Insulation Calculator

R-value (thermal resistance) measures how well a material resists heat flow. A higher R-value means better insulation — more resistance to heat moving through the material. Definition: R-value = temperature difference (ΔT) × area (A) / heat flow rate (Q). In practical terms: R = thickness (inches) / thermal conductivity. For a given assembly: Q (heat loss, BTU/h) = (ΔT × A) / R-value. Where: ΔT = temperature difference between inside and outside (°F). A = area of the surface (sq ft). R-value = total R-value of the assembly (wall, roof, floor). Example: a 1,000 sq ft attic with R-30 insulation; inside 70°F, outside 10°F (ΔT = 60°F): Q = (60 × 1,000) / 30 = 2,000 BTU/h of heat loss through the attic. If you upgrade to R-60: Q = (60 × 1,000) / 60 = 1,000 BTU/h — exactly half the heat loss. Stacking R-values: when insulation layers are stacked, R-values add directly. Example: R-13 batt in 2×4 wall + R-5 exterior foam board = R-18 total. This additivity is a key advantage of R-value as a measurement. R-value per inch: different materials have different R-values per inch of thickness. Closed-cell spray foam: ~R-6.5 per inch (highest). Polyiso rigid board: ~R-6.2 per inch. Fiberglass batt: ~R-3.2 per inch. Blown cellulose: ~R-3.5 per inch. This allows comparison: 3" of closed-cell foam (R-19.5) provides similar resistance to 6" of fiberglass batt (R-19). Important: R-value only measures conductive heat resistance. A high-R wall with large air leaks can still lose substantial energy — air sealing and R-value work together and cannot substitute for each other.

Attic insulation is one of the highest-return home energy improvements because heat naturally rises and exits through the ceiling. The U.S. Department of Energy recommends specific R-values for attic insulation based on climate zone. DOE recommended attic R-values: Zones 1–2 (hot, South): R-30 to R-49. Zone 3 (mixed, mild): R-38 to R-60. Zones 4–8 (cool to cold, North): R-49 to R-60. Most of the US outside the deep South should target R-49 to R-60 for attic insulation. How much insulation to add: determine your current R-value: locate your current insulation; determine its type. Fiberglass batts: measure depth × 3.2 per inch. Blown cellulose: measure depth × 3.5 per inch. Blown fiberglass: measure depth × 2.5 per inch. Calculate additional insulation needed: target R-value − current R-value = additional R needed. Determine how many inches to add based on the material you're using. Example: current attic has 4" of blown cellulose (R-14). Target R-49. Additional R needed: R-49 − R-14 = R-35. Using blown fiberglass (R-2.5/inch): 35 / 2.5 = 14 more inches needed. Installation considerations: before adding insulation: seal all air leaks (around can lights, plumbing and electrical penetrations, attic hatch). Air sealing is often more impactful than adding insulation. Ensure attic has adequate ventilation (soffit and ridge vents should not be blocked by insulation). Install baffles to maintain airflow from soffits to the ridge. For DIY: blown cellulose can be rented with a blowing machine from many home improvement stores. Fiberglass batts are easier for DIY installation. Closed-cell spray foam requires professional installation. Cost vs. savings: attic insulation improvements typically have a payback period of 3–7 years, depending on energy prices and climate. Energy Star estimates 15% average savings on heating and cooling with proper air sealing and insulation.

Wall insulation selection depends on whether you're insulating new construction or retrofitting existing walls, your climate, and your budget. New construction (open walls): best overall: mineral wool (rock wool) batts or dense-pack cellulose. Mineral wool: R-3.1–3.3 per inch. Fire-resistant (melts at 1,000°C, not just moves heat). Better sound attenuation than fiberglass. More moisture-resistant. Slightly more expensive than fiberglass (~30% premium). Fiberglass batts: R-2.9–3.8 per inch. Most affordable and widely available. Most common choice for new construction. Adequate performance when properly installed. Spray foam (closed-cell): highest R-value per inch (R-6.0–6.5). Also acts as a vapor barrier and air seal. Most expensive option (3–5× the cost of batts). Best for extremely cold climates or for combining with exterior insulation. Existing walls (retrofit): dense-pack cellulose (blown-in): loose cellulose blown into wall cavities through small holes (drilled, then patched). Good R-value (R-3.5–3.8/inch) and good air-sealing properties. Best for retrofitting without opening walls. Blown-in fiberglass: similar approach to dense-pack cellulose; slightly lower R-value per inch. Injection spray foam: newer technique; can be injected through small holes; fills cavity completely; good air seal. More expensive than blown-in fiber. Wall assembly upgrades: continuous exterior insulation (rigid foam over sheathing): eliminates thermal bridging through studs. Studs have lower R-value than insulation (~R-1 per inch for wood); they create "thermal bridges" that reduce effective wall R-value by 15–25%. Adding 1–2" of exterior foam (R-5 to R-10) dramatically reduces thermal bridging. Interior insulation board: add on the inside of exterior walls; reduces interior space but significantly improves R-value without exterior work.

Calculating insulation savings requires estimating the reduction in heat loss and translating that to energy cost savings. Step 1 — Calculate current heat loss: Q_current = (ΔT_avg × Area × Hours) / R_current. ΔT_avg = average temperature difference between inside and outside (°F) during heating/cooling season. Area = surface area being insulated (sq ft). Hours = heating/cooling hours per year (typically 4,000–7,000 hours depending on climate). R_current = current R-value. Step 2 — Calculate heat loss after adding insulation: Q_new = (ΔT_avg × Area × Hours) / R_new. Step 3 — Calculate savings: heat saved (BTU) = Q_current − Q_new. Step 4 — Convert to fuel savings: for natural gas: 1 therm = 100,000 BTU (at ~80% furnace efficiency: effective = 80,000 BTU/therm). For electricity (heat pump or electric heat): 1 kWh = 3,412 BTU (for resistance heat; heat pumps deliver 3× more). For fuel oil: 1 gallon = ~140,000 BTU (at ~85% efficiency: ~119,000 BTU/gallon). Example calculation: 1,000 sq ft attic, R-20 → R-50. Average ΔT: 40°F (heating season, Chicago). Heating season: 5,500 hours. Current loss: (40 × 1,000 × 5,500) / 20 = 11,000,000 BTU/year. New loss: (40 × 1,000 × 5,500) / 50 = 4,400,000 BTU/year. Saved: 6,600,000 BTU/year. At 80% furnace efficiency: effective therms needed = 6,600,000 / 80,000 = 82.5 therms. At $1.50/therm: annual savings ≈ $124/year. Typical attic insulation project cost: $1,500–$3,500 DIY to professional. Payback period: $2,500 / $124 ≈ 20 years (longer in mild climates; shorter in cold climates or with high energy prices). Federal tax credits: as of 2024, the IRS offers a 30% tax credit (up to $1,200/year) on qualifying home insulation improvements — which significantly improves the effective payback period. Always check current IRS Publication 5797 or consult a tax professional.