Insulation is sold by the bag or bundle with a stated square foot coverage, but the number that governs the job is the R-value your climate zone and cavity depth call for. This calculator sizes the order and shows what R-value your assembly can physically hold.
Framing first? Size studs with the Stud Calculator.
Insulation Calculator
Enter the area to insulate and choose your cavity depth and product type.
Recommended R-Values by Climate Zone
US Department of Energy guidance. Higher zone numbers are colder climates requiring more insulation.
| Climate Zone | Example Region | Attic (uninsulated) | Attic (has 3–4 in) | Floor |
|---|---|---|---|---|
| Zone 1 | South Florida, Hawaii | R-30 | R-25 | R-13 |
| Zone 2 | Gulf Coast, S. Texas | R-49 | R-38 | R-13 |
| Zone 3 | N. Florida, Georgia, Arizona | R-49 | R-38 | R-19 |
| Zone 4A & 4B | Virginia, Kansas, Tennessee | R-60 | R-49 | R-19 |
| Zone 4C, 5, 6 | Oregon, Illinois, Minnesota | R-60 | R-49 | R-30 |
| Zone 7 & 8 | N. Minnesota, N. Maine, Alaska | R-60 | R-49 | R-38 |
Source: EPA ENERGY STAR recommended insulation levels for retrofitting existing wood-framed buildings, based on the 2021 International Energy Conservation Code (IECC) Residential Provisions, Chapter 4, Table R402.1.3. Note that the recommended attic value depends on what is already there — an uninsulated attic targets a higher figure than one that already holds 3–4 inches.
Wall and Basement Insulation Recommendations
ENERGY STAR frames existing-wall guidance around adding insulative sheathing when siding comes off, rather than around batt R-values. This is the relevant table for a retrofit.
| Assembly | Zone 3 | Zone 4A & 4B | Zone 4C & 5–8 |
|---|---|---|---|
| Uninsulated wood-frame wall | Blow cavity + R-5 sheathing | Blow cavity + R-5 to R-10 | Blow cavity + R-5 to R-10 |
| Already-insulated 2×4 wall | — | Add R-10 sheathing | Add R-10 sheathing |
| Basement / crawlspace wall | R-5 sheathing or R-13 batt | R-10 sheathing or R-13 batt | R-15 sheathing or R-19 batt |
Cavity batt values for new framing differ from these retrofit recommendations: a 2×4 cavity physically accepts R-13 to R-15 and a 2×6 cavity accepts R-19 to R-21. Use the cavity-depth table further down this page when framing new walls, and the table above when re-siding an existing house.
Attic work gives the best return in almost every climate, because the ceiling plane is where the largest share of heat loss occurs in a typical house. Walls are far harder to upgrade once siding and drywall are in place.
Insulation Type Comparison
| Type | R per Inch | Cost per Sq Ft | Best For | Drawback |
|---|---|---|---|---|
| Fiberglass batt | R-3.1 to R-3.4 | $0.40–$1.00 | New framing, DIY walls | Gaps if poorly fitted |
| Mineral wool batt | R-3.7 to R-4.2 | $0.90–$1.80 | Sound, fire resistance | Higher cost, heavier |
| Blown-in fiberglass | R-2.2 to R-2.7 | $0.60–$1.30 | Attics, retrofit | Settles over time |
| Blown-in cellulose | R-3.2 to R-3.8 | $0.70–$1.50 | Attics, dense-pack walls | Moisture sensitive |
| Open-cell spray foam | R-3.5 to R-3.8 | $1.20–$2.50 | Odd cavities, air sealing | Requires pro install |
| Closed-cell spray foam | R-6.0 to R-7.0 | $1.80–$4.50 | Max R in thin cavity, rim joists | Highest cost |
| Rigid foam board (XPS) | R-5.0 | $0.80–$1.80 | Basement walls, exterior | Seams need taping |
| Rigid polyiso board | R-5.6 to R-6.5 | $1.00–$2.20 | Roof, exterior sheathing | Performance drops when cold |
What R-Value Fits in a Cavity
Cavity depth limits achievable R-value with batt insulation. Compressing a batt reduces its rated performance.
| Framing | Actual Depth | Fiberglass Batt | Mineral Wool | Closed-Cell Foam |
|---|---|---|---|---|
| 2 × 4 wall | 3-1/2 in | R-13 to R-15 | R-15 | R-21 to R-24 |
| 2 × 6 wall | 5-1/2 in | R-19 to R-21 | R-23 | R-33 to R-38 |
| 2 × 8 joist | 7-1/4 in | R-25 | R-30 | R-43 to R-50 |
| 2 × 10 joist | 9-1/4 in | R-30 | R-38 | R-55 to R-64 |
| 2 × 12 joist | 11-1/4 in | R-38 | R-45 | R-67 to R-78 |
| Attic (blown, open) | Unlimited | R-38 at 13 in | — | — |
Never compress a batt to fit a shallower cavity. An R-19 batt squeezed into a 2×4 wall delivers roughly R-13, so you pay for R-19 and receive R-13. Buy the batt rated for the cavity you actually have.
Common Insulation Mistakes
Compressing Batts Into Undersized Cavities
R-value depends on trapped air, and compressing a batt removes exactly that. Forcing R-19 into a 2×4 bay yields about R-13 while costing more than the correct R-13 batt would have. Buy the product rated for your actual cavity depth, and if you need more R than the cavity allows, add continuous rigid foam over the framing instead.
Leaving Gaps Around Wiring and Boxes
Fiberglass performance collapses at gaps, because air moves through the void and bypasses the insulation entirely. Split the batt around wires rather than compressing it behind them, and cut carefully around electrical boxes. A wall with 10% of its area poorly fitted loses well over 10% of its rated performance, because air moves through the gaps instead of being stopped by them.
Insulating Without Air Sealing First
Insulation slows conductive heat transfer but does almost nothing to stop air movement. Top plates, can lights, plumbing penetrations, and the attic hatch all leak air straight through a bed of insulation. Sealing those penetrations with foam and caulk before insulating typically delivers more comfort improvement per dollar than the insulation itself.
Blocking Soffit Vents in the Attic
Pushing insulation out to the eaves blocks the soffit vents that ventilate the roof deck, which leads to moisture accumulation and ice damming in cold climates. Install baffles at each rafter bay to hold the insulation back and keep a clear air channel from soffit to ridge. This is a code requirement in most jurisdictions, not merely good practice.
About these price ranges
Pricing varies widely by region, season, project size, and site conditions. Treat every figure on this page as a planning estimate for budgeting only — not as a quote. Coastal metro markets commonly run 20–40% above national ranges while rural inland markets can fall 10–20% below. Always collect at least three written quotes from local contractors before committing to a budget.
Sourced figures: EPA ENERGY STAR recommended R-values (based on 2021 IECC Table R402.1.3).
Unsourced estimates: All per-square-foot material prices and bundle coverage figures. These are informed planning figures rather than data drawn from a published price index, and they carry a correspondingly wider margin of error.