Insulation Thickness vs. R-Value: What Do You Need?

R-value measures how well insulation resists heat moving through it. The higher the number, the better it insulates. Thickness is just how many inches you install. The link between them is the R-value per inch, which varies by material and product.

That's why the same R-value takes different depths depending on what's installed. Here's how R-38 compares across common materials:

For a San Antonio attic, ENERGY STAR's retrofit guidance for Climate Zone 2 recommends adding R-49 to an uninsulated attic, or adding R-38 when the attic already has 3 to 4 inches. Treat those as reference points rather than a rule for your house. The right level depends on what's already there and how the attic is built.

What R-Value Means

R-value is thermal resistance. Two things determine it:

• The material and its density, which set the R-value per inch.
• The installed thickness.

Multiply them and you get the total R-value of the layer. Layers add up. R-19 already in the attic plus R-19 added on top makes roughly R-38.

What matters for performance is the total R-value, not the per-inch number. R-38 of fiberglass and R-38 of cellulose resist heat equally. The per-inch number only matters when space is limited.

Insulation R-Value Per Inch

ENERGY STAR's shortcut for the common attic materials (fiberglass, cellulose, and mineral wool) is about R-3 to R-3.5 per inch. That's close enough for estimating what you already have.

R-30, R-38, and R-49 Insulation: How Thick?

Fiberglass batts are the simplest, because each batt is labeled. Owens Corning lists:

• R-30: 9½ inches
• R-38: 12 inches
• R-49: 14 inches

Denser cathedral ceiling versions are thinner: R-30C at 8¼ inches and R-38C at 10¼ inches.

Blown-in insulation depends on the product's coverage chart. Under the Federal Trade Commission's R-Value Rule, every loose-fill product has to publish a chart showing the minimum settled thickness, weight per square foot, and coverage per bag at each R-value. The examples at R-38:

• Owens Corning ProPink L77 blown fiberglass: at least 13¼ inches
• GreenFiber loose-fill cellulose: 11.7 inches when installed, settling to 10½ inches

For R-30 or R-49 with blown-in insulation, use the chart on the specific product being installed. Brands differ enough that a generic number can leave you short.

Spray foam is installed by thickness. Johns Manville rates its open-cell foam at about R-3.8 per inch and its closed-cell foam at about R-7.0 per inch. At those ratings, R-38 would take roughly 10 inches of open-cell or a little over 5 inches of closed-cell.

Recommended Insulation R-Value for San Antonio

San Antonio is in IECC Climate Zone 2. ENERGY STAR's recommended levels for adding insulation to existing wood-framed homes in Zone 2 are:

• Attic, uninsulated: add R-49
• Attic, with 3 to 4 inches already there: add R-38
• Floors: R-13

These are cost-effectiveness recommendations for retrofits, not a requirement for your specific home. Building code requirements generally apply to new construction and permitted work, and depend on the assembly. The appropriate level for your attic depends on the existing insulation, the space available, and whether the attic is vented or insulated at the roofline. Our attic insulation service starts by measuring those.

How to Estimate What You Have Now

1. Look across the attic. ENERGY STAR's quick check: if the insulation is level with or below the tops of the floor joists, you should add more. If it's well above them and even, you probably have enough.
2. Measure the depth in several spots, including near the eaves and the hatch.
3. Multiply the inches by about 3 for a rough R-value, per ENERGY STAR. Six inches is roughly R-18. Twelve inches is roughly R-36.

Why Thickness Doesn't Always Equal R-Value

Compression. Squeezing a batt into less space lowers its R-value. Owens Corning notes its R-19 batt, which is 6¼ inches thick, delivers R-18 when installed in a 5½ inch cavity. Stepping on insulation, storing boxes on it, or stuffing batts behind pipes has the same effect.

Density. The same blown fiberglass product that delivers about R-2.9 per inch loose in an attic is listed by Owens Corning at R-15 in a 3½ inch wall cavity when dense-packed. That's more than R-4 per inch.

Settling. Loose-fill insulation compacts after installation. DOE data puts cellulose settling at 10 to 20 percent and fiberglass at 2 to 4 percent. Coverage charts account for this, which is why cellulose is installed deeper than its settled depth.

Gaps. Uncovered spots let heat bypass the insulation around them. Low areas near the eaves and hatch are the usual culprits.

Common Mistakes

1. Comparing R-value per inch instead of total R-value when choosing a material.
2. Measuring cellulose to its settled depth on installation day instead of the initial installed depth.
3. Compressing batts to fit a shallow space.
4. Assuming the whole attic matches the depth by the hatch. Check several spots.
5. Adding depth without sealing air leaks underneath.

Whether you're working with fiberglass insulation or cellulose insulation, the target is the same: an even layer at the total R-value your attic needs.

Frequently Asked Questions

How many inches of insulation do I need for R-49?

Fiberglass batts at R-49 are 14 inches thick (Owens Corning). Blown-in insulation varies by product. Check the coverage chart on the specific product, since the required depth differs between fiberglass and cellulose and between brands.

Is thicker insulation always better?

Only if it keeps its R-value. Thicker, properly installed insulation adds R-value. Compressed or unevenly spread insulation may be thick in places without delivering its rated performance.

What's the difference between R-30 and R-38?

R-38 resists heat about 27% more than R-30. In fiberglass batts, that's roughly 12 inches instead of 9½ inches. Whether the extra depth is worth it depends on what's already in the attic and how much room there is, especially near the eaves.

What R-value does my 4 inches of old insulation have?

Using ENERGY STAR's shortcut of about R-3 per inch, 4 inches is roughly R-12. Older insulation that has compressed or settled may be less. ENERGY STAR's Zone 2 guidance for attics with 3 to 4 inches is to add R-38.

The Takeaway

Insulation performance comes from total R-value:

• Thickness only tells you the R-value once you know the material. Batts, blown fiberglass, cellulose, and spray foam all reach the same R-value at different depths.
• For blown-in insulation, the bag's coverage chart is the real answer.
• For San Antonio attics, ENERGY STAR's Zone 2 retrofit levels (add R-49 to a bare attic, or R-38 over 3 to 4 inches) are a useful reference.

The right target for your home comes from measuring what's already there. Get an Estimate and we'll measure your attic and recommend a level that fits.

References

• ENERGY STAR: Recommended Home Insulation R-Values and Well-Insulated and Sealed Attic. energystar.gov
• ENERGY STAR: A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR. energystar.gov
• U.S. Department of Energy, Building America Solution Center (PNNL): Loose-Fill Insulation. basc.pnnl.gov
• Federal Trade Commission: Labeling and Advertising of Home Insulation (R-Value Rule), 16 CFR Part 460. ftc.gov
• Owens Corning: EcoTouch PINK Fiberglas Insulation Submittal Sheet and Product Data Sheet and PROPINK L77 Loosefill Insulation Submittal Sheet and Fact Sheet. owenscorning.com
• GreenFiber: Loose Fill Cellulose Insulation Fact Sheet (INS551LD). greenfiber.com
• Johns Manville: JM Corbond Open-cell Spray Foam and JM Corbond Closed-cell and Open-cell SPF. jm.com