Insulation Calculator — Attic R-Value & Blown-In Cost

Find your recommended attic R-value by climate zone, then calculate blown-in insulation depth and installed cost for fiberglass or cellulose.

July 9, 2026

Climate Zone

I already have roughly 3-4 inches of existing insulation

Recommended Target R-Value

R-49

Zone 2-3 — Gulf Coast, Southeast, coastal California, most of Texas · uninsulated attic

Targets follow DOE/ENERGY STAR climate-zone recommendations (2021 IECC basis). "Uninsulated" targets apply when your attic has little to no existing insulation; "adding to existing" targets are lower because a few inches of existing insulation already contributes some R-value before you add more.

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Reference Values

Last verified: Jul 9, 2026

Category	Range	What It Means	Status
Zone 1 — South Florida, Hawaii, southern Texas Gulf coast	R-30 uninsulated / R-25 adding to existing 3-4"	Warmest US climate zone. Even here, an uninsulated attic should be brought up to R-30 — attic insulation still pays back through summer cooling savings, not just winter heating.	Okay
Zone 2-3 — Gulf Coast, Southeast, coastal California, most of Texas	R-49 uninsulated / R-38 adding to existing 3-4"	Mixed-humid and hot-humid climates. This is the most common zone band for new-construction attic insulation minimums.	Good
Zone 4A/4B/4C, 5, 6 — Mid-Atlantic, Midwest, Pacific Northwest, Mountain West ★	R-60 uninsulated / R-49 adding to existing 3-4"	Cold and very-cold climates. DOE recommends the highest attic R-value tier here to limit winter heat loss.	★ Best
Zone 7-8 — Northern Minnesota, North Dakota, Alaska, far northern border states ★	R-60 uninsulated / R-49 adding to existing 3-4"	Subarctic and arctic climates. Same top-tier R-value target as Zone 4-6 — DOE does not recommend going higher than R-60 for attics since returns diminish sharply past this point.	★ Best
Fiberglass blown-in — R-value per inch	2.2-2.7 per inch (2.5 typical)	Loose-fill fiberglass R-per-inch varies by manufacturer and installed (settled) density. 2.5 is a reasonable mid-range default for estimating.	Good
Cellulose blown-in — R-value per inch ★	3.2-3.8 per inch (3.5 typical)	Recycled-paper cellulose insulates slightly better per inch than fiberglass, so it needs less depth to hit the same target R-value.	★ Best
Fiberglass blown-in — installed cost	$1.00-$3.00 per sq ft	Material plus labor, installed. Actual price depends on region, attic accessibility, and local contractor rates.	Good
Cellulose blown-in — installed cost	$1.00-$2.50 per sq ft	Material plus labor, installed. Cellulose is often marginally cheaper per R-value delivered since less depth is needed for the same target.	Good

Source: R-value-by-climate-zone targets from ENERGY STAR 'Recommended Home Insulation R-Values' (based on 2021 IECC climate zone map); blown-in R-per-inch and installed cost ranges aggregated from HomeGuide 'Blown-In Insulation Cost' and InsulationRValues.com 'Blown-In Insulation Guide'. Costs vary by region, material grade, and attic accessibility — get a local quote before budgeting.

Worked Examples

Uninsulated Attic in a Mixed-Humid Climate

Climate Zone: Zone 2-3 (Gulf Coast, Southeast)
Existing Insulation: None (uninsulated)

R-49 target

No existing insulation in Zone 2-3 uses the full uninsulated target of R-49, not the lower 'topping up' target.

Topping Up Existing Insulation in a Cold Climate

Climate Zone: Zone 4-6 (Mid-Atlantic, Midwest, Mountain West)
Existing Insulation: Yes, roughly 3-4 inches already in place

R-49 target

Zone 4-6's uninsulated target is R-60, but since 3-4 inches already exists, the 'adding to existing' target of R-49 applies instead.

Blown-In Fiberglass Depth & Cost

Target R-Value: R-49
Attic Area: 1,000 sq ft
Material: Fiberglass (2.5 R per inch)

19.6 inches deep, $1,000-$3,000

Inches = 49 ÷ 2.5 = 19.6 inches. Cost = 1,000 sq ft × $1.00-$3.00/sq ft = $1,000-$3,000 installed.

