Fiberglass vs. Cellulose Insulation:
Which Is Better for Maryland Attics?
Both are blown into attics by the same machines, both qualify for the same rebates — but they don't perform the same way. A Maryland BPI-certified contractor breaks down what actually matters when you choose between them.
Both Work. The Real Question Is Whether They're Installed Correctly.
If you've gotten quotes for blown-in attic insulation, you've probably been offered both options: fiberglass (spun glass fibers, usually pink, white, or yellow) or cellulose (recycled paper treated with borate, gray and slightly chunky). Same blowing machine. Same attic floor. Same crew. So why does the choice matter?
Honestly? Less than most marketing material would suggest. If the attic is properly air sealed first and the insulation is blown to its correct depth, both materials perform well. The factors that wreck attic insulation performance — air leaks at top plates and penetrations, insulation that settles below the rated R-value, missing baffles at the eaves — affect cellulose and fiberglass equally.
That said, we install more cellulose than fiberglass at Leonard Home Performance, and it's worth explaining why. The differences are real, just smaller than you'd think. This page covers them honestly: R-value per inch, settling, fire and pest resistance, sound, cost, and rebate eligibility — so you can make the call with the actual tradeoffs in front of you.
The thing both materials share — and the thing that actually drives performance: insulation alone won't fix a leaky attic. Air sealing has to come first. Adding any insulation on top of an unsealed attic is one of the most common (and expensive) mistakes we see homeowners make. Get the air sealing right, get the depth right, and either material gives you a comfortable, efficient attic.
Quick Comparison Table
Here's how cellulose and fiberglass stack up on the factors that actually matter for a Maryland attic. Sources for the technical numbers are linked throughout the article.
| Factor | Cellulose | Fiberglass |
|---|---|---|
| R-Value Per Inch | R-3.2 to R-3.8 (typically R-3.5) | R-2.2 to R-2.7 (typically R-2.5) |
| Inches Needed for R-49 | ~13–15 inches | ~18–22 inches |
| Weight on Ceiling | Heavier — drywall integrity should be verified before install | Lighter — minimal load on ceiling |
| Dust During Install | Dustier — requires sealing off living space | Less dusty, but glass fibers irritate skin/lungs |
| Settling Over Time | ~15–20% (installed thicker to compensate) | ~2–4% (less, but starts thicker) |
| Fire Rating | Class A (borate-treated, self-extinguishing) | Class A (non-combustible glass) |
| Pest Resistance | Borate deters insects & rodents | Inert — rodents nest in it readily |
| Recycled Content | ~85% (recycled newsprint) | 30–60% (recycled glass) |
| Sound Dampening | Better — denser material | Moderate |
| BGE/PEPCO Rebate Eligible? | ✓ Yes | ✓ Yes |
| Best Application | Most Maryland attic floors, dense-pack walls, sound control | Open framing, low-load ceilings, vented cathedral assemblies |
R-value figures based on the U.S. Department of Energy's published insulation R-value ranges and ENERGY STAR insulation guidance.
What Actually Drives Attic Insulation Performance
Before getting into the differences between cellulose and fiberglass, it's worth being clear about what really determines whether an attic insulation job works. Most attic problems we walk into aren't a material problem — they're an installation problem.
1. Air sealing comes first, period
Insulation slows heat that moves by conduction. It does almost nothing to stop heat that moves by air leakage — and in a typical Maryland home, air leakage carries away a significant portion of the heating and cooling you've paid for. Top plates, plumbing penetrations, recessed light cans, the attic hatch, chases around chimneys and ductwork — all of these need to be sealed before insulation goes down. Skip this step and the best material in the world won't save the bills. More on attic air sealing →
2. Proper depth, measured and verified
R-49 in Maryland means R-49, not "we blew some insulation in there." Cellulose at R-3.5/inch needs roughly 14 inches of depth; fiberglass at R-2.5/inch needs roughly 20 inches. Both materials are typically over-blown slightly to account for settling. We mark depth gauges at the design height and walk the attic to verify coverage end to end before calling a job done. An attic that's R-30 in the middle and R-15 at the edges is a job that wasn't installed properly — regardless of which bag the material came out of.
3. Baffles, dams, and protected ventilation
Insulation needs to stop short of the soffit vents to allow attic ventilation airflow, and it needs a dam around the attic hatch and around any recessed lights or chimney penetrations. Without these, blown-in insulation drifts into the soffit (blocking ventilation and causing moisture issues) or against heat-producing fixtures (a fire and damage risk). Done right, this is invisible. Done wrong, it shows up as ice dams, mold, and complaints about ventilation a year or two after the install.
If you get those three things right — air sealing first, proper depth, protected ventilation — both cellulose and fiberglass will perform well. The material choice matters at the margins, and that's what the rest of this page is about. But it's a tiebreaker, not the main event.
Where Each One Belongs
Both materials have their place. The honest version of "which is better" depends on what part of the home you're insulating and what climate you live in.
