Blown-in insulation is the most cost-effective way to upgrade an attic to modern R-value requirements, and most homeowners can do it in a weekend with a rented blower. Understanding the difference between cellulose and fiberglass, how many bags you actually need, and what the code requires in your climate zone makes the difference between a smooth project and an expensive re-do.

Cellulose vs. Fiberglass: Which Material Is Right for Your Attic

Cellulose and fiberglass blown-in insulation are both effective, but they perform differently in the field. Cellulose is made from 80–85% post-consumer recycled paper treated with borate-based fire retardants and delivers R-3.8 per inch. Its higher density — about 1.5 lb/cu ft installed — provides better air blocking than fiberglass at the same installed depth. That air-blocking property is critical in attics because even a small gap lets warm air bypass the insulation layer entirely, degrading effective R-value. Fiberglass blown-in delivers R-2.5 per inch at roughly 0.5 lb/cu ft installed density. It is lighter, easier to blow long distances, inorganic (won't support mold or insects), and non-combustible. However, it requires about 52% more installed depth to achieve the same R-value as cellulose, and it settles less — only about 5% compared to 15% for cellulose. For most attic upgrades, cellulose is the better choice for R-value per bag and R-value per dollar. Fiberglass is preferable where moisture risk or mold is a concern.

How to Read the Coverage Chart on the Bag

Every bag of blown-in insulation carries a coverage chart on the label — this is the most important document for your project. The chart shows how many bags per 1,000 square feet you need at each target R-value, along with the initial installed thickness to achieve that R-value after settling. Never calculate bags using just the R/inch value — density varies by product, and some manufacturers calibrate for higher or lower density installation. Always match the bag count to the chart for your specific product at your target R-value. When using a big-box store machine, set the feed speed and blower pressure as instructed on the chart's equipment settings section. Underfilling by even 10% can drop your effective R-value by one or two R-points across the whole attic. Buy two or three extra bags and return any unopened ones — most stores allow returns of unused sealed bags when purchased with their blower loan program.

Preparing Your Attic Before You Blow

Proper prep work makes or breaks a blown-in installation. Before adding any insulation, seal all air bypasses at the attic floor — top plates, plumbing penetrations, electrical chase openings, and recessed light cans are the most common pathways for warm air to bypass insulation. Use fire-rated caulk or spray foam to seal gaps, and install airtight covers over any non-ICAT recessed lights. Install wind baffles (rafter vents) at every rafter bay along the eaves to maintain a 1-inch air channel from the soffit to the ridge — blocking soffit ventilation will cause moisture problems and can rot roof sheathing over time. Add a cardboard dam at the attic hatch, and install a folding attic stair insulator cover if accessible. These preparatory steps are where most DIYers cut corners — and where most air leakage energy losses occur. Spending an extra two hours sealing will save more energy than the insulation itself in a leaky attic.

Climate Zone R-Value Requirements

IECC 2021 sets minimum attic insulation R-values by climate zone, and many state energy codes have adopted equal or stricter requirements. Zone 1 (Hawaii, far south Florida): R-30. Zone 2 (coastal Florida, south Texas): R-38. Zone 3 (Carolinas, central Texas, coastal California): R-38. Zone 4 (mid-Atlantic, Pacific Northwest, lower Midwest): R-49. Zone 5 (Great Lakes, northern Plains, New England coast): R-49. Zone 6 (Minnesota, northern Wisconsin, high elevation Rockies): R-49 to R-60. Zone 7–8 (Alaska, extreme northern climates): R-60 minimum. These are code floors, not performance targets. Energy modeling consistently shows that R-60 in Zone 5 pays back the incremental cost over R-49 in under seven years at current energy prices, making it a smart upgrade whenever the equipment is already set up. If you are insulating an attic from scratch, always install to at least the next R-increment above code minimum — the marginal material cost of additional blown-in depth is very low once the blower machine is running and labor is sunk.

Common Mistakes That Reduce Effectiveness

The most common blown-in installation mistake is installing to the target R-value at the wrong density — blowing too fast reduces density below spec, which lowers R-value per inch and accelerates settling over time. Set your blower speed to achieve the manufacturer's recommended pounds per square foot per inch of depth and spot-check the depth with a ruler gauge as you go. A second frequent error is covering soffit vents: always install rafter baffles before blowing, and never let insulation block the free ventilation area at the eave. Blocking even one bay of soffit ventilation concentrates moisture in that rafter cavity and can lead to roof sheathing rot within just a few years. Third, many DIYers forget to check for knob-and-tube wiring before starting. Knob-and-tube wiring is designed to dissipate heat into free open air; covering it with blown-in insulation creates a fire hazard and voids most homeowners' insurance policies. Have a licensed electrician verify the wiring type before insulating any pre-1950 attic.