Skip to main content
Residential Free Pro Features Available

Insulation Calculator

Determine insulation type, quantity, and R-value requirements based on your climate zone, cavity depth, and local energy code.

Enter your location, wall or attic cavity dimensions, and target R-value to get a material estimate for fiberglass batts, blown cellulose, spray foam, or mineral wool. The calculator checks that the chosen insulation fits the available cavity depth, applies settling factors for blown-in products, and flags when a vapor barrier is required based on IECC climate zone guidelines.

Pro Tip: In climate zones 5 through 8, the IRC requires a Class I or II vapor retarder on the warm-in-winter side of insulated walls. Kraft-faced batts satisfy this requirement. If you use unfaced batts, you need a separate polyethylene sheet or a smart vapor retarder membrane installed before drywall.

Need to know your wall cavity depth? Calculate your framing layout first.

Lumber & Framing Calculator →

Closing up walls after insulating? Estimate drywall sheets and finishing materials.

Drywall Estimator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Insulation Calculator
Pop Out

How It Works

  1. Select Your Climate Zone

    Enter your ZIP code or select your IECC climate zone (1 through 8). This determines the minimum R-value required by code for walls, ceilings, and floors.

  2. Choose the Assembly

    Select what you are insulating: exterior wall, attic floor, cathedral ceiling, basement wall, crawlspace, or rim joist. Each has different R-value targets and installation considerations.

  3. Enter Cavity Dimensions

    Input the wall height and total length (or attic area in square feet), plus the cavity depth. Standard 2x4 walls have 3.5" cavities, and 2x6 walls have 5.5" cavities.

  4. Select Insulation Type

    Choose fiberglass batts (R-3.2/inch), mineral wool batts (R-3.8/inch), blown cellulose (R-3.5/inch settled), or closed-cell spray foam (R-6.5/inch). The calculator checks whether the chosen product achieves the target R-value in the available cavity depth.

  5. Review Results

    See the number of bags or rolls needed, installed R-value, whether a vapor barrier is required, and any warnings if the cavity is too shallow for the target R-value.

Built For

  • Homeowners insulating an attic who need to know how many bags of blown cellulose to buy for a specific R-value.
  • Contractors bidding on insulation jobs who need material quantities broken down by assembly type.
  • DIYers choosing between fiberglass batts and mineral wool for a basement finishing project.
  • Energy auditors calculating whether existing insulation meets current IECC code requirements for a home's climate zone.
  • Builders verifying that 2x4 vs 2x6 wall cavities can achieve the prescribed R-value with the chosen insulation product.

Assumptions

  • R-value per inch: fiberglass batts R-3.2, mineral wool R-3.8, blown cellulose R-3.5 (settled), closed-cell spray foam R-6.5.
  • Cellulose settling factor of 20% applied to initial blow depth.
  • Vapor barrier requirements based on 2021 IECC Table R702.7.

Limitations

  • Does not calculate continuous exterior insulation (ci) requirements for advanced wall assemblies.
  • Does not model thermal bridging through studs, which reduces effective whole-wall R-value by 10-25% compared to cavity-only R-value.

References

  • R-value requirements from the 2021 International Energy Conservation Code (IECC) Tables R402.1.2 and R402.1.4.
  • R-value per inch data from the DOE Building Energy Codes Program insulation fact sheet and manufacturer product data (Owens Corning, CertainTeed, Rockwool).

Frequently Asked Questions

The 2021 IECC requires R-13 cavity insulation for walls in climate zones 1-3, R-20 in zones 4-5, and R-20+5ci (continuous insulation) or R-13+10ci in zones 6-8. Check your local code adoption, since some jurisdictions still use the 2015 or 2018 IECC. Your climate zone is determined by county and can be found on the DOE climate zone map.
The 2021 IECC requires R-30 in climate zones 1-2, R-38 in zone 3, R-49 in zones 4-5, and R-60 in zones 6-8. Attic insulation is the most cost-effective upgrade because heat rises and losses through the ceiling are proportionally large. Blown cellulose at R-3.5 per inch needs about 17 inches to reach R-60.
Yes, blown cellulose settles approximately 15-20% after installation. Installers compensate by blowing to an initial depth that accounts for settling. For example, to achieve R-38 (about 10.5 inches settled), the initial blow depth should be around 12.5 inches. The settled R-value is what matters for energy code compliance.
IECC climate zones 5 through 8 (most of the northern US) require a Class I or II vapor retarder on the interior (warm) side of exterior walls. Kraft-faced insulation batts count as a Class II retarder. In climate zones 1-4, a vapor barrier is generally not required on walls and can actually trap moisture in humid climates. Always follow local code.
No. R-19 fiberglass batts are 6.25 inches thick, but a 2x4 cavity is only 3.5 inches deep. Compressing batts reduces their R-value because the trapped air pockets are what provide insulation. A compressed R-19 batt in a 3.5" cavity performs at roughly R-13, so you are better off buying R-13 batts designed for the cavity and saving money.
Closed-cell spray foam provides R-6.5 per inch, roughly double fiberglass, and it acts as both an air barrier and a vapor retarder. It is most cost-effective in rim joists, cathedral ceilings, and other areas where air sealing is critical and cavity depth is limited. For standard wall cavities where air sealing can be achieved with caulk and foam at penetrations, fiberglass or mineral wool batts are more cost-effective.
Mineral wool (also called rock wool or stone wool) provides R-3.8 per inch versus R-3.2 for fiberglass. It is denser, more fire-resistant (rated to 2,000F), and better at blocking sound. It also holds its shape in the cavity without sagging. The tradeoff is higher cost, roughly 25-50% more per square foot than fiberglass batts of the same R-value.
Disclaimer: Verify R-value requirements against your local energy code, which may differ from the base IECC edition cited here. Installation quality significantly affects real-world thermal performance.

Learn More

Residential

Wall Framing Basics: Studs, Headers, and Cut Lists

Precut stud lengths, header sizing per IRC, cripple and trimmer stud accounting, and how to build a lumber yard order from a framing plan.

Read Guide →
Residential

Insulation by Climate Zone: Types, R-Values, and Vapor Barriers

IECC R-value targets by zone, insulation type comparison, settling factors for blown-in, vapor barrier rules, and common mistakes to avoid.

Read Guide →

Related Tools

Residential Live

Paint Coverage Calculator

Calculate gallons of primer and topcoat needed for any room or project. Surface-aware coverage rates, color change logic, multi-room batch mode, and spray waste factors.

Launch Tool →
Residential Live

Mulch, Topsoil & Gravel Calculator

Calculate cubic yards, bags, and truck loads for mulch, topsoil, gravel, sand, and decorative stone. Includes compaction factors, weight estimates, and bulk vs bag cost comparison.

Launch Tool →
Residential Live

Tile & Flooring Calculator

Calculate tile count, boxes, thinset, grout, and cost for floors, walls, backsplashes, and showers. Pattern-aware waste factors for herringbone, diagonal, chevron, and more.

Launch Tool →