Your home loses heat through every surface — attic, walls, floor, and foundation. R-value is the single number that tells you how well each assembly resists that heat flow. Getting it right means knowing your climate zone, your assembly type, and the real performance of the insulation material you choose, not just the marketing claims on the bag.

What R-Value Means and How It Is Measured

R-value stands for thermal resistance and is measured in units of °F·ft²·h/BTU. A higher number means the material slows heat flow more effectively. To put it in practical terms: one square foot of R-19 fiberglass batt allows about 1/19th of a BTU to pass through per hour for every degree of temperature difference across the assembly. Double the R-value and you cut that heat loss in half. R-values are additive — R-19 batts in a 2×6 wall cavity plus R-5 rigid foam on the exterior gives an assembly R-value of R-24 before accounting for framing losses at studs. The IECC sets minimum R-values by climate zone and assembly type, with attics requiring the highest values (R-38 to R-60 depending on zone) because the attic is typically the largest surface area and experiences the greatest temperature differential in both heating and cooling seasons. The U-factor — the inverse of R-value — is used for windows and doors because frames and glazing layers complicate a simple R-based comparison.

IECC Climate Zones and Code Minimums

The International Energy Conservation Code divides the US into eight climate zones based on heating degree days and humidity. Zone 1 covers southern Florida and Hawaii — the hottest, most humid areas — while Zone 8 covers arctic Alaska. The zone determines the minimum R-values required by building code for new construction and major renovations. In Zone 5 (including Chicago, Denver, and Boston), the IECC 2021 requires R-49 in the attic, R-20 in 2×6 walls, and R-30 in floors over unconditioned spaces. In Zone 7 (northern Minnesota and Montana), those minimums jump to R-60 in the attic, R-21 plus R-5 continuous in walls, and R-38 in floors. Your local jurisdiction may have adopted an older IECC edition (2018, 2015, or even 2012) or added state-specific amendments — always verify with your local building department before planning an insulation retrofit. Energy-code enforcement for existing homes also varies by state: some states require code upgrades when you renovate more than a threshold percentage of the floor area, while others only enforce the minimums for new construction. The calculator uses IECC 2021 defaults, which you can override manually.

Comparing Insulation Materials

Different insulation types perform very differently per inch of thickness. Closed-cell spray polyurethane foam leads at R-6.5 per inch, making it the best choice for tight spaces and assemblies where thickness is limited, such as cathedral ceilings or rim joists. Open-cell spray foam delivers R-3.5 per inch but provides excellent air sealing at a lower material cost. Rigid extruded polystyrene (XPS) board runs R-5 per inch and is moisture-resistant, making it the right choice for below-grade applications and exterior continuous insulation. Mineral wool batts deliver R-4.2 per inch, are fire-resistant, vapor-open, and dimensionally stable. Fiberglass batts average R-3.5 per inch — inexpensive and widely available, but sensitive to gaps, voids, and compression, all of which dramatically reduce effective in-service R-value. Blown cellulose achieves R-3.7 per inch, fills irregular cavities well, and is made primarily from recycled paper treated with borate fire retardant. For attics, blown fiberglass or cellulose is the most cost-effective way to reach high R-values quickly; for walls, mineral wool batts or rigid foam continuous insulation eliminates the thermal bridging through studs that cavity-only batts cannot address.

Calculating Bags, Depth, and Cost

Estimating blown insulation requires three inputs: target R-value, material R-per-inch rating, and bag coverage at depth. First, calculate required depth by dividing the target R by the material's R/in rating — for R-49 blown fiberglass at R-2.5/in, you need 19.6 inches. Next, check the bag coverage chart on the product packaging, which lists square feet per bag at various settled depths; do not rely on a single coverage number, because it changes with depth. Divide your area by the coverage rate at your target depth to get bag count. Add 10% waste for irregular edges and joists. For bagged fiberglass at 1,200 sq ft and a coverage rate of 25 sq ft per bag at 20 inches, you need about 48 bags plus 5 extra for waste. Material cost runs roughly $0.50–$1.25 per square foot installed for blown attic insulation, depending on depth and local labor rates. Most homeowners see payback periods of 3–8 years when upgrading an uninsulated or minimally insulated attic.

Common Installation Mistakes to Avoid

The biggest R-value mistake is not missing the target depth — it is air leakage. An air gap as small as 1% of a wall's area can account for 50% of total heat loss because air moves heat far more efficiently than conduction through a material. Always air-seal penetrations (electrical boxes, plumbing penetrations, top plates) before adding insulation. In attics, seal the top-plate gaps between the ceiling and the attic before blowing insulation over them. A second common mistake is compressing batt insulation to fit in a thinner-than-rated cavity — a 2×4 cavity is 3.5 inches deep, but compressing R-19 batts (nominally 6.25 inches) into that space drops the effective R-value to roughly R-13. Use the correct batt thickness for your cavity depth. Third, covering soffit baffles with blown insulation blocks attic ventilation, causing moisture buildup and ice dams. Install rafter baffles (insulators) at every rafter bay before blowing insulation to maintain a clear 1-inch airway from the soffit to the ridge.