plainly: loose-fill cellulose and fiberglass are typically the lower-cost path for a standard vented attic floor, and closed-cell foam is a premium option whose cost is justified specifically in unvented assemblies, cathedral ceilings, and attics with ducts or equipment in the conditioned space, the cases our unvented hot roof guide covers in depth.

06 / THE DECISIONMatching the material to your attic, not the other way around

The honest way to choose is to start with the attic you have, not the material a salesperson prefers to sell. A standard vented attic floor over living space, in a house without ducts or equipment up there, is the easy case: loose cellulose or fiberglass over a properly air-sealed ceiling plane, brought up to the zone 5 depth target, solves the problem well and at the lower end of the cost range. A cathedral ceiling, a knee-wall attic, or an attic carrying HVAC ducts is a harder case where the vented, loose-fill approach fights the geometry of the house, and that is where the unvented, foam-based approach earns a serious look despite the higher price.

Air sealing comes first regardless of which material you choose. Piling a premium material over unsealed bypasses at the hatch, the recessed lights, or the top plates wastes the material's real advantage, whichever one it is. If you have not read it yet, our air sealing versus insulation guide lays out that sequencing directly, and the R-value reference guide gives you the depth target to aim for once the bypasses are handled.

If your attic insulation question came up because of ice dams, stained ceilings, or a roof that is due for replacement anyway, that is worth mentioning up front, because a reroof is the cheapest moment to correct ventilation and insulation together while the deck is already exposed. Our roof replacement service page and replacement cost guide cover that side of the ledger, including the published $9,000 to $18,000 installed range for a typical asphalt job, with most homes landing $12,000 to $16,000. The cost calculator runs those ranges against your own roof size in about a minute, and the instant estimator does the same from the homepage.

A free inspection is the fastest way to get a straight answer on your specific attic. We look at the depth and condition of what is already up there, the ventilation path, and whether the geometry of your attic calls for a vented, loose-fill approach or a case for the unvented route, and we will tell you plainly which one fits rather than steering you toward whichever material happens to be on the truck. See the full honest pricing page for how we price the roofing side of the work, and browse the rest of the guides library for the full winter science series.