A typical 1,500 square foot attic can swallow 40-50 bags of loose-fill insulation in under an hour with the right machine. That speed is why blown in fiberglass insulation dominates attic retrofits across the country. It fills every joist bay, every corner, and every gap that batt insulation tends to miss. Done right, it drops heating and cooling bills 15-25%. Done wrong, it settles, compresses, and leaves cold spots that eat at your energy savings for decades.
How Blown In Fiberglass Compares to Other Insulations
Fiberglass loose-fill competes mainly with cellulose and spray foam in the attic market. Each has distinct tradeoffs.
- Blown fiberglass: R-2.5 per inch, lightweight, mold-resistant, lowest cost per R-value
- Blown cellulose: R-3.5 per inch, denser, better sound dampening, made from recycled paper
- Open-cell spray foam: R-3.7 per inch, air-sealing built in, 3-4x the cost
- Closed-cell spray foam: R-6.5 per inch, vapor barrier, 5-7x the cost
- Fiberglass batts: R-3.1 per inch but installation quality varies widely
Fiberglass loose-fill wins on price, weight, and ease of install. Cellulose wins on per-inch R-value and sound reduction. Spray foam wins on air sealing. For most retrofits where the attic floor already has batts, topping up with blown fiberglass is the cheapest route to modern R-values.
Target R-Value by Climate Zone
The Department of Energy recommends attic R-values based on climate. Warmer zones require less, colder zones more.
- Zone 1 (southern Florida, Hawaii): R-30 to R-49
- Zones 2-3 (Gulf states, Texas, California): R-30 to R-60
- Zone 4 (mid-Atlantic, Tennessee): R-38 to R-60
- Zones 5-6 (Midwest, New England): R-49 to R-60
- Zones 7-8 (northern plains, Alaska): R-49 to R-60
At R-2.5 per inch, blown fiberglass reaches R-38 at about 15 inches of depth and R-60 at 24 inches. Manufacturer bag labels list the coverage per bag at each target R-value, which makes ordering straightforward.
Cost Breakdown for Typical Projects
Material runs $0.30-0.45 per square foot at R-30 and $0.50-0.80 per square foot at R-60. Professional installation adds $0.50-1.25 per square foot depending on region, attic accessibility, and whether existing insulation needs removal. A 1,500 square foot attic topped up from R-19 to R-49 typically costs $1,800-2,800 installed.
DIY saves roughly half. Home Depot and Lowe’s rent insulation blowers free with a purchase of 10-20 bags, depending on the store. Bags run $35-50 each and cover 40-60 square feet at R-30. Total DIY cost for the same 1,500 square foot attic: around $900-1,400 in material plus a Saturday of labor.
Preparing the Attic for Installation
Good insulation starts with prep work. Skipping these steps leaves energy on the table even if the install goes perfectly.
- Seal every penetration with caulk or spray foam: wiring holes, plumbing stacks, chimney chases, recessed lights
- Install baffles at every soffit vent to prevent insulation from blocking airflow into the attic
- Build dams around attic hatches, whole-house fans, and any knee-wall openings
- Mark the top of joists with tape or rafter rulers so you can verify depth after blowing
- Check recessed lights for IC-rated labels; non-IC cans need a box built around them to prevent fire
Air sealing delivers roughly 15-20% of the total savings. Blown fiberglass itself does not stop air leaks; it only slows heat transfer. The combination of air sealing plus insulation is what produces noticeable utility bill drops.
Running the Blower Machine
The blower machine has two operators: one feeds bags into the hopper, one directs the hose in the attic. Use walkie-talkies or a cell phone on speaker because the machine is too loud for voice communication. The operator in the attic wears a respirator (N95 minimum, P100 better), safety glasses, long sleeves, and gloves. Fiberglass particles irritate skin and lungs for hours after exposure.
Work from the far end of the attic toward the hatch. Sweep the hose in even passes, maintaining consistent depth. Overlap passes by about 6 inches to prevent gaps. Check depth frequently with a rafter ruler and add more where needed. Blown insulation settles 10-20% over the first year, so target 1-2 inches above the final R-value marker.
Safety and Health Considerations
Modern fiberglass is classified as a non-hazardous irritant rather than a carcinogen, reversing concerns from earlier decades. Still, the fibers cause itching and respiratory irritation at install time. Full PPE is non-negotiable. After the install, shower in cool water first to close pores, then warm water to wash the fibers away. Hot water immediately opens pores and drives fibers deeper.
Once settled and covered, fiberglass is chemically stable and releases no ongoing emissions. It does not attract pests, does not support mold growth, and does not lose R-value unless it gets wet. Wet fiberglass compacts permanently and must be removed and replaced.
Common Problems After Installation
The most common complaint after an attic blow-in is a slight drafty feeling that never goes away. This usually means air sealing was inadequate, not that the insulation failed. A thermal imaging camera pass on a cold winter morning will reveal the leaking spots around wiring penetrations, plumbing stacks, and exterior wall top plates.
The second common problem is insulation migration. Wind-washing in poorly baffled attics can push loose-fill away from the eaves, creating cold spots above exterior walls. Installing proper baffles at each soffit vent fixes this permanently.
Return on Investment
A well-installed blown in fiberglass insulation upgrade typically pays back in 3-8 years through energy savings, depending on climate and fuel costs. Colder climates and electric-heat homes see the fastest payback. Federal tax credits through 2032 cover 30% of insulation project costs up to $1,200 per year, which further shortens payback. Utility rebate programs in many states add another $100-500 off the total.
For a low-cost, high-impact home improvement, blown in fiberglass insulation is hard to beat. Fast to install, easy to budget, and backed by decades of performance data in every climate zone. Pair it with proper air sealing and you will feel the difference the first cold night after the hose comes out of the attic.
