Can you compress R-20 into a 2x4 cavity?

Short answer: no. Compressing higher-R batts into a shallower cavity reduces their rated R-value; the insulation becomes denser but loses performance due to reduced trapped air. To achieve R-20 in a 2x4 wall, use continuous exterior rigid foam or high-R/in spray foam rather than compressing batts.

Is continuous exterior foam better than thicker batts?

Continuous exterior foam reduces thermal bridging by covering studs and sheathing, improving whole-wall U-factor even if cavity R is lower. This often yields better whole-wall performance than cavity-only increases and can allow a 2x4 wall to match a 2x6 cavity's thermal performance with less total thickness. Proper WRB and flashing detailing are required for durability; see the California code guidance on prescriptive approaches for examples: https://www.energy.ca.gov/sites/default/files/2020-09/2019%20Insulation%20and%20QII%20Requirementsv3ADA.pdf.

How much energy will I actually save by moving from R-15 to R-20?

Energy savings vary by climate, airtightness, and the rest of the building envelope. Typical modeled reductions in heat flow are roughly 15–25% moving from R-15 to R-20 in the wall cavity, but actual annual energy savings depend on heating degree days and existing air leakage. Use the insulation savings calculator with local weather data to estimate payback for your project.

Are there moisture risks with rigid foam or spray foam?

Yes—moisture management depends on climate, assembly, and material. Closed-cell spray foam can act as a vapor retarder and is appropriate in some conditions, while polyiso exterior foam can trap moisture if not detailed with a proper drainage plane. For detailed design, consult code guidance and manufacturer technical data sheets and consider vapor-permeability in your climate zone when selecting materials.

Which option is best for a tiny cabin?

Tiny cabins often benefit most from high R-per-inch systems: closed-cell spray foam or rigid foam plus batt/hybrid approaches because wall depth is limited. For many tiny builds, a thin layer of closed-cell spray for air sealing plus cavity fill with fiberglass or cellulose balances cost, thickness, and thermal performance; see our tiny-structure insulation notes in the best insulation for sheds guide.

R-15 vs R-20 Insulation: Which Do You Need?

R-Value Comparisons

Compare R-15 and R-20 insulation — performance, cost, installation, and which is best for your walls, attic, or retrofit project.

By Graham Mann | Published: 6/2/2026

R-15 vs R-20 Insulation: Which Do You Need? R-15 vs R-20 insulation is a common decision for budget-conscious DIY self-builders deciding between 2x4 and 2x6 wall assemblies, retrofit options, or higher-R thin-wall strategies. This article compares what each R-value actually means in real wall assemblies, how to achieve R-15 or R-20 with batts, dense-pack, rigid foam, or spray foam, and which choice makes sense by climate, wall depth, and budget. Readers will get clear installation tips, common mistakes to avoid, and scenario-based recommendations so they can pick the most cost-effective path for their project. TL;DR: - R-15 matches a standard 2x4 batt (≈3.5"), suitable for mild climates and interior partitions; choose when wall depth or budget is limited. - R-20 is typical for 2x6 cavities (≈5.5") or achieved with rigid foam or spray-foam hybrids; it reduces heat flow roughly 15–25% vs R-15, so favor it in cold climates and new builds. - If cavity depth is constrained, use continuous r...

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