Blown-In Cellulose Depth & Cost

Target R-Value: R-38
Attic Area: 800 sq ft
Material: Cellulose (3.5 R per inch)

10.9 inches deep, $800-$2,000

Inches = 38 ÷ 3.5 = 10.86 ≈ 10.9 inches. Cost = 800 sq ft × $1.00-$2.50/sq ft = $800-$2,000 installed.

Combined Workflow: Warm Climate Recommendation Into the Depth Calculator

Climate Zone: Zone 1 (South Florida)
Existing Insulation: None
Attic Area: 600 sq ft
Material: Fiberglass (2.5 R per inch)

R-30 target → 12.0 inches deep, $600-$1,800

Zone 1 uninsulated target is R-30. Carried into the depth calculator: 30 ÷ 2.5 = 12.0 inches. Cost = 600 sq ft × $1.00-$3.00/sq ft = $600-$1,800 installed.

How to Use This Calculator

1

Pick your climate zone

Select the zone that matches your region on the "Recommended R-Value by Zone" tab — brief region examples are shown for each zone.
2

Note whether you have existing insulation

Check the box if you already have roughly 3-4 inches in your attic — this lowers your target R-value since existing material already contributes some insulation.
3

Read your target R-value

The recommendation updates instantly based on zone and existing-insulation status.
4

Send it to the depth calculator

Click "Use this R-value in the Blown-In Depth & Cost Calculator" to carry your target straight into the second tab, or enter a target R-value manually.
5

Enter attic area and material

Add your attic's square footage and choose fiberglass or cellulose to see the inches of depth needed and an estimated installed cost range.

What Each Value Means

Target R-Value (R-value): The recommended thermal resistance rating for attic insulation, set by DOE/ENERGY STAR climate-zone guidance and depending on whether the attic is currently uninsulated or already has roughly 3-4 inches in place.
Blown-In Depth (inches): The depth of loose-fill insulation material needed to reach a target R-value, calculated as target R-value divided by the material's R-value per inch.
Installed Cost Estimate (USD): A rough material-plus-labor cost range for professionally installed blown-in insulation, calculated as attic area multiplied by a per-square-foot cost range that varies by material and region.

Frequently Asked Questions

How much attic insulation do I need for my climate zone?

It depends on whether your attic already has insulation. For an uninsulated attic, DOE/ENERGY STAR recommends R-30 in Zone 1 (South Florida, Hawaii), R-49 in Zones 2-3 (Gulf Coast, Southeast), and R-60 in Zones 4-8 (most of the northern and mountain US). If you already have roughly 3-4 inches of existing insulation, the target drops to R-25, R-38, and R-49 respectively, since the existing material already contributes some R-value. Use the "Recommended R-Value by Zone" tab above to get your exact target.

How many inches of blown-in insulation do I need to reach my target R-value?

Divide your target R-value by the R-value per inch of your chosen material. Fiberglass loose-fill averages about 2.5 R per inch, so reaching R-49 needs roughly 19.6 inches. Cellulose averages about 3.5 R per inch, so the same R-49 target needs only about 14 inches. Cellulose's higher R-per-inch is why it often reaches the same target with less depth — useful in attics with limited clearance.

Is fiberglass or cellulose insulation better for an attic?

Both are widely used and code-approved; the right choice depends on your priorities. Cellulose delivers slightly more R-value per inch (3.2-3.8 vs fiberglass's 2.2-2.7), which matters in attics with limited joist depth, and it's usually made from recycled paper treated with fire retardant. Fiberglass doesn't absorb moisture the way cellulose can and is often marginally cheaper installed. Both are blown in with similar equipment, so the cost difference mostly comes down to material price and local contractor rates, not installation complexity.

How much does blown-in attic insulation cost?

Installed cost typically runs $1.00-$3.00 per square foot for fiberglass and $1.00-$2.50 per square foot for cellulose, including both material and labor. A 1,000 sq ft attic reaching R-49 with fiberglass lands around $1,000-$3,000. Your actual price depends on region, attic accessibility (a cramped or obstructed attic costs more to work in), whether old insulation needs removal first, and local contractor rates — get a few local quotes before budgeting.

Do I need to remove my old insulation before adding more?

Usually no. If your existing insulation is dry, not moldy, and not contaminated by pests, blown-in insulation can typically be added directly on top of it — that's exactly what the "adding to existing 3-4 inches" target on this calculator assumes. Removal is only necessary when existing insulation is wet, moldy, compressed beyond usefulness, or contains vermiculite (which may contain asbestos and requires professional abatement, not a DIY removal).