Where Cellulose Shines
Higher R-Value Per Inch
At roughly R-3.5 per inch versus fiberglass at R-2.5, cellulose hits R-49 in about 14 inches versus 20 for fiberglass. In tight attics or low-clearance bays, that depth difference can be the deciding factor.
Topping Off Existing Insulation
Blowing cellulose over older fiberglass or cellulose is a common, effective way to reach R-49 without removing what's already there — provided the existing insulation is dry, intact, and uncontaminated.
Dense-Pack in Walls
Cellulose is the only blown-in material commonly dense-packed into existing wall cavities. It's installed at 3.5 lbs/ft³ from small drilled holes — a minimally invasive way to insulate older Maryland homes without gutting the walls.
Sound-Sensitive Spaces
Bedrooms over garages, ceilings under home offices, party walls between townhomes. Cellulose's density makes it noticeably better at attenuating airborne sound than fiberglass.
Older Homes With Pest History
If your attic has any history of mice or insects, the borate treatment in cellulose is a meaningful deterrent. We've pulled out fiberglass full of rodent nests; we've never pulled cellulose out for the same reason.
Rooms Above Garages
Dense-packing the floor cavity between an unconditioned garage and a finished room above is one of cellulose's strongest applications. See: rooms above garages →
Where Fiberglass Still Makes Sense
Older Ceilings With Compromised Drywall
Cellulose at R-49 weighs roughly 2–3 times what fiberglass weighs at the same R-value. On older homes with cracked drywall, sagging ceilings, undersized joists, or any signs of structural fatigue, fiberglass is the safer choice. We always check ceiling integrity before specifying cellulose — and if there's any doubt, fiberglass wins.
Dust-Sensitive Households
Blowing cellulose creates a noticeable amount of fine dust during installation. We seal off the work area with plastic and run negative-pressure ventilation, but the cleanup is more involved than fiberglass. For homes with severe allergies, asthma, or someone who can't easily relocate during the install, fiberglass is a cleaner installation experience.
Vented Cathedral Assemblies
In a vented cathedral assembly, fiberglass batts (paired with proper vapor management) can be the cleaner detail. Cellulose has more moisture risk in unvented cathedral cavities.
Tight Budgets at Low Target R-Values
If budget is the absolute binding constraint and you're only getting to R-30 or so, fiberglass squeezes a little more thermal resistance per installed dollar. Above R-49, the gap closes considerably.
Open Framing With Easy Access
For new construction or major renovations where the cavity is wide open and the depth target is high, fiberglass is fast, clean, and effective. The handicap of needing more inches matters less when there's plenty of room to install them.
What About Fire, Mold, and Rodents?
These are the three concerns we hear most often when homeowners are weighing cellulose. They almost always trace back to outdated information — so it's worth getting clear on what's actually true.
"Doesn't cellulose burn? It's basically newspaper."
Modern cellulose insulation is approximately 85% recycled paper and 15% borate-based fire retardant. The borate is what makes the difference. Cellulose insulation in the U.S. is required to meet the ASTM C739 standard for fire resistance — which means it carries a Class A flame-spread rating, the same as fiberglass and the highest available rating. Treated cellulose is self-extinguishing: pull a flame source away and it stops burning. Building inspectors and fire marshals across Maryland approve it for attic installation precisely because of these standards. The "newspaper insulation" concern is from untreated product that hasn't been on the market in decades.
"Won't paper insulation get moldy?"
Borate isn't just a fire retardant — it's also fungistatic, meaning it suppresses the growth of mold and fungi. Cellulose can absorb and release small amounts of moisture without losing performance, which actually makes it more forgiving than fiberglass in attics that experience high humidity (a real issue in Maryland summers — see our guide to humidity in Maryland homes). The main moisture risk for either material is a sustained leak from above, and in that scenario both materials need to be removed and replaced. Routine humidity isn't a problem for properly installed cellulose.
"What about rodents and insects?"
This is where cellulose has its clearest advantage. Borate is naturally toxic to insects — termites, cockroaches, ants — and unappetizing to rodents. Fiberglass, by contrast, is biologically inert. It doesn't feed pests, but it makes excellent nesting material, and we routinely find mouse trails, droppings, and nests when we remove old fiberglass from Maryland attics. If your home has any history of pests, the deterrent property of cellulose is a real benefit, not a marketing line.
If your existing fiberglass shows signs of pest activity — droppings, nests, tunnels, or a strong ammonia smell — it should be removed before any new insulation is installed. Blowing fresh material on top of contaminated insulation traps the contamination and can create indoor air quality issues. Our energy audit identifies whether removal is needed. More on attic insulation removal →
Why This Matters Specifically for Maryland Attics
Most "fiberglass vs. cellulose" articles online are written for a generic North American homeowner. That's fine if you live in San Diego — but the calculation looks different in Baltimore, Towson, Columbia, Silver Spring, or any of the counties we serve. A few Maryland-specific factors are worth pointing out.
