Wall Insulation: Complete Guide to R-Values, Methods & Cost

Quick Answer: Wall insulation requirements range from R-13 in climate zones 1–2 to R-20+5ci in zones 5–8 per the 2021 IECC (Table R402.1.3). The 2021 code now requires continuous insulation (ci) in cold climates — because cavity insulation alone delivers 19–28% less than its label R-value due to thermal bridging through wood framing. Retrofit options for existing homes include dense-pack cellulose at $1.50–$3.00/sq ft or adding rigid foam during a re-siding project at $3.00–$8.00/sq ft.

Table of Contents

• Cavity Insulation vs Continuous Insulation
• The Thermal Bridging Problem
• R-Value Requirements by Zone
• Wall Assembly R-Value: The Full Picture
• New Construction Wall Options
• Retrofit: How to Insulate Existing Walls
• Material Options for Wall Cavities
• Cost by Method
• Steel Stud and Masonry Walls
• Common Mistakes
• Key Takeaways
• FAQ

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Cavity Insulation vs Continuous Insulation

Every wall insulation strategy boils down to two components — and most homes need both.

Cavity insulation fills the space between studs: fiberglass batts, mineral wool batts, cellulose, or spray foam. This is what most people picture when they think of wall insulation. Cavity insulation is limited by stud depth — a 2×4 wall gives you 3.5 inches, a 2×6 wall gives you 5.5 inches.

Continuous insulation (ci) covers the exterior of the framing — typically rigid foam board (EPS, XPS, or polyiso) or mineral wool board (Rockwool Comfortboard 80). It runs unbroken over studs, headers, corners, and window framing. The "continuous" part is what matters: it breaks the thermal bridge created by the framing members.

The 2021 IECC now requires continuous insulation in zones 4–8. This is the biggest shift in residential wall energy code in the last decade — and it's driven by the thermal bridging data we'll cover next.

Our exterior wall / continuous insulation guide covers material selection, attachment, and moisture management in detail.

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The Thermal Bridging Problem

This is the most underappreciated concept in residential insulation. Wood framing conducts heat approximately 3–4× faster than the insulation between it. In a standard wall framed 16 inches on center, 23–25% of the wall area is lumber — studs, top plates, bottom plates, headers, corners, and cripples. Building Science Corporation's research on wall performance consistently shows that this framing significantly degrades whole-wall R-value.

The pattern is clear: cavity insulation alone loses 19–28% of its label R-value to thermal bridging. Adding even R-5 of continuous insulation over a 2×6 wall jumps the whole-wall R-value from ~R-15.5 to ~R-19 — a 23% improvement. Adding R-10 ci to a 2×4 wall delivers ~R-21 whole-wall, outperforming a much thicker 2×6 wall with cavity insulation only.

ORNL's whole-wall R-value calculator models these assemblies including framing factors, headers, corners, and penetrations. Use it — or our R-value calculator — to understand your actual wall performance.

Pro Tip: Thermal bridging through framing accounts for an estimated 10–15% of total residential energy use in the US. If you're building new or re-siding, adding 1–2 inches of continuous exterior insulation is the single most impactful wall upgrade you can make. In a 1,500 sq ft home, it adds $1,500–$4,000 to the project but typically saves $150–$300/year in energy costs — a 5–10 year payback before factoring in comfort improvements.

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R-Value Requirements by Zone

The 2021 IECC provides multiple compliance paths for walls. Here are the options by zone:

ci = continuous insulation. Per 2021 IECC Table R402.1.3.

What this means practically:

Zones 1–2 are simple: R-13 in a 2×4 wall meets code. Standard fiberglass or mineral wool batts.

Zone 3 introduces a choice: either R-20 in a 2×6 cavity (R-21 high-density fiberglass or R-23 mineral wool) or R-13 cavity + R-5 continuous exterior insulation. Both approaches meet code — the second is often easier with standard 2×4 framing plus 1 inch of rigid foam.

Zones 4 and above are similar to zone 3 but with a performance path option (U-0.045) that allows flexibility.

Zones 5–8 require both cavity and continuous insulation for the first time. The common approach: fill the 2×6 cavity with R-20 or R-21 insulation, then add R-5 continuous rigid foam or mineral wool board on the exterior. The alternative — R-13 cavity + R-10 ci — uses a 2×4 wall with thicker exterior foam. Either works.

