Insulation & Air Quality
What Is R-Value? Insulation Thermal Resistance Explained
R-value is a material's resistance to conductive heat flow; the higher the number, the better it insulates. Layers in series simply add their R-values, and heat loss is area times temperature difference divided by total R. Framing, gaps and compression mean a wall's effective R-value is usually well below the number on the insulation label.
What R-value measures
R-value is thermal resistance: how strongly a layer resists heat conducting through it. In US units it is measured in ft²·°F·h/BTU. An R-1 layer lets 1 BTU per hour pass through each square foot for every 1 °F of temperature difference; an R-10 layer lets only a tenth of that through.
Heat flow (BTU/h) = Area (ft²) × ΔT (°F) ÷ R
U-factor = 1 ÷ R
U-factor is used for windows and whole assemblies. Lower U means less heat flow. A window rated U-0.30 is about R-3.3.
Metric R-values (RSI, m²·K/W) are smaller numbers for the same material: multiply RSI by 5.68 to get the US R-value.
Worked example: heat loss through an attic
A 1,000 sq ft attic floor at R-38, with 70 °F inside and 10 °F in the attic (ΔT = 60 °F):
1,000 × 60 ÷ 38 = 1,579 BTU/h
The same attic at R-19 loses 1,000 × 60 ÷ 19 = 3,158 BTU/h, exactly twice as much. Going from R-38 to R-60 cuts the loss to 1,000 BTU/h. Notice the pattern: each extra R removes less heat than the last, so the first few inches of insulation matter most. This is why the insulation R-value chart targets differ so much by climate. These heat-loss numbers feed directly into a heating BTU calculation.
R-values add in series
When heat flows through several layers one after another, their R-values add. A typical insulated 2×4 wall, measured through the insulated cavity:
| Layer | Approx. R-value |
|---|---|
| Inside air film | 0.68 |
| 1/2-inch drywall | 0.45 |
| 3.5-inch fiberglass batt (R-13) | 13.0 |
| 7/16-inch OSB sheathing | 0.62 |
| Siding (vinyl or wood, approx.) | 0.6 |
| Outside air film | 0.17 |
| Total through cavity | about 15.5 |
Air films are thin layers of still air at each surface that add a little resistance. An air gap only counts if it is enclosed; ventilated gaps add almost nothing.
Effective versus nominal R-value
The label on insulation is its nominal R-value: the material alone, installed perfectly. The effective R-value of the whole wall or ceiling is lower, for three main reasons.
- Thermal bridging: wood studs are only about R-1.25 per inch, so a 3.5-inch stud is roughly R-4.4. Framing covers around 20 to 25% of a typical wall, and heat takes that easier path.
- Installation gaps and compression: voids, compressed batts and gaps around wiring can reduce performance noticeably.
- Air movement: fibrous insulation does not stop air. Air leaking through or around it carries heat that R-value does not account for.
For the 2×4 wall above, heat flows in parallel through the cavity path (R-15.5) and the stud path (about R-6.9). With 25% framing:
U = 0.25 ÷ 6.9 + 0.75 ÷ 15.5 = 0.0846
Effective R = 1 ÷ 0.0846 ≈ 11.8
So an "R-13 wall" performs closer to R-12 overall, including all the layers and air films. Continuous insulation outside the studs (rigid foam or mineral wool board) is the most effective way to close that gap.
R-value per inch by material
| Material | R per inch (typical) |
|---|---|
| Closed-cell spray foam | 6.0–7.0 |
| Polyisocyanurate board (foil-faced) | 5.6–6.5 |
| Extruded polystyrene (XPS) | 5.0 |
| Expanded polystyrene (EPS) | 3.6–4.2 |
| Open-cell spray foam | 3.5–3.8 |
| Mineral wool batt or board | 3.0–4.2 |
| Fiberglass batt | 2.9–4.3 |
| Blown cellulose | 3.2–3.8 |
| Blown fiberglass | 2.2–2.9 |
| Softwood lumber | about 1.25 |
| Brick | about 0.1–0.2 |
| Concrete | about 0.08 |
Polyiso's R-value falls in very cold weather, and some foams lose a little over time as blowing agents diffuse out. A full list by product type is in the R-value chart.
What R-value does not cover
R-value is only about conduction. Radiant barriers work by reflecting radiant heat and are rated differently. Air sealing, which stops convective heat loss, often delivers as much savings as adding insulation, and should come first. Thermal mass, as in thick masonry walls, slows heat flow in time but does not show up in R-value. When you upgrade insulation, smaller heating and cooling loads may allow smaller equipment next time; see the AC tonnage calculator and furnace sizing calculator.
Using R-value to compare upgrades
Because heat flow is proportional to 1 ÷ R, the useful way to compare upgrades is the change in U-factor, not the change in R. Adding R-11 to an uninsulated R-3 wall cavity cuts U from 0.33 to 0.071, a reduction of about 0.26. Adding R-11 on top of an R-38 attic cuts U from 0.026 to 0.020, a reduction of only 0.006. Same added R-value, more than 40 times the benefit in the first case.
Multiply the U-factor change by area, by the temperature difference and by heating hours to estimate energy saved, then compare that with the installed cost. In practice this means: insulate empty spaces first, seal air leaks, and only then add depth to areas that are already reasonably insulated.
Frequently asked questions
Is a higher R-value always better?
For insulating performance, yes, but returns diminish. Going from R-11 to R-30 saves far more than going from R-49 to R-60, so compare cost per unit of heat saved.
Can I stack two batts to add R-values?
Yes, as long as neither is compressed. Two R-19 layers give roughly R-38. Use unfaced batts for the top layer to avoid trapping moisture between vapor retarders.
What is the R-value of a window?
Much lower than a wall. Single-pane glass is about R-1, double-pane about R-2, and good low-e, gas-filled double or triple glazing about R-3 to R-5 (U-0.33 to U-0.20).
Last reviewed 2026-10-28