What Is R-Value? Insulation R-Value Explained Simply

Quick Answer: R-value measures how well a material resists the flow of heat — the higher the number, the better it insulates. An R-30 attic insulates twice as well as R-15. But the R-value printed on the label is tested in perfect lab conditions; real-world performance depends on installation quality, air sealing, moisture, temperature, and thermal bridging.

Table of Contents

• R-Value Definition
• How Heat Moves Through Your Home
• R-Value vs. U-Value
• How R-Values Add Up
• 7 Factors That Change Real-World R-Value
• The FTC R-Value Rule
• Label R-Value vs. Whole-Wall R-Value
• R-Value Myths Debunked
• Key Takeaways
• FAQ

---

R-Value Definition

R-value is thermal resistance — a material's ability to slow the transfer of heat from the warm side to the cold side. The "R" stands for resistance.

The formula is straightforward:

R = ΔT / Q

Where ΔT is the temperature difference across the material (in °F) and Q is the heat flux (BTU per hour per square foot flowing through it). A higher R-value means less heat gets through for the same temperature difference.

In plain terms: if you put R-19 insulation between a 70°F room and a 30°F attic (ΔT = 40°F), the heat loss through one square foot of that insulation is 40 ÷ 19 = 2.1 BTU per hour per square foot. Upgrade to R-49 and that drops to 40 ÷ 49 = 0.82 BTU/hr/sq ft — less than half the heat loss.

You don't need to do the math yourself. The practical takeaway is this: R-value is a standardized way to compare all types of insulation on the same scale. An R-13 fiberglass batt provides the same thermal resistance as R-13 of mineral wool or R-13 of spray foam — on paper. Real-world performance is where things diverge, and we'll get to that.

For a side-by-side comparison of every insulation material's R-value, see our R-value insulation chart.

---

How Heat Moves Through Your Home

Heat moves in three ways, and insulation primarily addresses just one of them.

Conduction is heat transferring through solid materials — through the drywall, studs, sheathing, and insulation in your walls. This is what R-value directly measures. Higher R-value = slower conduction.

Convection is heat carried by moving air. A drafty wall with R-19 insulation but a gap at the top plate can lose more heat through air leakage than a sealed wall with R-13. This is why air sealing is not optional — it's arguably more important than the R-value itself. The DOE's insulation guide estimates proper air sealing reduces heating and cooling costs by 15–25%.

Radiation is heat transferred through electromagnetic waves — think of the sun heating your roof, or a hot roof deck radiating heat down into your attic. Standard insulation doesn't block radiation well. That's where radiant barriers come in, which are effective in hot climates (zones 1–3) but do little in cold climates. See our types of insulation guide for more on radiant barriers.

The bottom line: R-value only tells you about conductive resistance. A complete insulation strategy addresses all three — insulation for conduction, air sealing for convection, and radiant barriers (where applicable) for radiation.

---

R-Value vs. U-Value

R-value and U-value measure the same thing from opposite directions.

• R-value = thermal resistance (higher is better)
• U-value (U-factor) = thermal transmittance (lower is better)

The conversion is simple: U = 1/R and R = 1/U

A wall assembly with a total R-value of R-20 has a U-factor of 1/20 = 0.05. A window with a U-factor of 0.30 has an R-value of 1/0.30 = R-3.3.

In practice, R-value is used for insulation products and opaque assemblies (walls, ceilings, floors). U-factor is used for windows, doors, and whole-assembly performance in energy code compliance. The 2021 IECC provides R-value and U-factor compliance paths — you can use either one. Most contractors think in R-values; most energy modelers work in U-factors.

---

How R-Values Add Up

R-values are additive across layers. Every material in a wall or ceiling assembly contributes its R-value to the total.

Here's a real example — a standard 2×6 exterior wall assembly:

Now add R-5 continuous polyiso insulation over the sheathing:

Our R-value calculator lets you build any assembly layer by layer and get the total. This is especially useful for code compliance — the 2021 IECC zone 5 wall requirement of "R-20+5ci" means you need both cavity and continuous insulation.

Pro Tip: The air film R-values (R-0.68 interior, R-0.17 exterior) seem trivial but they're standardized values used in all energy calculations. Including them makes about a 3.5% difference in a typical wall — not huge, but it matters when you're right at code minimums.

---

7 Factors That Change Real-World R-Value

The number printed on the insulation label is tested under perfect conditions. Here's what happens in your house.

1. Installation Quality

This is the single biggest variable. RESNET (Residential Energy Services Network) grades insulation installation on a three-tier scale:

• Grade I: Complete fill, uniform thickness, no gaps, no compression, no voids around obstructions. Material is in full contact with all six sides of the cavity.
• Grade II: Minor imperfections — small gaps, slight compression in places, up to 2% void area.
• Grade III: Visible defects — missing insulation, compression around wiring/plumbing, gaps, and voids.