Check insulation code requirements for your state-specific amendments. Energy Star's Seal and Insulate program provides simplified retrofit recommendations by ZIP code.

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Wall Assembly R-Value: The Full Picture

Every layer in a wall contributes to the total R-value. Here's a walkthrough for a typical 2×6 wall in zone 5 (code: R-20+5ci):

That's the R-value through the cavity. Through the stud path (23–25% of the wall area), the R-21 batt is replaced by R-6.88 for the 2×6 stud — so the stud-path R-value is approximately R-14.9. Blending both paths gives a whole-wall R-value of approximately R-24–R-25.

The R-5 continuous insulation contributes equally to both paths, which is why ci improves whole-wall performance disproportionately. Our R-value calculator automates this math for any assembly.

For a deeper explanation of how R-values work across layers, What Is R-Value? covers the fundamentals.

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New Construction Wall Options

Cavity materials:

• Fiberglass batts — cheapest at $0.30–$1.50/sq ft, but sensitive to installation quality
• Mineral wool batts — 40–70% premium, but higher R-per-inch (R-23 vs R-19 in a 2×6), plus fire and sound benefits
• Dense-pack cellulose — blown in, R-3.5–3.8/inch, provides partial air sealing
• Closed-cell spray foam — R-6.0–7.0/inch, provides air barrier + vapor retarder, but 3–5× more expensive

Continuous insulation materials:

• Polyiso — highest R-per-inch (R-5.6–6.5) but derated in cold climates. Best in zones 3–4.
• XPS — R-4.5–5.0/inch, excellent moisture resistance. Good for all zones.
• EPS — R-3.6–4.4/inch, cheapest, most temperature-stable. Our pick for zones 5+ exterior.
• Mineral wool board (Comfortboard 80) — R-4.2/inch, non-combustible, vapor-permeable. Premium but excellent for fire zones and moisture-sensitive assemblies.

For the full material comparison, our types of insulation guide covers all options with costs and performance data.

Choosing between a thicker wall or adding continuous insulation is one of the first framing decisions in new construction. A 2×6 wall with cavity fill alone can meet code through zone 4, but zones 5+ require adding continuous insulation regardless of wall depth. Even in zones where ci isn't code-mandated, adding R-5 of exterior foam to a standard 2×4 wall often outperforms a deeper 2×6 cavity because it eliminates thermal bridging at every stud, header, and corner — the exact spots where cavity insulation can't help.

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Retrofit: How to Insulate Existing Walls

Insulating existing walls without demolition is harder and more expensive than attic insulation — but the energy savings are real, especially if your walls are currently uninsulated.

Dense-pack cellulose is the most common and cost-effective wall retrofit. An installer drills 2–3 inch holes through the siding (one per stud bay), inserts a fill tube, and blows cellulose to 3.0–3.5 lb/ft³ density. The holes are plugged, patched, and painted. A 1,500 sq ft home with ~1,000 sq ft of wall area runs $1,500–$3,000 for the cellulose fill. The material provides R-12 to R-13 in a 2×4 cavity plus meaningful air-sealing improvement. Our guide on insulating walls without removing drywall walks through the process in detail.

Adding rigid foam during re-siding is the gold-standard retrofit — and the only practical way to add continuous insulation to an existing home. If you're re-siding anyway, adding 1–2 inches of rigid foam board under the new siding adds R-5 to R-10, breaks thermal bridges, and costs only $1.50–$4.00/sq ft more than siding alone (the siding crew is already there). We strongly recommend this for any home in zones 4+ that's getting new siding.

For old houses with unique wall configurations (balloon framing, plaster on lath, no sheathing), our insulation for old houses guide covers the specific challenges and solutions.

Pro Tip: Wall insulation retrofit payback is typically 5–10 years — longer than attic insulation (2–5 years). Prioritize your attic first, then walls, then basement/crawl space. The exception: if your walls are completely uninsulated (common in pre-1960 homes), the improvement from R-0 to R-13 is dramatic — you'll feel the comfort difference immediately, and energy savings often exceed 15% on heating costs alone.

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Material Options for Wall Cavities

Quick comparison of what works in wall cavities — with links to each material's full guide:

Fiberglass batts: The default choice for new construction. R-13 to R-15 in a 2×4, R-19 to R-21 in a 2×6. Cheapest at $0.30–$1.50/sq ft. Sensitive to installation quality — a Grade III install (common) drops real-world performance by 30%+.

