Insulation Calculator – R-Value, Thickness & Cost

The DOE recommends R-30 for climate zones 1–2 (Florida, South Texas), R-38 for zone 3 (most of CA, GA, NC), and R-49 for zones 4–8 (Midwest, Northeast, Northwest, Alaska). Most existing US homes have attic insulation in the R-11 to R-25 range — well below current recommendations. The DOE estimates that upgrading attic insulation to R-49 can reduce heating and cooling costs by up to 15% annually.

R-value measures thermal resistance — the higher the number, the better the insulation. Thickness is the physical depth of the insulation layer. The relationship between them depends on the material: fiberglass batt provides R-3.2 per inch, blown-in cellulose R-3.5 per inch, and closed-cell spray foam R-6.5 per inch. To achieve R-49 in an attic with blown-in cellulose, you need approximately 14 inches of material.

Blown-in cellulose is almost always the cheapest option for attic floors: materials cost $0.05–$0.08 per sq ft per inch of insulation, and blower rental is free at Home Depot or Lowe's when you buy 10+ bags. A 1,000 sq ft attic brought from R-11 to R-49 (38 additional R-value, ~11 inches of cellulose) costs roughly $250–$400 in materials for a DIYer. Professional installation adds $0.50–$1.00/sq ft in labor.

Closed-cell spray foam (R-6.5/inch) is worth it in specific situations: crawl spaces where moisture control is critical, rim joists, and tight cavities where you need maximum R-value per inch. At $1.50–$3.00/sq ft installed (for 2 inches = R-13), it costs 3–5× more than fiberglass batt for the same R-value. Open-cell spray foam (R-3.6/inch) is better priced and provides excellent air sealing — ideal for cathedral ceilings and irregular framing.

Yes, for attic floors you can simply blow or lay new insulation over existing material — there is no need to remove old insulation unless it is wet, moldy, or contaminated with vermiculite (asbestos risk). For walls, adding insulation typically requires opening the wall cavity (injection foam or blown-in through drilled holes) or installing continuous rigid foam on the exterior. Never cover existing faced fiberglass batts with more faced batts — use unfaced insulation on top.

The DOE's 8-zone climate map divides the US based on heating and cooling degree days. Zone 1 (hottest) covers Hawaii and South Florida. Zone 5 covers most of the Midwest and Northwest (Illinois, Ohio, Pennsylvania, Washington). Zone 6 covers Minnesota, Wisconsin, Montana. Zones 7–8 cover most of Alaska. You can look up your exact county at energycodes.gov. This matters because the DOE's recommended R-values jump significantly between zones — R-30 attic in zone 2 vs R-49 in zone 5.

Yes. Under the Inflation Reduction Act (IRA), homeowners can claim a 30% tax credit (up to $1,200/year) for insulation that meets 2021 IECC standards. This applies to attic, wall, floor and basement insulation — both materials and installation costs are eligible. Spray foam always qualifies; batt and blown-in insulation must meet the IRS bulk insulation requirements. File IRS Form 5695. Many states also offer additional rebates — check DSIRE.org for your state programs.

Faced insulation has a kraft paper or foil vapor barrier on one side. Unfaced has none. For exterior walls in cold climates, install faced batts with the vapor barrier facing the warm-in-winter side (toward living space). Unfaced is used when adding insulation over existing insulation (to avoid double vapor barriers that trap moisture), for interior partition walls, and in hot/humid climates where the vapor barrier direction reverses. Never install two vapor barriers facing each other — that traps moisture and causes mold.