Calculate R-value by climate zone, coverage in batts or bags, and estimate your annual energy savings — with a live wall cross-section preview.
R-value measures thermal resistance — the higher the R-value, the better the material resists heat flow. Insulating your home to the correct R-value for your climate zone is one of the highest-return investments in home energy efficiency, typically saving 10–50% on annual heating and cooling costs depending on your starting point. The U.S. Department of Energy (DOE) publishes recommended minimum R-values by climate zone for each building assembly — attic, walls, floors, and basement walls — and these recommendations have increased significantly over the past decade as energy costs have risen and better insulation products have become widely available.
| Zone | Examples | Attic | Wall Cavity | Floor | Basement Wall |
|---|---|---|---|---|---|
| Zone 1 | Miami, Honolulu | R-30 to R-49 | R-13 | R-13 | R-0 to R-10 |
| Zone 2 | Houston, Phoenix | R-30 to R-60 | R-13 to R-15 | R-13 to R-19 | R-0 to R-10 |
| Zone 3 | Atlanta, Dallas | R-30 to R-60 | R-13 to R-15 | R-19 to R-25 | R-5 to R-10 |
| Zone 4 | St. Louis, Seattle | R-38 to R-60 | R-13 to R-21 | R-25 to R-30 | R-10 to R-15 |
| Zone 5 | Chicago, Denver | R-49 to R-60 | R-21 to R-25 | R-25 to R-30 | R-10 to R-15 |
| Zone 6 | Minneapolis, Burlington | R-49 to R-60 | R-21 to R-25 | R-25 to R-30 | R-10 to R-15 |
| Zone 7 | Fairbanks, Duluth | R-49 to R-60+ | R-21+ | R-25 to R-30 | R-15+ |
R-value is additive — you can stack layers of insulation and their R-values add together. The R-value of any insulation layer equals the R-value per inch of that material multiplied by the thickness in inches. To find the depth of insulation needed to reach a target R-value, divide the target by the R-value per inch of your chosen insulation: Depth (inches) = Target R ÷ R per inch. If you already have existing insulation, subtract the existing R-value from the target first: Additional R needed = Target R − Existing R. Only then divide by R per inch to find the additional depth required.
| Type | R/inch | Form | Typical Cost | Best For |
|---|---|---|---|---|
| Fiberglass Batt | R-3.1 | Batt / roll | $0.30–0.65/sqft | Wall cavities, attic floors |
| Blown Fiberglass | R-2.2–2.7 | Loose-fill | $0.40–0.90/sqft | Attic floors, irregular spaces |
| Blown Cellulose | R-3.7 | Loose-fill | $0.45–0.85/sqft | Attic floors, retrofit walls |
| Mineral Wool (Rockwool) | R-3.7 | Batt | $0.70–1.20/sqft | Walls, fire resistance, soundproofing |
| Spray Foam — Open Cell | R-3.6 | Spray | $0.50–0.65/sqft/in | Rim joists, complex shapes |
| Spray Foam — Closed Cell | R-6.5 | Spray | $1.00–2.00/sqft/in | Tight spaces, vapor barrier needed |
| Rigid Foam — EPS | R-3.8 | Board | $0.25–0.45/sqft/in | Foundation walls, continuous insulation |
| Rigid Foam — XPS | R-5.0 | Board | $0.45–0.75/sqft/in | Below-grade, high-moisture areas |
| Rigid Foam — Polyiso | R-6.0 | Board | $0.55–0.85/sqft/in | Roofs, exterior walls |
The calculation is straightforward once you know your target R-value, existing R-value, and the R-value per inch of your chosen insulation. Determine your existing R-value by measuring the depth of existing insulation and multiplying by the R-value per inch of that material — older blown fiberglass typically runs R-2.2 to R-2.7 per inch; older fiberglass batts run R-3.1; old cellulose may have settled to R-3.2 to R-3.5. Subtract existing R from target R to find additional R needed. Divide by R per inch of new insulation to get inches of insulation to add. Multiply area by thickness to get cubic feet, then convert to bags or rolls using manufacturer coverage charts.
Choosing the right insulation type involves balancing R-value per inch, cost, moisture resistance, air permeability, ease of installation, and building code requirements for your specific application. Here is a detailed breakdown of the most common insulation options for residential projects.
Fiberglass batt insulation is the most widely installed insulation in the United States — it is affordable, widely available, and easy for DIY installation in open stud and joist cavities. Standard R-13 batts (3.5 inches thick) fit between 2×4 studs; R-15 or R-19 batts (3.5 to 6.25 inches) fit in 2×6 walls. In attics, fiberglass batts can be stacked in perpendicular layers to build up R-value without gaps at the seams. The main weakness of fiberglass batts is that they do not air seal — they must be combined with a separate air barrier to prevent convective heat loss through the insulation cavity. Unfaced batts are used where a vapor barrier is already present; kraft-faced batts provide a vapor retarder on one side and are stapled to stud faces.
Cellulose is made from recycled paper (primarily newsprint) treated with borate fire retardant. It is the greenest insulation option by recycled content and has excellent performance per dollar in attic applications. Blown cellulose settles approximately 20% over time, so installers add extra depth to account for settling. At R-3.7 per inch, cellulose provides good thermal performance and — unlike fiberglass — has enough density to reduce air movement within the insulation layer, improving real-world performance beyond just R-value. Professional installation requires a blowing machine, but most big-box stores rent machines for free with bag purchase. Dense-pack cellulose can be blown into existing closed wall cavities through small holes, making it ideal for retrofitting existing homes without opening walls.
Spray polyurethane foam (SPF) is the premium insulation option for tight spaces, complex geometries, and applications requiring both air sealing and insulation in a single product. Open-cell (OC) spray foam expands to approximately 100 times its liquid volume, creating a soft spongy material at R-3.6 per inch. It air seals excellently but is vapor-permeable, making it appropriate in most climate zones without a separate vapor barrier. Closed-cell (CC) spray foam expands to about 30 times its volume, creating a dense rigid material at R-6.5 per inch — the highest R-value per inch of any common insulation. CC spray foam is also a vapor barrier and adds structural rigidity to wall assemblies. Both require professional installation, protective equipment during application, and a 24-hour curing period before occupancy.
Mineral wool (sold as Rockwool or Roxul) is made from basalt rock and recycled steel slag spun into fibers. It has become increasingly popular as an alternative to fiberglass batts due to several performance advantages: it provides the same R-3.7 per inch as cellulose, it is inherently fire resistant (melting point above 2,000°F), it does not absorb water and retains its R-value when damp, and its higher density provides better soundproofing than fiberglass. Mineral wool batts are stiffer than fiberglass and hold their shape in the cavity without friction-fitting pressure, which simplifies installation around wiring and plumbing. The main disadvantage is cost — mineral wool typically costs 50–100% more per square foot than comparable fiberglass batts.