Insulation Calculator — How Much Insulation Do I Need?
Calculate how much batt insulation you need for walls, attics, or floors. Enter your area dimensions to get the number of bags for R-13 and R-19 insulation.
Materials You'll Need
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Owens Corning R-13 Kraft Faced Fiberglass Batt Insulation (15 in × 93 in)
Standard R-13 batt insulation for 2×4 walls — fits 16-inch on-center framing. Based on your calculated bag count.
Shop on AmazonOwens Corning R-19 Kraft Faced Fiberglass Batt Insulation (15 in × 93 in)
R-19 batt insulation for 2×6 walls and attic floors — higher R-value for better energy efficiency. Based on your calculated bag count.
Shop on AmazonInsulation Knife / Utility Knife with Extra Blades
A sharp blade is essential for cleanly cutting batts around outlets, pipes, and irregular framing
Shop on Amazon3M Insulation Safety Kit (Respirator, Goggles, Gloves)
Fiberglass insulation irritates skin, eyes, and lungs — always wear a respirator, goggles, gloves, and long sleeves
Shop on AmazonAssumptions
- 5% waste factor for trimming around outlets, pipes, and irregular framing
- R-13 kraft-faced batt insulation (3.5 inch thick, 15 inch wide): approximately 40 sq ft per bag
- R-19 kraft-faced batt insulation (6.25 inch thick, 15 inch wide): approximately 50 sq ft per bag
- Batts are designed for standard 15-inch cavities (16-inch on-center framing) or 23-inch cavities (24-inch on-center framing)
- Cavity count is calculated along the length dimension — measure along the direction studs or joists run
- Does not account for headers, fire blocking, or other framing that reduces cavity space
- For walls, enter the wall length as Area Length and wall height as Area Width
- Choose R-13 for 2×4 walls (3.5-inch cavity) or R-19 for 2×6 walls and attic floors (5.5-inch+ cavity)
Your Insulation Project Guide
Insulating your home is one of the highest-return improvements you can make — properly insulated walls, attics, and floors can cut heating and cooling costs by 20 to 30 percent, and most batt insulation projects are well within DIY territory. This insulation calculator helps you figure out how many bags of R-13 or R-19 batt insulation you need based on the area you're covering.
The 'R' in R-value stands for thermal resistance — the higher the number, the better the insulation resists heat flow. R-13 insulation is 3.5 inches thick and fits perfectly in standard 2×4 wall cavities. R-19 is 6.25 inches thick and is designed for 2×6 wall cavities and attic floors between joists. For most of the United States, building codes require at least R-13 in exterior walls and R-38 to R-60 in attics. In cold climates (zones 5 through 8), R-20 or higher is required for exterior walls.
Batt insulation comes in two forms: kraft-faced (with a paper vapor barrier on one side) and unfaced (no barrier). Use kraft-faced batts on exterior walls and any surface where warm, moist interior air meets a cold surface — the paper faces toward the living space to prevent moisture from condensing inside the wall cavity. Use unfaced batts when adding a second layer over existing insulation, or when a separate vapor barrier is already installed.
Installing batt insulation is straightforward but takes attention to detail. The batts should fill the entire cavity without being compressed — squishing insulation actually reduces its R-value because you're eliminating the air pockets that provide the thermal resistance. Cut batts to fit snugly around electrical boxes, pipes, and wiring rather than stuffing them behind obstacles. A sharp utility knife or insulation knife makes clean cuts easy.
For attic insulation, you have a choice between batts and blown-in insulation. Batts work well when joists are evenly spaced and accessible, and you can stack layers to reach the R-value you need (two layers of R-19 gives you R-38). Blown-in insulation (cellulose or fiberglass) is better for attics with irregular joist spacing, lots of obstacles, or hard-to-reach areas. Many home improvement stores loan out blowing machines for free when you buy a minimum number of bags.
Safety matters with fiberglass insulation. The tiny glass fibers irritate your skin, eyes, and respiratory system. Always wear a dust mask or respirator, safety goggles, work gloves, and long sleeves. Work in a well-ventilated area, and shower immediately after you finish. If you're insulating an attic, lay down temporary plywood walkways across the joists — stepping between joists onto the drywall ceiling below is a costly and painful mistake.
Frequently Asked Questions
What is the difference between R-13 and R-19 insulation?
R-13 insulation is 3.5 inches thick and fits in 2×4 wall cavities. R-19 is 6.25 inches thick and fits in 2×6 wall cavities or between attic floor joists. R-19 provides about 46% more thermal resistance than R-13. Use R-13 for interior walls and standard exterior walls, R-19 for 2×6 exterior walls and attic floors.
How much insulation do I need for a 1,000 square foot attic?
For a 1,000 sq ft attic with R-19 batt insulation and 5% waste, you need about 21 bags (at ~50 sq ft per bag). For R-38 (code minimum in most climate zones), you would install two layers of R-19 and need about 42 bags.
Should I use faced or unfaced insulation?
Use kraft-faced (paper-backed) insulation on exterior walls with the paper facing the warm side (interior). Use unfaced insulation when adding a layer over existing insulation, or when a separate polyethylene vapor barrier is being installed. Never install two vapor barriers — trapped moisture causes mold.
How do I know what R-value I need?
R-value requirements depend on your climate zone. In zones 1-3 (Southern US), exterior walls need R-13 and attics R-30 to R-38. In zones 4-5 (Central US), walls need R-13 to R-20 and attics R-38 to R-49. In zones 6-8 (Northern US), walls need R-20+ and attics R-49 to R-60. Check ENERGY STAR's recommendations for your zip code.