R-value is a measure of how well insulation resists heat moving through it. The higher the number, the better the insulation performs. When you see a product labeled R-13 or R-49, that number tells you its thermal resistance: an R-49 batt will slow heat transfer far more effectively than an R-13 batt.
But R-value isn’t just a number on a package. How much insulation you actually get depends on the material, how it’s installed, and even the moisture in your walls. Understanding what drives R-value helps you make smarter choices when insulating your home.
How R-Value Works
Heat naturally flows from warm spaces to cool ones. In winter, it escapes through your walls, attic, and floors. In summer, it pushes inward from the hot exterior. Insulation slows that transfer, and R-value quantifies how much resistance a material provides.
The math behind it is straightforward: R-value equals the thickness of the insulation (in inches) divided by the material’s thermal conductivity. A material that conducts heat poorly (like trapped air pockets in fiberglass) will have a higher R-value per inch than one that conducts heat easily (like solid wood). Double the thickness of a given material and you double the R-value.
R-Value Per Inch by Material
Not all insulation materials deliver the same thermal resistance per inch of thickness. That difference matters when you’re working with limited cavity depth in a wall or trying to hit a target R-value in your attic.
- Fiberglass batts: R-3.1 per inch
- Mineral wool batts: R-3.1 per inch
- Blown cellulose (walls): R-3.7 per inch
- Blown fiberglass (attic): R-2.2 per inch
- Closed-cell spray foam (polyurethane): R-6.25 per inch
- Foil-faced polyiso rigid board: R-7.2 per inch
Fiberglass, mineral wool, and cellulose all cluster around R-3 to R-3.7 per inch. Spray foam and rigid foam boards deliver roughly double that, which is why they’re used in tight spaces where every inch counts. The tradeoff is cost: high-performance foams are significantly more expensive per square foot.
Why the Number on the Label Isn’t Always What You Get
The R-value printed on a package is the “nominal” value, tested under ideal lab conditions. What your wall actually delivers, called the “effective” R-value, is often lower. Two factors are primarily responsible: thermal bridging and compression.
Thermal Bridging
Wood studs in a typical wall have an R-value of about 5, compared to R-13 for the fiberglass batts between them. Heat takes the path of least resistance, flowing more readily through those studs. The result is an effective R-value for the whole wall assembly that’s lower than R-13.
Metal studs are far worse. Steel conducts heat so efficiently that metal framing spaced 16 inches apart can reduce a wall’s effective R-value by 63%. A wall filled with R-13 fiberglass but framed with metal studs performs dramatically below its labeled insulation value. This is why building codes allow a lower cavity R-value if you add continuous insulation on the exterior of the framing. A wall with R-13 cavity insulation plus R-5 continuous foam sheathing performs as well as an R-20 wall, because the continuous layer blocks the thermal bridge entirely.
Compression
Fiberglass batts are designed for a specific cavity depth. When you stuff a thick batt into a shallow cavity, you compress the fibers and lose R-value. For example, an R-49 batt (designed for a 14-inch cavity) drops to about R-44 when squeezed into an 11⅞-inch space. An R-38 batt compressed from 12 inches into an 11¼-inch cavity delivers only R-37. The losses get steeper with more compression. This is a common mistake in attics and walls where the framing doesn’t match the batt you bought.
Moisture Degrades Performance
Wet insulation is poor insulation. When moisture gets into insulation materials, it replaces the trapped air pockets that slow heat transfer. Water conducts heat about 25 times better than still air, so even modest moisture content can meaningfully increase heat flow through a wall.
Research on common building insulation materials shows that thermal conductivity (the inverse of R-value) rises with both temperature and humidity. In extremely hot and humid climates, the actual heat transfer through an insulated wall can be 1.5 to 1.9 times greater than what you’d calculate using the dry, lab-tested R-value. Beyond thermal performance, moisture retention also damages insulation integrity over time and can support mold growth. Proper vapor barriers and drainage planes matter as much as the R-value number itself.
Foam Insulation Ages
Closed-cell spray foams and rigid foam boards start with high R-values partly because the gas trapped inside their cells conducts heat poorly. Over time, that gas slowly diffuses out and is replaced by regular air, reducing thermal performance. This aging process is well-documented, and industry standards exist to predict how foam will perform years after installation.
The most widely used testing methods predict performance at 5 years or extrapolate out to 25 years based on accelerated aging of thin slices. When you see an R-value listed for closed-cell foam, it’s typically the “long-term thermal resistance” value rather than the initial freshly sprayed number. Fiberglass, mineral wool, and cellulose don’t have this problem because they rely on trapped air rather than special gases.
How Much R-Value You Need
The right amount of insulation depends on where you live. The U.S. Department of Energy divides the country into climate zones, with colder regions requiring higher R-values. As a general guide:
- Attics: R-30 in warm southern climates up to R-49 or R-60 in cold northern zones
- Exterior walls: R-13 to R-21 for cavity insulation, often supplemented with continuous exterior insulation
- Floors over unconditioned spaces: R-19 to R-30 depending on climate
These are minimums from energy codes and federal recommendations. Adding insulation beyond code requirements continues to reduce energy loss, though the returns diminish. Going from no insulation to R-19 in your attic makes a dramatic difference in your energy bills. Going from R-38 to R-60 saves less in absolute terms, though it still pays off over the life of the insulation in cold climates.
R-Value vs. U-Value
You’ll sometimes see windows and wall assemblies rated by U-value instead of R-value. They measure the same thing from opposite directions. R-value measures resistance to heat flow (higher is better), while U-value measures how easily heat passes through (lower is better). The conversion is simple: U-value equals 1 divided by R-value. A wall assembly with an effective R-value of 20 has a U-value of 0.05. Building codes often specify wall performance in U-values because it accounts for the entire assembly, including framing, sheathing, and air films, not just the insulation in the cavities.