The performance hit from Grade III installation is 30% or more compared to Grade I. An R-19 wall installed at Grade III performs like R-13 or worse. We cannot overstate this: poor installation is the #1 reason insulation underperforms, and it's the issue we see on the majority of home inspections.

2. Compression

Insulation R-value depends on its full designed thickness. Compress an R-19 batt (6.25" thick) into a 3.5-inch 2×4 cavity and you get approximately R-13 — not R-19. The R-value per inch actually increases slightly under compression, but the total R-value drops because you've lost thickness.

Check our insulation thickness chart for the correct product/cavity pairings. This is one of the most common mistakes in insulation — and it's completely avoidable.

3. Moisture

Wet insulation is terrible insulation. Water conducts heat about 23× faster than still air, and since most insulation works by trapping tiny pockets of still air, moisture destroys performance. Fiberglass can lose 40% or more of its R-value when damp. Closed-cell spray foam and mineral wool are hydrophobic and resist this much better. Proper vapor barrier design prevents this problem entirely.

4. Temperature

The FTC R-Value Rule requires testing at 75°F mean temperature. Most insulation types hold their R-value across a wide temperature range — except polyiso. Building Science Corporation's research on temperature-dependent R-values shows polyiso's R-value drops from R-6+ per inch at 75°F to R-3.5–R-4.5 per inch when mean temperature falls below 25°F. That's a loss of up to 25% when mean temperatures drop below 50°F.

XPS foam slightly improves in cold. EPS stays the most stable across all temperatures. Fiberglass and mineral wool are unaffected.

5. Air Movement (Wind Washing)

Air moving through or over insulation dramatically reduces its effectiveness. We call this "wind washing" — common in attics where soffit vents blow air over the top of insulation or through gaps at the eaves. Loose-fill fiberglass is particularly susceptible because of its low density. Installing baffles between rafters and ensuring insulation isn't exposed to airflow is essential. Dense materials like dense-pack cellulose and spray foam resist wind washing far better.

6. Thermal Bridging

Wood conducts heat about 3–4× faster than insulation. In a standard 2×6 wall framed 16 inches on center, 23–25% of the wall surface is wood framing, not insulation. That framing acts as a thermal "bridge," reducing the effective R-value by 14–18%.

A 2×6 wall with R-23 batts and standard framing factor delivers a whole-wall R-value of about R-18 — not R-23. Adding R-5 continuous insulation over the exterior breaks the thermal bridge and brings the whole wall to approximately R-19. See our thermal bridging guide for the full analysis.

7. Settling

Cellulose insulation settles approximately 20% in the first few years after installation. If you install to R-49 depth (about 14 inches), it may settle to R-39 territory. Reputable installers account for this by over-filling — installing to 17–18 inches so the settled depth still meets the target R-value. Dense-pack cellulose in wall cavities settles much less because the higher density (3.5 lbs/cu ft vs. 1.5 lbs/cu ft for loose-fill) resists compression.

---

The FTC R-Value Rule

The Federal Trade Commission's R-Value Rule (16 CFR 460) governs how insulation manufacturers test, label, and advertise R-values in the United States. Here's what it requires:

• All R-value testing must be conducted at 75°F mean temperature using ASTM C177 or C518 test methods.
• Manufacturers must state the R-value at its installed thickness, not per inch, for products sold at specific thicknesses (like batts).
• Blown-in insulation must include a coverage chart showing bags per 1,000 sq ft needed to achieve each labeled R-value, at the specified installed thickness and settled density.
• "R-value" claims must refer only to resistance to conductive heat flow — manufacturers cannot claim R-value benefits from air sealing or radiant barrier properties.
• Retailers must have R-value fact sheets available at point of sale.

The Rule hasn't been substantially updated since 1979. The 75°F testing temperature is a known limitation — it doesn't reflect real-world cold-climate performance of materials like polyiso. Several building science organizations have pushed for temperature-dependent testing, but as of 2026, the 75°F standard remains. You can verify labeled R-values against manufacturer data sheets — for example, Owens Corning's insulation product catalog and Rockwool's technical specifications publish tested R-values for each SKU.

---

Label R-Value vs. Whole-Wall R-Value

Here's the uncomfortable truth: the R-value on the insulation label is always higher than what your wall actually delivers.

Label R-value is tested on the insulation product alone — a perfect specimen, no framing, no penetrations, no windows. Whole-wall R-value accounts for the entire wall assembly including framing members, headers, corners, window and door framing, and electrical boxes.

The difference is substantial:

The continuous insulation (ci) option narrows the gap because the exterior foam covers framing and reduces thermal bridging. This is exactly why building codes in cold climates now require cavity + continuous insulation in combination.

