What R-value do I need for my attic?

IECC 2021 requires R-38 to R-60 for attics depending on climate zone. Zone 1-2 (Florida, Hawaii): R-38. Zone 3 (Carolinas, Texas): R-38. Zone 4 (mid-Atlantic, Pacific NW): R-49. Zone 5-6 (Midwest, northern states): R-49 to R-60. Zone 7-8 (Alaska, northern Minnesota): R-60.

How many bags of blown-in insulation do I need?

Divide the area in square feet by the coverage per bag at your desired R-value. Coverage is listed on the bag. For a 1,500 sq ft attic at R-49 with fiberglass: typically 25–35 bags. For cellulose: 30–45 bags. This calculator computes bags automatically from your area and R-value.

What is the R-value of a 2x4 wall vs. 2x6?

A standard 2x4 wall cavity (3.5 inches) holds R-13 to R-15 fiberglass batt. A 2x6 wall (5.5 inches) holds R-19 to R-21 batt. With exterior continuous insulation added, 2x4 walls can reach R-20+ total. IECC 2021 requires R-20 in zones 3-4 and R-21 in zones 5-8 for wood-framed walls.

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Insulation R-Value Calculator

IECC climate zone requirements, material quantities & energy payback

R-49 Required R-value

0 Bags / rolls needed

$0 Material cost

0 yr Energy payback

Location & Assembly

IECC climate zone

Assembly location

Area to insulate

sq ft

Insulation type

Existing R-value (0 if none)

Cost Inputs

Price per bag / roll

Current heating/cooling bill

$/mo

Insulation Requirements

R-49 required

Existing: R-0

Required: R-49

49 R-value to add

0" Depth / thickness

0 Bags / rolls

$0 Material cost

$0/yr Est. savings

0 yr Payback period

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R-Value vs. Material Type (this area)

IECC 2021 Climate Zone Map (by state)

Zone	States / Regions	HDD base 65°F
1	Hawaii, Puerto Rico, Guam	<2,000
2	S. Florida, S. Texas	2,000–3,000
3	N. Florida, Carolinas, Georgia, Texas, Arizona, Los Angeles	3,000–4,000
4	Mid-Atlantic, Tennessee, Pacific NW, N. California	4,000–5,000
5	Ohio, Indiana, Illinois, New England, Colorado	5,000–6,500
6	Minnesota, N. New England, Montana, Wyoming	6,500–8,000
7	N. Minnesota, N. Alaska coastal	8,000–9,500
8	Arctic Alaska	>9,500

IECC 2021 Minimum R-Values by Zone

Zone	Attic	Wall (cavity)	Floor
1	R-30	R-13	R-13
2	R-38	R-13	R-13
3	R-38	R-20 or 13+5ci	R-19
4	R-49	R-20 or 13+5ci	R-19
5	R-49	R-20 or 13+5ci	R-30
6	R-49	R-20 or 13+5ci	R-30
7	R-49	R-21	R-38
8	R-49	R-21	R-38

Key IECC 2021 Sections

Section	Description
R402.1.2	Insulation minimum requirements — Table R402.1.2
R402.2.1	Ceilings with attic space — R-38 min in zones 1–4
R402.2.2	Ceilings without attic — continuous insulation or air barrier
R402.2.5	Floors — minimum R-19 depending on zone
R402.2.6	Basement walls — separate requirements from floors
R402.2.8	Crawl space walls — R-5 to R-19 depending on zone
R402.4	Air leakage — 3–5 ACH50 blower door requirement
R402.5	Fenestration — window U-factor and SHGC limits

R-Value per Inch by Material

Material	R/inch	Notes
Fiberglass batt	3.1–3.8	Most common; R-13 in 2×4, R-19 in 2×6
Fiberglass blown	2.2–2.7	Attics: check bag coverage chart
Cellulose blown	3.2–3.8	Higher density, better air blocking
Mineral wool batt	3.0–3.3	Fire-resistant; excellent sound control
Open-cell spray foam	3.5–3.8	Vapor permeable; good for 2×6+ cavities
Closed-cell spray foam	6.0–7.0	High R/in; vapor barrier; most expensive
Rigid EPS (foam board)	3.6–4.2	Continuous insulation; exterior use
Rigid XPS (foam board)	4.5–5.0	Higher R; blue/pink foam board

How to Use This Calculator

1Select your IECC climate zone from the dropdown, or find your zone on the map. The calculator sets required R-values automatically based on your zone.

