Is R-30 twice as good as R-15 (and how does R-21 fit in)?

R-value is a linear measure of thermal resistance: R-30 is not twice R-15, it's double the R-value number but the heat flow reduction is proportional to the reciprocal of R. In simple terms, R-30 will cut conductive heat loss roughly in half compared with R-15 in the same assembly, but real-world gains depend on air sealing and thermal bridges. R-21 sits between R-15 and R-30; its effectiveness depends on where it’s used (wall vs attic) and the assembly details.

Can I compress R-30 batts into a 2x6 wall cavity?

Compressing a batt reduces its R-value; manufacturers rate batts at full thickness. Trying to stuff an R-30 batt into a 2x6 cavity will lower the installed R and often creates gaps. If you need higher R in a shallow cavity, consider exterior continuous foam, closed-cell spray foam, or hollow-fill solutions designed for retrofit applications.

Will adding attic insulation cause moisture problems?

Moisture risk depends on vapor profiles, ventilation, and how the insulation is installed. Adding insulation to the attic floor generally reduces heat transfer and can lower condensation risk in the conditioned space below. But for insulated roof assemblies or when changing venting strategy, evaluate vapor control and ventilation to avoid trapping moisture. Consult prescriptive guidance for your climate and follow manufacturer and code recommendations. If you’re unsure, sealing air leaks and addressing ventilation before adding insulation reduces the chance of moisture issues.

How do I know if my house needs more insulation?

Start with an energy bill review and a simple home audit: feel for drafts, check ceiling insulation depth, and inspect rim joists and attic access. Use a blower-door test or hire an energy auditor to quantify air leakage and thermal imaging to find cold spots. For a quick financial check, run your numbers through the insulation savings calculator to estimate payback for upgrading from R-21 to R-30.

R-21 vs R-30 Insulation: Which Do You Need?

R-Value Comparisons

Compare R-21 and R-30 insulation options, costs, and when the upgrade makes sense for different climates and DIY projects.

By Graham Mann | Published: 6/3/2026

R-21 vs R-30 Insulation: Which Do You Need? If you’re deciding whether to stop at R-21 or push to R-30, this article answers that exact question for DIY builders and self-builders. The comparison covers what assemblies commonly hit R-21 and R-30, which materials achieve those R-values, installation trade-offs, and a straightforward way to estimate incremental cost and payback. Readers will learn when R-21 is sufficient, when R-30 is a sensible upgrade, and how air sealing and assembly details change the outcome. TL;DR: - R-30 provides about 43% more thermal resistance than R-21 (R‑30 ÷ R‑21 ≈ 1.43), making it the better choice for cold-climate attics and where cavity depth allows. - If your project has shallow cavities, choose exterior continuous insulation or spray foam to meet R-30 without compressing batts; use an insulation savings calculator(/tools/insulation-savings-calculator) to compare payback. - R-21 is often enough for 2x6 wall cavities in mild climates or for budget retrofi...

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