Blown-In Cellulose vs. Fiberglass: Which Survives Florida Humidity?

The one-sentence answer for South Florida

In a humid Boca Raton attic, cellulose outperforms loose-fill fiberglass. It is denser, settles less, holds its R-value when humidity spikes, and is treated with borate fire retardant that also deters pests. We install cellulose on most blown-in jobs across the corridor.

Fiberglass blown-in is cheaper and has a place in very dry attics, but in South Florida's 75–90% humidity, it loses real-world performance faster than the label suggests.

Cellulose vs. loose-fill fiberglass

| Factor | Cellulose | Loose-fill fiberglass | |---|---|---| | R-value per inch | R-3.8 | R-2.2–R-2.7 | | Settling | 15–20% then stable | More, continues | | Humidity performance | Holds R-value | Loses up to 40% when damp | | Fire resistance | Class 1 (borate-treated) | Non-combustible but air-permeable | | Pest resistance | Borate deters pests | Pests nest in it | | Cost per sq ft | $1.00–$2.00 | $0.80–$1.50 |

Why humidity changes the math

Fiberglass insulation works by trapping pockets of still air. When humidity spikes, those pockets fill with moist air — which conducts heat better than dry air. The result: a fiberglass attic rated R-30 can perform like R-18 to R-22 on a July afternoon in Boca.

Cellulose is denser and its fibers hold together, so it resists that humidity-driven R-value loss. In a coastal Delray or Boca attic, that difference compounds over a 10–15 year service life.

Settling and longevity

Both materials settle after install. Cellulose settles 15–20% in the first year, then stabilizes. Loose-fill fiberglass can continue to settle and lose depth over time. We account for settling by blowing to a depth that lands at the target R-value after settling — not before.

Realistic service life in a South Florida attic: 10–15 years for cellulose, 7–12 years for loose-fill fiberglass before a top-up is worth doing.

The fire and pest angle

Cellulose is treated with borate fire retardants — it has a Class 1 fire rating and actually resists flame spread better than untreated fiberglass. The borate also deters rodents and insects, which is a real advantage in mature Boca neighborhoods where roof rats nest in attics.

Fiberglass is non-combustible but air-permeable, and pests readily nest in loose-fill fiberglass. If your attic has had rodent activity, cellulose is the better replacement.

When fiberglass still makes sense

Loose-fill fiberglass is the right call when the attic is very dry, the budget is the overriding factor, or it is a short-term hold before a larger retrofit. For most Boca, Delray, and Boynton attics, cellulose is the better lifetime value.

The right next step

We install both, and we will tell you straight which fits your attic. A free FLIR inspection shows the existing condition and moisture level so the recommendation is based on your attic, not a brochure. See your city page — Boca Raton, Delray Beach, Boynton Beach — or our near-me page.

Call (561) 654-7698 — 5.0 stars on Google.

Frequently asked questions

Is cellulose or fiberglass better for a Boca Raton attic? In Boca's humidity, cellulose outperforms loose-fill fiberglass. Cellulose is denser, settles less, holds its R-value better when humidity spikes, and is treated with borate fire retardant that also deters pests. We install cellulose on most blown-in jobs in the corridor.

Does blown-in fiberglass lose R-value in Florida humidity? Yes. Loose-fill fiberglass can lose up to 40% of its rated R-value when humidity climbs above 70% — which is most afternoons in Boca. Cellulose handles humidity better and is our default for South Florida attics.

Is cellulose insulation fire-resistant? Yes. Cellulose is treated with borate fire retardants and has a Class 1 fire rating. It also resists pests and mold better than loose-fill fiberglass in a humid attic.

How much more does cellulose cost than fiberglass? Cellulose runs about 10–20% more than loose-fill fiberglass per bag, but it settles less and holds R-value longer in Florida — so the lifetime cost is usually lower. On a 2,000 sq ft attic the difference is typically $300–$600.