2Choose the assembly type — attic floor, cathedral ceiling, exterior wall, floor over crawlspace, or basement wall. Each has different minimum R-value requirements.

3Select your insulation material (fiberglass batt, blown fiberglass, cellulose, mineral wool, closed-cell foam, or rigid board). The calculator uses the material's R/inch value to compute required depth.

4Enter the existing R-value of insulation already in place (0 if none). The calculator shows only the additional R-value needed.

5Enter the area in square feet. For blown-in, the calculator also shows number of bags needed and estimated material cost.

Key Terms

R-Value — A measure of thermal resistance. Higher R-value means better insulation performance. Measured in °F·ft²·h/BTU. Required minimums are set by the IECC for each climate zone and assembly type.

U-Factor — The inverse of R-value (U = 1/R). Measures the rate of heat transfer — lower is better. Windows and doors are typically rated by U-factor rather than R-value.

IECC Climate Zone — The International Energy Conservation Code divides the US into 8 climate zones (1 = hottest, 8 = coldest). Each zone has minimum R-value requirements for attics, walls, floors, and basements.

Thermal Resistance per Inch (R/in) — The R-value delivered per inch of material thickness. Closed-cell spray foam leads at R-6.5/in; fiberglass batt averages R-3.5/in.

Vapor Retarder — A material that slows moisture movement through assemblies. Class I = sheet poly or foil (≤0.1 perm); Class II = kraft-faced batts (0.1–1 perm). Required on the warm-in-winter side in cold climates.

Thermal Bridging — Heat flowing through structural members (studs, joists) that bypass insulation in the cavity. Continuous insulation (CI) on the exterior face eliminates thermal bridging and improves whole-wall R-value.

Frequently Asked Questions

Can I add insulation on top of existing insulation?

Yes — in attics you can layer new insulation over existing. If existing insulation is damaged, wet, or contains vermiculite (asbestos risk), remove it first. For blown-in over batt: lay batt perpendicular to joists first, then blow over. Do not cover soffit vents — maintain an air channel from eaves to ridge.

What is the difference between R-value and U-factor?

R-value measures thermal resistance — higher is better. U-factor measures thermal conductance (heat flow rate) — lower is better. U = 1/R. Windows are rated by U-factor because their performance is dominated by frames and glazing, not just insulation. Walls and ceilings use R-value because they are primarily insulation assemblies.

Does more insulation always save more money?

No — there are diminishing returns. Going from R-0 to R-19 saves far more energy than going from R-38 to R-57. The IECC minimum requirements represent the point where additional insulation stops having a quick payback. For most retrofits, hitting code minimum is the right economic target.

Do I need a vapor barrier with insulation?

It depends on climate zone and assembly. In cold climates (zones 5–8), a Class II vapor retarder (kraft-faced batt or 6-mil poly) is typically required on the warm-in-winter side of insulation in walls. In hot/humid climates (zones 1–3), vapor barriers go on the exterior. Mixed climates can be tricky — consult a building scientist or local code.

What is the best insulation for an attic floor vs. roof deck?

Attic floor (most common, ventilated attic): blown-in fiberglass or cellulose — easy to install to high R-values. Roof deck / cathedral ceiling (unventilated): closed-cell spray foam is the most reliable choice for air sealing and thermal performance in tight spaces. Rigid foam + air sealing is an alternative for new construction.