What is R-value in insulation?

R-value measures thermal resistance — how well insulation resists heat flow. Higher R-values mean better insulating performance. R-value is additive: two layers of R-19 = R-38 total.

What R-value do I need for my attic?

Required R-value depends on your climate zone. Most US homes in Climate Zones 4–7 need R-49 to R-60 for attic insulation. Warmer zones (1–3) require R-30 to R-49. Use our calculator to find your specific requirement by zip code.

How do I measure my existing attic insulation R-value?

Use a ruler to measure the depth in inches, then multiply by the R-value per inch for your insulation type. Blown-in fiberglass is about R-2.5/inch, cellulose is R-3.5/inch, and fiberglass batts are R-3.2/inch on average.

Can I add more insulation on top of existing insulation?

Yes — in most cases you can add blown-in insulation directly on top of existing batts or loose-fill. The R-values add together. No removal is needed unless the old insulation is water damaged, moldy, or contaminated.

Attic Insulation R-Value Guide 2026

Everything homeowners need to know about R-values — what they mean, what you need for your climate, and how to measure what you have.

What is R-Value?

R-value is a measure of thermal resistance — how well a material resists the flow of heat. The higher the R-value, the better the insulation performs. R-value is determined per inch of material thickness, and when you stack layers of insulation, the R-values add together.

Key Rule: R-values are additive

6 inches of R-19 fiberglass batts + 8 inches of blown-in cellulose (R-28) = R-47 total

R-value is not the same as thickness — different materials achieve different R-values per inch. Closed-cell spray foam reaches R-6 to R-7 per inch, while standard fiberglass blown-in delivers R-2.5 per inch.

R-Value per Inch by Material

Material	R-Value/Inch	Inches for R-49	Inches for R-60
Closed-Cell Spray Foam	R-6.5	8"	10"
Polyisocyanurate (Polyiso)	R-6.5	8"	10"
XPS Foam Board	R-5	10"	12"
EPS Foam Board	R-3.8	13"	16"
Open-Cell Spray Foam	R-3.6	14"	17"
Sheep's Wool	R-3.65	14"	17"
Blown-in Cellulose	R-3.5	14"	18"
Blown-in Mineral Wool	R-3.15	16"	20"
Mineral Wool Batts	R-3.6	14"	17"
Cotton Denim Batts	R-3.35	15"	18"
Fiberglass Batts	R-3.35	15"	18"
Blown-in Fiberglass	R-2.5	20"	24"

R-Value Requirements by Climate Zone (IECC 2021)

The International Energy Conservation Code (IECC) sets minimum R-value requirements for attic insulation. ENERGY STAR requirements are equal to or exceed code minimums for optimal performance.

Zone	Example Locations	Code Min.	ENERGY STAR	Details
Zone 1	Hawaii, Miami, Key West	R-30	R-38	Full guide →
Zone 2	Houston, Phoenix, Tampa	R-49	R-49	Full guide →
Zone 3	Atlanta, Dallas, Las Vegas	R-49	R-60	Full guide →
Zone 4	NYC, Philadelphia, DC	R-60	R-60	Full guide →
Zone 5	Chicago, Boston, Denver	R-60	R-60	Full guide →
Zone 6	Minneapolis, Milwaukee	R-60	R-60	Full guide →
Zone 7	Duluth, Fargo	R-60	R-60	Full guide →
Zone 8	Fairbanks, Barrow (AK)	R-60	R-60	Full guide →
Zone 4-marine	Seattle, Portland (OR)	R-60	R-60	Full guide →

How to Measure Your Existing Insulation

1
Access your attic safely
Use a sturdy ladder and wear an N95 mask and safety glasses. Most insulation is not harmful but particles can irritate.
2
Identify the material
Fluffy gray/white = blown-in fiberglass. Yellow/pink batts = fiberglass rolls. Gray loose material = cellulose. Stiff boards = rigid foam.
3
Measure the depth
Use a ruler or tape measure. Push it down to the attic floor (top of the ceiling drywall). Read the depth in inches.
4
Calculate your R-value
Multiply depth × R-value per inch for your material. Example: 8 inches of cellulose = 8 × 3.5 = R-28.
5
Compare to your zone target
Use our calculator or the table above to see your requirement. If your R-value is below the code minimum, an upgrade is recommended.

Frequently Asked Questions

Is higher R-value always better?

Higher R-value does reduce heat loss more, but returns diminish at very high levels. Going from R-11 to R-30 saves far more energy than going from R-49 to R-60. Focus on reaching at least the code minimum first, then consider the ENERGY STAR target for the best cost/benefit ratio.

Does R-value decrease over time?

Some insulation types settle and lose effective R-value. Blown-in cellulose typically settles 15–20% over the first few years. Fiberglass batts can lose R-value if compressed. Spray foam and rigid boards maintain their R-value for the lifetime of the building.

Does air sealing affect R-value?

Air sealing does not change the R-value rating, but it dramatically improves real-world performance. Even R-60 insulation performs poorly if air can bypass it through gaps around light fixtures, plumbing, or attic hatches.

What if my existing insulation has a higher R-value but is old?

Old insulation that is in good condition (no water damage, mold, or pest activity) can still perform well. Measure the depth and identify the material to estimate current R-value, then top up if needed.

Detailed Zone Guides

Zone 1: R-30 Zone 2: R-49 Zone 3: R-49 Zone 4: R-60 Zone 5: R-60 Zone 6: R-60 Zone 7: R-60 Zone 8: R-60 Zone 4-marine: R-60

Find Your R-Value Target in 60 Seconds

Enter your zip code and current insulation depth. Our calculator determines your climate zone, calculates your R-value gap, and recommends the best material and depth to close it.

Calculate My R-Value Gap