What is R-Value for Insulation?
R-value is the standard measure of how well an insulation material resists heat flow. Higher is better. A 2×4 wall cavity packed with R-13 fiberglass batt resists about 13 units of heat flow per square foot; an attic with R-49 blown-in cellulose resists nearly four times as much. R-value is the inverse of U-factor (R = 1 / U), and IECC code minimums for Virginia and Maryland (Climate Zone 4) are R-13 to R-15 in walls, R-49 in attics, and R-19 in floors over unheated crawl spaces.
Three framing cavities, three R-values. A 2×4 wall holds R-13. An older attic with R-30 (cyan, shown for reference) is below code in our climate. A current code-minimum attic in Virginia or Maryland is R-49, which takes 16 inches of fluffy blown-in insulation.
What is a good R-value for insulation in Virginia and Maryland?
Virginia and Maryland sit in IECC Climate Zone 4 (mixed-humid). The current code minimums, set by the 2021 IECC and adopted by both states, are:
| Location | IECC CZ4 Minimum | Recommended Upgrade Target |
|---|---|---|
| Attic / ceiling | R-49 | R-60 (modest premium, big payback) |
| Wall (2×4 framing) | R-13 to R-15 cavity | R-13 plus R-5 continuous exterior |
| Wall (2×6 framing) | R-19 to R-21 cavity | R-21 plus R-5 continuous |
| Floor over unheated crawl | R-19 | R-30 if budget allows |
| Crawl space wall | R-10 continuous | R-15 continuous |
| Basement wall | R-15 continuous | R-19 continuous |
Top-up attic insulation is the single highest-payback energy upgrade for most homes in our region. Going from an older R-30 attic to a current R-49 to R-60 attic typically saves 10 to 20 percent on the heating and cooling bill and qualifies for the IRA 25C insulation tax credit (up to $1,200 per year).
Is R-13 or R-30 insulation better?
R-30 resists heat flow more than twice as well as R-13. But the question is usually really “which one should I put where?” R-13 is the standard for a 2×4 wall cavity (because 3.5 inches of fiberglass batt fits the cavity and hits R-13). R-30 is the standard for some older attics with 8 to 10 inch joists where deeper insulation will not fit. Neither product is universally “better”; they are sized to the assembly they go into.
If you can choose between an R-13 install and an R-30 install in the same space, R-30 is better. If the cavity is 3.5 inches deep, R-13 is the right product because R-30 will not physically fit.
Can you put R-30 in a 2×4 wall?
No. R-30 fiberglass batt is about 9 inches thick; a 2×4 wall cavity is 3.5 inches deep. The batt would have to be compressed by more than half to fit, and compressing fiberglass dramatically reduces its R-value (compressed R-30 in a 3.5 inch space performs at about R-12, no better than the R-13 product that was designed for the cavity).
The right way to get higher wall R-value is either to use 2×6 framing (which holds R-19 to R-21) or to add continuous exterior insulation (typically R-5 rigid foam board) over the sheathing. Both techniques add R-value without crushing the cavity insulation.
How thick is R-60 insulation?
Depends on the material. Approximate thickness for common products:
- Blown-in cellulose: About 17 inches at the manufacturer-spec installed density (R-3.5 per inch).
- Blown-in loose-fill fiberglass: About 22 inches at installed density (R-2.7 per inch).
- Fiberglass batt: About 19 inches stacked (R-3.2 per inch for high-density batt).
- Closed-cell spray foam: About 9 inches (R-6.5 to R-7 per inch).
- Open-cell spray foam: About 17 inches (R-3.5 per inch).
In most Virginia and Maryland attics, blown-in cellulose at 17 inches is the cost-effective way to reach R-60. The IRA 25C insulation tax credit applies at R-49 minimum for the deduction, so anything above R-49 qualifies.
What is the relationship between R-value and U-factor?
They are mathematical inverses: R = 1 / U, and U = 1 / R. R-value measures resistance to heat flow (higher = more resistance = better insulation). U-factor measures the rate of heat flow (lower = slower transfer = better insulation). Industry convention uses R-value for insulated assemblies (walls, attics, floors) and U-factor for windows and doors.
A wall with R-19 has a U-factor of about 0.053. A window with U-factor 0.27 has an R-value of about 3.7. Notice that the R-value of even a high-quality window is much lower than the R-value of a typical insulated wall; this is why most heat loss in a typical home is through the windows and air leaks, not through the wall insulation itself.
What DreamHome installs
DreamHome installs R-49 attic insulation (current IECC CZ4 code minimum) by default and R-60 as the recommended upgrade tier. The base product is blown-in cellulose with a class III vapor retarder. We always pair the insulation install with targeted air sealing at the attic plane (a BPI Building Analyst diagnostic identifies the leak points first), because air leaks degrade insulation performance disproportionately.
For walls, we install code-minimum R-13 to R-15 batt in 2×4 cavities and R-19 to R-21 in 2×6 cavities, with continuous exterior R-5 rigid foam when feasible. For crawl spaces, we install R-19 floor insulation or R-10 continuous on the crawl walls (depending on whether the crawl is vented or conditioned).
Red flags on someone else’s insulation quote
- R-value not specified per cavity. Every quote should list the target R-value for attic, walls, floors, and crawl separately.
- R-30 attic on a current code job. Below CZ4 minimum (R-49). Does not pass code inspection on new construction; on retrofit work, it is leaving a lot of performance on the table.
- Compressed batt insulation. R-19 stuffed into a 2×4 cavity does not perform at R-19; it performs at about R-12. The label is misleading.
- No air sealing before insulation. Air leaks short-circuit insulation. A reputable insulation install always includes targeted air sealing (gaps around can lights, attic hatches, plumbing penetrations, top plates) before the insulation goes in.
- Loose-fill fiberglass priced like cellulose. Fiberglass is roughly 25 percent less effective per inch than cellulose. Either material is fine, but the R-value math should be checked against the installed thickness.