Understanding R-Values and Insulation Types
R-value measures thermal resistance: higher numbers mean better insulation. Each inch of fiberglass provides about R-3.2, cellulose about R-3.7, closed-cell spray foam about R-6.5, and open-cell spray foam about R-3.7. To achieve R-38 in an attic, you need about 12 inches of fiberglass or 10 inches of cellulose. The Department of Energy recommends R-38 to R-60 for attics depending on climate zone, R-13 to R-21 for walls, and R-25 to R-30 for floors over unheated spaces.
Fiberglass batts are the most common wall insulation, sold in pre-cut lengths to fit between standard 16 or 24-inch on-center framing. Blown-in cellulose (made from recycled newspaper) is the standard for attic retrofits because it fills gaps and voids that batts miss. Spray foam provides the highest R-value per inch and also acts as an air barrier, but costs 3-5 times more than fiberglass. Rigid foam board (XPS, EPS, polyiso) is used on exterior walls, basement walls, and continuous insulation applications. For energy audits, a roof pitch calculator helps determine attic floor area from the building footprint.
Insulation Cost by Type and Application
Fiberglass batts cost $0.30-1.00 per sq ft for R-13 to R-30 products. They are the cheapest option for new construction and remodeling when walls are open. Blown-in cellulose costs $0.50-1.50 per sq ft installed, or $0.30-0.70 for DIY with a rental blowing machine ($70-100/day). Blown-in fiberglass is similar in price to cellulose but does not settle as much over time. Open-cell spray foam runs $0.50-1.00 per sq ft per inch of thickness. Closed-cell spray foam costs $1.00-2.00 per sq ft per inch but provides both insulation and moisture barrier in one step.
Professional insulation installation adds $0.50-1.50 per sq ft on top of material costs. Many utility companies and state energy offices offer rebates of $200-1,500 for insulation upgrades that meet Energy Star requirements. The average attic insulation project (R-11 to R-38 upgrade) costs $1,200-2,500 professionally installed and saves $200-600 per year in energy costs, paying for itself in 3-7 years. For whole-house insulation during construction, budget $3,000-8,000 depending on home size and climate zone. Pair insulation upgrades with paint calculator estimates for the walls you open during the retrofit.
Attic Insulation Guide
Start by measuring the existing insulation depth in your attic. If you have less than 6 inches (roughly R-19), you are losing significant energy and should upgrade. The target for most homes is R-38 to R-49 (12-16 inches of fiberglass or 10-14 inches of cellulose). Before adding new insulation, seal all air leaks around plumbing penetrations, electrical boxes, chimney chases, and the attic hatch with caulk or spray foam. Air leaks account for 25-40% of energy loss, and insulation alone cannot stop air movement.
For blown-in insulation, rent a machine from the home center ($70-100/day) and buy bags of cellulose or fiberglass. A typical 1,000 sq ft attic going from R-11 to R-38 needs about 20-25 bags of cellulose at $6-10 per bag. The machine operator feeds bags while a helper in the attic directs the hose for even coverage. Maintain consistent depth using depth markers (yardsticks taped to joists at regular intervals). Keep insulation at least 3 inches away from recessed light fixtures unless they are IC-rated. Install baffles at the eaves to maintain soffit ventilation airflow.
Wall Insulation Methods for Existing Homes
Insulating existing walls without removing drywall is done through drilled-and-filled method. Small holes (2-3 inches) are cut in the exterior siding or interior drywall, insulation is blown in through a tube, and the holes are patched. Dense-pack cellulose is the standard material, installed at 3.5 lbs per cubic foot to prevent settling. Costs run $1.00-2.50 per sq ft of wall area. A typical 1,500 sq ft home with R-0 walls needs about 3,000 sq ft of wall insulation at $3,000-7,500.
Injection foam (a low-expansion polyurethane) is an alternative to cellulose for existing walls. It fills voids more completely and also air-seals, but costs 50-100% more. For homes undergoing major renovations where walls are opened, fiberglass batts or mineral wool are the easiest and cheapest options. Mineral wool (Rockwool) costs 20-30% more than fiberglass but provides better fire resistance and sound dampening. Use our siding calculator to coordinate exterior work if you are drilling through siding for blown-in insulation.
Common Insulation Mistakes
Compressing fiberglass batts to fit a smaller cavity is the most common mistake. A 6-inch R-19 batt compressed into a 4-inch cavity performs like an R-13, not R-19. Use the correct thickness for your cavity depth, or use a higher-density product rated for thinner spaces. The second mistake is leaving gaps around electrical boxes, plumbing, and framing irregularities. Even a 1-inch gap in a wall batt reduces the effective R-value of the entire wall section by 20-30%. For the wall framing that determines cavity depth, our wall framing calculator helps plan the stud layout.
Installing faced batts with the facing on the wrong side creates a moisture trap that leads to mold and rot. In cold climates, the paper facing goes toward the interior (warm side). In hot-humid climates, use unfaced batts with a separate vapor retarder only if required by local code. Blocking soffit vents with insulation in the attic prevents proper ventilation, leading to moisture buildup and ice dams in winter. Always install rafter baffles at the eaves to maintain airflow from soffit to ridge. Finally, not air-sealing before insulating wastes 25-40% of the potential energy savings regardless of R-value.
For related calculations, try the concrete slab to estimate materials for your project.