Maryland's housing stock is older and leakier than the national average
A large share of the homes we work on in Baltimore, Howard, Montgomery, and Anne Arundel counties were built between the 1950s and 1980s. That housing stock comes with two characteristics that favor cellulose at the margins: more existing top-plate and rim-joist leakage that needs careful sealing before insulation goes down, and more pest history, where the borate treatment in cellulose provides an ongoing deterrent that fiberglass can't match.
Code requires R-49 — and the depth difference is real
Maryland's code minimum for attic insulation in Climate Zone 4 is R-49. With cellulose, that's about 14 inches of depth. With fiberglass, it's closer to 20 inches. In a low-pitch roof, knee-wall attic, or any attic where the eaves crowd in fast, the extra six inches of fiberglass can be hard to fit. Cellulose's higher R-per-inch is genuinely useful in tight spaces — and a lot of Maryland attics are tight.
Both BGE and PEPCO rebate programs treat the materials equally
Some homeowners assume choosing fiberglass might unlock a different rebate or a better price tier. It doesn't. Both BGE (in Baltimore Gas & Electric territory — Baltimore City, Baltimore County, Howard, Anne Arundel, Carroll, and Harford) and PEPCO (in Montgomery and Prince George's) treat blown-in fiberglass and blown-in cellulose identically for rebate eligibility through the Home Performance with ENERGY STAR program. The choice is purely a performance and cost decision — the rebate side is neutral. See full Maryland service area details →
Bottom line for Maryland homeowners: the older housing stock, the depth advantage in tight attics, and the rebate-program neutrality nudge things toward cellulose for many of the homes we see. But a properly air-sealed attic insulated to R-49 with fiberglass is still a good attic. The material is the smaller decision; the install quality is the bigger one.
So Which One Do We Install?
Across the homes we work on in Baltimore, Howard, Anne Arundel, Montgomery, Prince George's, Carroll, and Harford counties, we lean toward borate-treated blown-in cellulose as our default. A few reasons drive that:
1. Higher R-value per inch. Hitting R-49 in 14 inches instead of 20 matters in a lot of Maryland attics where space at the eaves is tight. Less depth, same performance.
2. Pest deterrence. The borate treatment in cellulose discourages mice, insects, and mold growth. With the age of the housing stock we work on, that's a real ongoing benefit — not a guarantee, but a meaningful difference compared to inert fiberglass.
3. Recycled content and sound. Cellulose is roughly 85% recycled newsprint, and the higher density attenuates airborne sound noticeably better than fiberglass. Both are nice-to-have advantages, but they're real.
That said — and we want to be honest about this — a fiberglass install done right is a perfectly good attic. If a homeowner has a strong preference for fiberglass, has an installer they trust who specializes in it, or has a specific assembly that argues for it (cathedral ceilings, very low ceiling load capacity, tight budget at a low target R-value), we install fiberglass and the result is a comfortable, efficient home. The bigger decision is whether the air sealing was done first and whether the depth was installed and verified correctly. Get those two things right and the material choice is a smaller-stakes call.
The Honest Cons of Cellulose
Marketing material rarely mentions the downsides of the material it's selling, so it's worth being direct about cellulose's two real drawbacks:
It's heavier. Cellulose at R-49 weighs roughly 2–3 times what fiberglass weighs at the same R-value. On a typical 1,500 sq ft attic that can mean an additional 1,500–2,000 pounds of load on the ceiling. For modern construction with engineered trusses sized to 2010-and-later code, this is a non-issue. For older homes with undersized ceiling joists, sagging drywall, hairline cracks, or any prior signs of stress, we verify the ceiling can carry the load before recommending cellulose. If there's doubt, we either reinforce the ceiling first or specify fiberglass instead. This is part of the inspection that comes with our energy audit — we don't just recommend cellulose without checking the structure.
It's dustier during installation. Blowing cellulose generates a noticeable amount of fine paper dust. We seal off the work area with plastic, run negative-pressure ventilation back through the attic, and clean up thoroughly — but the install is messier than fiberglass, and there's a brief period where the home isn't ideal for someone with severe allergies or asthma. For most homeowners this is a few hours of inconvenience and nothing more. For dust-sensitive households, it's a real consideration, and fiberglass is a cleaner installation experience.
Neither of these is a dealbreaker for most homes. But they're real, and any contractor pretending cellulose has no downsides is overselling.
The biggest mistake either way is skipping air sealing. Pouring any amount of insulation onto a leaky attic floor is throwing money away — the air leakage carries heat away before the insulation has a chance to do its job. Our $100 energy audit always starts with a blower door test so we can quantify exactly where the leakage is and prioritize sealing before insulation goes down.
Frequently Asked Questions
Not Sure Which One Your Attic Needs?
A $100 energy audit gives you a blower door test, thermal imaging, and a written recommendation — so you know exactly which material makes sense for your home before spending a dollar on installation.
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