How Much Insulation Do I Need?
Quick answer: insulation is rated by R-value, and how much you need depends on your climate zone and the surface (walls vs attic). In most US climates, walls need R-13–R-23 and attics R-38–R-49. Measure the area, then use the bag's coverage at your target R-value to count bags.
- What R-value means
- R-value by surface & zone
- Wall vs attic math
- Coverage per bag
- Worked example: an attic
- Insulation types
- FAQ
What R-value means
R-value measures thermal resistance — the higher the number, the better it insulates. Building codes in the US recommend minimum R-values that rise with colder climates. The three main places to insulate are the attic/ceiling (biggest heat loss), exterior walls, and floors over unheated space.
R-value by surface & zone
| Surface | Typical R-value (most US zones) |
|---|---|
| Attic / ceiling | R-38 to R-49 |
| Exterior walls | R-13 to R-23 |
| Floors over unheated space | R-19 to R-30 |
| Crawl space / basement walls | R-11 to R-25 |
Colder zones need the higher end; check your local code or the Department of Energy's zone map for your area's recommendation.
Coverage per bag
Insulation is sold in batts (pre-cut for standard stud spacing) and bags of blown-in loose fill. Coverage per bag depends on R-value: a bag of blown-in fiberglass might cover ~40–70 sq ft at attic R-values and more at lower values. Always read the bag label for coverage at your target R-value, then divide your area by it.
Worked example: a 1,000 sq ft attic
- Attic floor = 1,000 sq ft
- Target = R-38 (typical)
- A blown-in bag covering ~71 sq ft at R-38 → 1,000 ÷ 71 = ~14 bags
- Check the bag's coverage at R-38 on the label before you buy
Insulation types
Common options: Fiberglass (batts or blown) is the standard, cheap, and effective. Cellulose (recycled paper) is eco-friendly and good for attics. Spray foam seals and insulates but costs more. Mineral wool resists fire and moisture well. Material prices range widely; labor is often a big part of the total if you hire an installer.
FAQ
An attic typically needs R-38 to R-49 (about 10–16 inches of fiberglass). For a 1,000 sq ft attic at R-38, you need roughly 14 bags of blown-in insulation (about 62 bags if every bag covers ~71 sq ft at that R-value). Check your climate zone.
In most US climates, walls need R-13 to R-23 and ceilings R-30 to R-60. Your exact R-value depends on your climate zone — colder zones need higher R-values.
Coverage depends on R-value: a bag of blown-in fiberglass may cover ~40 sq ft at high R-values and over 100 sq ft at lower ones. Check the bag label for coverage at your target R-value.
The R-value is thermal resistance — higher means better insulation. R-13 is typical for walls; R-38 or higher is common for attics and ceilings, which face the biggest heat loss in winter.