Insulation Calculator
Calculate how much insulation you need for walls, attics, or floors. Get results in batts, rolls, or bags of blown-in insulation.
How to Calculate Insulation for Your Home
To estimate insulation, measure the total area to be insulated (length times width in feet). Then choose the right R-value for your climate zone and application — walls, floors, or attics each have different recommended R-values. The amount of material depends on whether you use fiberglass batts, blown-in fiberglass, or blown-in cellulose.
Recommended R-Values by Application
| Application | R-Value | Typical Thickness |
|---|---|---|
| Exterior Walls (2x4) | R-13 | 3.5 inches |
| Floors / Crawl Spaces | R-19 | 6.25 inches |
| Attic (moderate climate) | R-30 | 9.5 – 10.25 inches |
| Attic (cold climate) | R-38 | 12 – 13 inches |
| Attic (very cold climate) | R-49 | 15.5 – 16.5 inches |
Batts vs. Blown-In Insulation
Fiberglass batts come in pre-cut widths designed to fit between studs (16" or 24" on center) and are easy for DIY installation. Blown-in insulation (fiberglass or cellulose) is ideal for attics and irregularly shaped spaces because it fills gaps and voids more completely. For a detailed comparison of the two blown-in options, see our fiberglass vs. cellulose insulation guide. Blown-in requires a blowing machine, which many home improvement stores rent for free with purchase.
Tips for Accurate Insulation Estimates
- Buy 5-10% more than calculated to account for trimming around outlets, pipes, and obstructions.
- Do not compress insulation to fit — compressed batts lose R-value and effectiveness.
- Check your local building codes for minimum R-value requirements in your climate zone.
- Seal air leaks around wiring, plumbing, and ductwork before installing insulation.
- Wear protective gear (gloves, mask, long sleeves) when handling fiberglass insulation.
Can I Add New Insulation Over Old?
Yes, you can add new insulation on top of existing insulation in most cases. This is common in attics where the original insulation no longer meets current energy codes. Make sure the existing insulation is dry and free of mold. Do not add a vapor barrier on top of existing insulation — only the layer closest to the heated space should have one. Blown-in insulation is the easiest to layer over existing material. Once your insulation is in place, you can close up the walls with drywall.
For a complete cost breakdown, see our insulation cost guide.
Frequently Asked Questions
The R-value you need depends on your climate zone and the area being insulated. Exterior walls typically need R-13 (warm climates) to R-21 (cold climates). Attics need R-30 in moderate climates, R-38 in cold climates, and up to R-49 in very cold regions. Check the U.S. Department of Energy's recommendations for your specific ZIP code to determine the right R-value.
Batts are pre-cut panels of fiberglass or mineral wool that fit between standard stud spacings. They are easier for DIY projects in open walls and new construction. Blown-in insulation consists of loose fiberglass or cellulose fibers that are machine-blown into cavities. Blown-in is better for attics, retrofitting existing walls, and filling irregular spaces where batts cannot reach.
R-30 fiberglass batts are about 9.5 to 10.25 inches thick. Blown-in fiberglass achieves R-30 at approximately 10.25 inches of depth, while blown-in cellulose reaches R-30 at about 8.3 inches. The exact thickness varies by manufacturer and product density. Always check the product label for the installed thickness needed to achieve the stated R-value.
Yes, in most cases you can add new insulation on top of existing insulation, which is a common way to upgrade attic insulation. Make sure the old insulation is dry, free of mold, and not damaged. Do not install a second vapor barrier over the existing layer. Blown-in insulation is the easiest type to add on top of existing material. If the old insulation is wet or moldy, remove it before adding new material.
Properly insulating your home can reduce heating and cooling costs by 15-30% depending on your current insulation levels, climate, and energy prices. Attic insulation typically offers the highest return on investment, often paying for itself within 2-4 years. Upgrading from R-19 to R-49 in a cold-climate attic can save $200-$500 per year on energy bills. The actual savings depend on local utility rates, home size, and HVAC efficiency.