Mineral wool batts: Premium option delivering R-15 (2×4) or R-23 (2×6). Costs 40–70% more than fiberglass but adds fire resistance (2,150°F+), superior sound (NRC 1.0+), and hydrophobic moisture handling. Our recommendation for any project where the budget accommodates the premium.

Dense-pack cellulose: R-3.5–3.8/inch blown to 3.0–3.5 lb/ft³. The only realistic option for retrofitting enclosed wall cavities without demolition. Provides meaningful air sealing — a major advantage over batts in retrofit applications. The fiberglass vs. cellulose comparison covers the tradeoffs.

Spray foam: Closed-cell at R-6.0–7.0/inch delivers R-21+ in a 2×4 — the highest R-value achievable in that cavity depth. Also provides air barrier and vapor retarder. Costs 3–5× more than batts. The "flash and batt" approach (2" closed-cell + fiberglass fill) offers a cost-effective middle ground. Compare options at spray foam vs. fiberglass and spray foam vs. cellulose.

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Cost by Method

New Construction Wall Insulation (2025–2026, per sq ft of wall area)

Retrofit Wall Insulation

Full pricing details at insulation cost per square foot and our insulation cost calculator.

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Steel Stud and Masonry Walls

Steel Stud Walls

Steel stud walls present a severe thermal bridging problem. Steel conducts heat approximately 400× faster than wood. A steel-framed 2×6 wall with R-19 fiberglass in the cavities delivers a whole-wall R-value of approximately R-6 to R-8 — a 58–68% performance loss. Building Science Corporation's guidance on steel framing is unequivocal: continuous exterior insulation is mandatory for steel stud walls in any climate where thermal performance matters. R-10 to R-15 of exterior ci is typical for steel-framed commercial and residential buildings.

The takeaway: cavity insulation alone is nearly useless in steel framing. The continuous insulation does the real work. We never specify a steel stud wall without a minimum of R-7.5 ci — and in climate zones 5+, R-10 to R-15 is standard.

Masonry Walls

Masonry walls (brick, block, stone) have R-values of R-0.20/inch for solid brick — a 12-inch masonry wall provides only R-2.4. Insulating masonry requires either interior rigid foam + a framed wall inside (common for basements and retrofits), or exterior continuous insulation with a cladding system.

For residential masonry walls, the most common retrofit approach is:

1. Apply 1.5–2 inches of closed-cell spray foam or rigid foam board directly against the interior masonry surface
2. Frame a 2×3 or 2×4 stud wall in front of the foam (for electrical, drywall attachment)
3. Optionally fill the stud wall with additional cavity insulation
4. Install drywall

This gives R-10 to R-20+ depending on foam thickness and cavity fill. The foam layer against masonry is critical for moisture control — never put fiberglass directly against masonry for the same reasons it fails against basement concrete.

Solid brick walls and moisture drying: Masonry walls manage moisture differently from framed walls. They absorb rain and rely on drying both inward and outward. Adding interior insulation slows inward drying, which can cause freeze-thaw damage in cold climates if the masonry stays wet longer. The building science approach is to use a vapor-open interior insulation system (mineral wool or open-cell spray foam against the masonry) rather than a vapor-closed one (foil-faced polyiso) in cold, rainy climates. For warm or dry climates, closed-cell spray foam or rigid foam against masonry works well because freeze-thaw isn't a concern.

For historic masonry buildings, our insulation for old houses guide covers additional considerations around moisture management, preservation requirements, and avoiding damage to irreplaceable materials.

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Common Mistakes

1. Ignoring thermal bridging. Filling a 2×6 cavity with R-21 fiberglass and calling the wall "R-21" overstates performance by 26%. The whole-wall R-value is ~R-15.5. The fix: add continuous insulation over the exterior. Even R-5 of ci makes a meaningful difference. Read more at our thermal bridging guide.

2. Wrong vapor strategy for the climate. In cold climates (zones 5+), interior vapor retarders (kraft-faced batts or poly sheeting) prevent interior moisture from entering the wall. In hot-humid climates (zones 1–2), putting poly on the interior traps summer moisture inside the wall. Mixed climates are the trickiest — consider a smart vapor retarder (like CertainTeed MemBrain) that adjusts permeability with humidity. Our vapor barrier guide has climate-specific recommendations.