Oak Ridge National Laboratory (ORNL) maintains a whole-wall R-value calculator that models different assemblies including framing factors, headers, corners, and penetrations. We recommend using it — or our R-value calculator — to understand actual performance rather than relying on label values.

Pro Tip: If someone tells you their walls are "R-21" because they used R-21 batts, the whole-wall R-value is actually closer to R-17. Always think in terms of assembly performance, not individual product labels.

---

R-Value Myths Debunked

Myth: Spray Foam Is Always Better Because It Has Higher R-Value

Closed-cell spray foam does deliver R-6.0–R-7.0 per inch — the highest of any common insulation. But in an open attic where you have unlimited depth, 16 inches of blown-in cellulose (R-49 to R-56) costs $0.60–$2.30/sq ft installed versus $1.50–$5.00/sq ft for the 7+ inches of closed-cell spray foam needed to reach the same R-value. Spray foam's advantage is in tight spaces, air sealing, and moisture control — not raw R-value per dollar. Check our spray foam cost guide for when it makes financial sense.

Myth: You Can't Over-Insulate

Technically true — more R-value always reduces heat flow. But there's a clear point of diminishing returns. Going from R-0 to R-19 cuts heat loss by ~95%. Going from R-19 to R-49 saves roughly half again. Going from R-49 to R-100 would cost 2–3× as much and save only a few percent more. Beyond code levels, your money is almost always better spent on air sealing, upgraded windows, or mechanical equipment.

Myth: R-Value Is the Only Number That Matters

R-value measures conductive resistance only. It says nothing about air leakage, moisture management, fire resistance, sound performance, or long-term durability. Mineral wool at R-4.2 per inch has lower R-per-inch than closed-cell spray foam, but it won't burn (rated to 2,150°F), it doesn't absorb water, and it delivers an NRC of 1.00–1.05 for soundproofing. Picking insulation on R-value alone is like buying a car based solely on horsepower.

Myth: Older Insulation Has Lost Its R-Value

Fiberglass and mineral wool don't degrade over time if they stay dry and uncompressed. A 40-year-old fiberglass batt that's in good condition still delivers its original R-value. The exception is cellulose (which settles ~20%) and some spray foams (which can lose blowing agents over years). If your old insulation is dirty, wet, compressed, or has animal damage — that's a problem with the conditions, not the R-value degrading.

---

Key Takeaways

• R-value measures thermal resistance — higher numbers mean less heat transfer. R = ΔT / Q.
• R-value and U-value are inverses: U = 1/R. R-value is used for insulation; U-factor is used for windows and whole-assembly code compliance.
• R-values add up across layers — every component from drywall to air films contributes.
• Seven real-world factors reduce performance below label R-value: installation quality, compression, moisture, temperature, air movement, thermal bridging, and settling.
• The FTC tests at 75°F mean temperature, which overstates polyiso performance in cold climates by up to 25%.
• Whole-wall R-value is 15–19% lower than label R-value due to thermal bridging through framing.
• R-value addresses conduction only — pair it with air sealing (convection) and radiant barriers (radiation) for complete thermal management.

---

FAQ

What does R-value mean in simple terms?

R-value is a number that tells you how well insulation blocks heat from passing through it. The higher the number, the better it works. R-30 blocks twice as much heat as R-15. Every type of insulation is rated with an R-value so you can compare them directly.

What R-value do I need for my walls?

It depends on your climate zone. The 2021 IECC requires R-13 for walls in zones 1–2, R-20 (or R-13+5ci) in zones 3–4, and R-20+5ci (or R-13+10ci) in zones 5–8. "ci" means continuous insulation applied over the exterior sheathing. See the complete requirements in our R-value chart or code requirements page.

Does R-value really matter, or is air sealing more important?

Both matter, but if we had to choose one, we'd prioritize air sealing first. A perfectly sealed wall with R-13 will often outperform a leaky wall with R-21. The DOE estimates air sealing alone reduces heating and cooling costs by 15–25%. Energy Star's Seal and Insulate guide walks through the priority order for homeowners. In practice, you need both — seal first, then insulate to the R-value your climate zone requires.

How is R-value tested?

Under the FTC's R-Value Rule, insulation R-value is measured using ASTM C177 (guarded hot plate) or ASTM C518 (heat flow meter) at a mean temperature of 75°F. The test measures how much heat energy (in BTU/hr) passes through a one-square-foot sample for each degree Fahrenheit of temperature difference. The result is the R-value. This standardized test allows fair comparison between materials — but remember, your home's conditions don't match the lab.

Can I just add more insulation on top of existing insulation?

Yes — R-values are additive. If you have R-19 in your attic and blow R-30 on top, you get R-49 total. A few caveats: don't mix faced insulation in the middle of an assembly (the facing can trap moisture), remove any existing vapor barrier between layers in the attic, and make sure you air seal the attic floor before adding more insulation. Old insulation that's wet, contaminated, or heavily compressed should be removed first.