Zone 4 Attic Insulation — R-49
The DOE recommends R-49 insulation for the attic in Climate Zone 4 (Mixed). The typical attic cavity depth is 16 inches. This page compares all insulation types showing the thickness needed, material cost per square foot, and whether each type fits within the standard cavity depth.
Quick Summary
Insulation Type Comparison for Zone 4 Attic
The following table compares all insulation types for achieving R-49 in the attic of a building in Climate Zone 4. Types marked as "recommended" are specifically suited for this building component. The cost estimates represent material costs only and do not include labor, which can add 30-100% depending on the insulation type and project complexity.
| Insulation Type | R/Inch | Thickness | Cost/SqFt | Cost/1000 SqFt | Fits Cavity | Recommended |
|---|---|---|---|---|---|---|
| Blown-In Cellulose | R-3.5 | 14" | $3.43 | $3,430 | Yes | Yes |
| Blown-In Fiberglass | R-2.5 | 19.6" | $3.92 | $3,920 | No | Yes |
| Vermiculite (Loose Fill) | R-2.1 | 23.3" | $4.90 | $4,900 | No | Yes |
| Fiberglass Batt | R-3.2 | 15.3" | $5.88 | $5,880 | Yes | Yes |
| Expanded Polystyrene (EPS) | R-3.9 | 12.6" | $5.88 | $5,880 | Yes | Yes |
| Mineral Wool Batt | R-3.3 | 14.8" | $7.35 | $7,350 | Yes | Yes |
| Cotton Denim (Recycled) | R-3.5 | 14" | $9.80 | $9,800 | Yes | Yes |
| Polyisocyanurate (Polyiso) | R-6 | 8.2" | $10.78 | $10,780 | Yes | Yes |
| Sheep Wool | R-3.5 | 14" | $17.15 | $17,150 | Yes | Yes |
| Extruded Polystyrene (XPS) | R-5 | 9.8" | $8.82 | $8,820 | Yes | - |
| Mineral Wool Board | R-4.3 | 11.4" | $12.25 | $12,250 | Yes | - |
| Open-Cell Spray Foam | R-3.7 | 13.2" | $19.60 | $19,600 | Yes | - |
| Closed-Cell Spray Foam | R-6.5 | 7.5" | $39.20 | $39,200 | Yes | - |
| Aerogel Blanket | R-10 | 4.9" | $147.00 | $147,000 | Yes | - |
Installation Guidance
Achieving R-49 in your Zone 4 attic requires approximately 12 to 16 inches of blown-in insulation or multiple layers of batts. This level of insulation addresses both significant heating and cooling loads characteristic of mixed and cool climates. Start with air sealing all penetrations through the attic floor — recessed light fixtures, plumbing vents, electrical boxes, and the top plates of interior walls. A properly air-sealed and insulated attic can reduce heating and cooling costs by 10 to 20 percent. When using blown-in insulation, mark the desired depth on several attic trusses or joists so you can verify uniform coverage. For batt insulation, install the first layer between joists and a second layer perpendicular across the top of the joists to cover the thermal bridges.
Construction Notes
Attic insulation is typically installed between and over ceiling joists (2x6, 2x8, 2x10, or 2x12) on the attic floor. In new construction, the cavity can be filled and additional insulation layered on top perpendicular to the joists. Loose-fill blown insulation is the most common method for achieving high R-values in attics because it can be applied to any depth. When adding insulation to an existing attic, do not compress existing insulation; instead add new material on top. Ensure that soffit vents are not blocked and install baffles to maintain airflow from soffit to ridge. The attic is typically the single most cost-effective location to add insulation because heat rises and the attic represents the largest area of heat loss in most homes.