3. Using fiberglass batts in basement/foundation walls. Fiberglass traps moisture, sags, and loses R-value when wet — and below-grade walls are perpetually exposed to soil moisture. We've torn out thousands of square feet of moldy, failed fiberglass from basements. Use closed-cell spray foam or rigid foam board against concrete. Full approach at our basement insulation guide.

4. Compressing batts in wall cavities. An R-19 batt (6.25" thick) stuffed into a 3.5" 2×4 cavity delivers approximately R-13, not R-19. Match the product to the cavity depth. Our insulation thickness chart has all the correct pairings.

5. Not air sealing at top and bottom plates. The top plate (where the wall meets the attic) and bottom plate (where the wall meets the subfloor) are major air leakage pathways. Seal these with caulk or foam before insulating. Check air sealing vs. insulation for the full priority list.

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Key Takeaways

• Wall insulation requirements range from R-13 (zones 1–2) to R-20+5ci (zones 5–8) per the 2021 IECC. Continuous insulation is now required in cold climates.
• Thermal bridging through framing reduces whole-wall R-value by 19–28% versus the cavity insulation label R-value. Adding continuous exterior insulation is the most effective countermeasure.
• New construction options: fiberglass or mineral wool batts in cavities + rigid foam or mineral wool board on the exterior.
• Retrofit options: dense-pack cellulose ($1.50–$3.00/sq ft) for filling existing cavities; rigid foam added during re-siding ($3.00–$8.00/sq ft) for continuous insulation.
• A 2×6 wall with R-21 cavity + R-5 continuous insulation delivers a whole-wall R-value of approximately R-19 — meeting code for most zones.
• Steel stud walls have severe thermal bridging — continuous exterior insulation is mandatory.
• Always air seal top and bottom plates, and choose the right vapor barrier strategy for your climate.
• Wall retrofit payback is typically 5–10 years. Prioritize attic insulation first, then walls.

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FAQ

What R-value do I need for my walls?

Per the 2021 IECC: R-13 for zones 1–2, R-20 (or R-13+5ci) for zones 3–4, and R-20+5ci (or R-13+10ci) for zones 5–8. The "ci" stands for continuous insulation — rigid foam or mineral wool board applied over the exterior framing. Use our climate zone map to find your zone and the R-value chart for a quick reference.

How much does wall insulation cost?

New construction: $0.30–$2.10/sq ft for cavity insulation (fiberglass cheapest, mineral wool most), plus $1.00–$3.50/sq ft for continuous exterior insulation in cold climates. Retrofit: dense-pack cellulose at $1.50–$3.00/sq ft for cavity fill, or $3.00–$8.00/sq ft for rigid foam + re-siding. A typical 1,500 sq ft home retrofit with dense-pack cellulose costs $1,500–$3,000 total. The insulation cost calculator provides project-specific estimates.

Can I insulate my walls without removing drywall?

Yes — dense-pack cellulose is blown into wall cavities through small 2–3 inch holes drilled from the exterior (or interior). The holes are plugged, patched, and repainted. This is the standard approach for retrofitting uninsulated walls and it's been done successfully on millions of homes. Injection foam is an alternative at higher cost ($2.50–$4.50/sq ft). Complete details at insulate walls without removing drywall.

Is continuous insulation really necessary?

In cold climates (zones 5+), the 2021 IECC requires it — and the physics support the requirement. Without continuous insulation, 23–25% of your wall is uninsulated wood framing acting as a thermal bridge. A 2×6 wall with R-21 batts delivers only ~R-15.5 whole-wall. Adding R-5 ci bumps that to ~R-19 — a 23% improvement. The cost for 1,000 sq ft of R-5 ci (1" rigid foam, installed) is $1,000–$2,500. The energy savings and comfort improvements make this the highest-value upgrade in cold-climate wall assemblies.

What's the best insulation for wall soundproofing?

Mineral wool leads with an NRC of 1.00–1.05 — measurably better than fiberglass (0.85–0.95) and cellulose (0.80–0.90). For maximum sound isolation, use mineral wool in the cavity combined with resilient channel and double drywall on at least one side. Closed-cell spray foam is actually one of the worst choices for sound — its rigid structure transmits vibration. Full details at our soundproofing guide.