Insulation coverage isn't a fixed number — the same bag covers dramatically different square footage depending on the R-value you're targeting. Buy based on square footage alone and you'll likely run short or overbuy. Here's how to get the number right.
Step 1: Pick Your Target R-Value
R-value measures resistance to heat flow — higher means better insulating performance. The right target depends on the area of the house and your climate zone.
| Area | Recommended R-Value |
|---|---|
| Attic (most US climates) | R-38 to R-60 |
| Exterior walls | R-13 to R-21 |
| Floors over unheated space | R-25 to R-30 |
| Basement walls | R-10 to R-19 |
Colder northern climates need the higher end of each range; warmer southern climates can typically use the lower end. Check your local energy code for the exact minimum in your area.
Step 2: Choose Blown-In or Batt
Blown-in insulation (fiberglass or cellulose) is blown into place with a machine and fills gaps and irregular spaces around joists, wiring, and obstructions more completely — it's the more common choice for attics. Batt (roll) insulation comes in pre-cut sections and is easier to install without special equipment, making it a common choice for open, regularly-shaped wall cavities you can access directly.
Step 3: Coverage Per Bag by R-Value
This is the part most people miss: coverage per bag drops significantly as target R-value rises, since a thicker layer means less material stretches across the same bag.
| R-Value | Blown-In Fiberglass | Blown-In Cellulose |
|---|---|---|
| R-30 | ~32.5 sq ft/bag | ~27 sq ft/bag |
| R-38 | ~25.7 sq ft/bag | ~21 sq ft/bag |
| R-49 | ~19.7 sq ft/bag | ~16 sq ft/bag |
| R-60 | ~16 sq ft/bag | ~13.5 sq ft/bag |
Bags needed = Area (sq ft) ÷ Coverage per bag at your target R-value
Worked Example
A 1,200 sq ft attic targeting R-38 with blown-in fiberglass: 1,200 ÷ 25.7 = about 47 bags. The same attic at R-49 instead would need 1,200 ÷ 19.7 = about 61 bags — 30% more material for the higher R-value target.
Seal Air Leaks First
Insulation slows heat transfer through a surface, but it doesn't stop air movement. Before adding insulation, seal gaps around recessed lighting, plumbing and wiring penetrations, and the attic hatch with caulk or expanding foam — an attic full of insulation over unsealed air leaks still loses significant energy.
Frequently Asked Questions
What R-value do I need for my attic? Most US climate zones recommend R-38 to R-60. Colder northern climates need the higher end; warmer southern climates can use R-30 to R-38.
How many bags of blown-in insulation do I need for an attic? It depends on your target R-value — roughly 32.5 sq ft per bag at R-30, but only about 19.7 sq ft per bag at R-49, since higher R-values need a thicker layer.
Is blown-in or batt insulation better for an attic? Blown-in fills irregular spaces more completely, making it the more common attic choice. Batt is easier to install without equipment and works well for open wall cavities.
Do I need to remove old insulation before adding new insulation? Not usually — new insulation can typically go directly on top of existing insulation, as long as it's dry and free of mold or pest damage.