Fiberglass Insulation: R-Values, Costs & Complete Buyer's Guide

Quick Answer: Fiberglass is the most common and affordable insulation in the US at $0.30–$1.50/sq ft installed (batts) or $0.50–$2.00/sq ft (blown-in). R-value ranges from R-3.0 to R-4.3 per inch depending on density. Available as batts (pre-cut blankets for standard stud cavities) or blown-in (loose-fill for attics and retrofit). Its biggest weakness: fiberglass is extremely sensitive to installation quality — a poorly installed R-19 wall can perform like R-13 or worse.

Table of Contents

• Batts vs Blown-In Fiberglass
• Complete Product Table
• Standard vs High-Density Fiberglass
• Kraft-Faced vs Unfaced
• Installation Quality — The Make-or-Break Factor
• Compression Effects
• Cost Analysis
• Fiberglass vs Other Materials
• Health & Safety
• Major Brands
• When Fiberglass Is the Right Choice
• Common Mistakes
• Key Takeaways
• FAQ

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Batts vs Blown-In Fiberglass

Fiberglass comes in two fundamentally different forms, and they perform very differently.

Batts are the product most people picture when they think of fiberglass. They come in standard widths — 15 inches for 16-inch on-center framing and 23 inches for 24-inch on-center — and are sized to friction-fit between studs. We use them primarily in new construction walls where cavities are open and accessible.

Blown-in fiberglass is loose-fill material applied with a blowing machine. In attic applications, the density is low (about 1 lb/ft³ at R-30 coverage) and the R-value per inch is the lowest of any insulation at R-2.2 to R-2.7. But depth compensates — at 18–22 inches, blown-in fiberglass hits R-44 to R-49 for $0.50–$2.00/sq ft installed. At higher densities (wall dense-pack), R-value improves to R-3.7–R-4.3 per inch.

Blown-in fiberglass also settles far less than cellulose — just 1–3% versus cellulose's ~20%. In an attic, that means blown-in fiberglass largely retains its installed depth over time.

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Complete Product Table

Every standard fiberglass batt product, with the thickness, cavity, and R-value data you need to specify the right product.

Source: Owens Corning product specifications. See our insulation thickness chart for a cross-material thickness comparison.

Notice the R-value per inch varies significantly. High-density products (R-15 at ~R-4.29/inch, R-21 at ~R-3.82/inch) pack more thermal resistance into the same cavity depth. Standard density products (R-19 at ~R-3.04/inch) deliver lower R-per-inch but are cheaper. See our R-value per inch chart for a full material ranking.

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Standard vs High-Density Fiberglass

This is one of the best-kept secrets in insulation. High-density fiberglass batts deliver significantly more R-value in the same cavity depth for a modest cost increase.

In a 2×4 wall, upgrading from R-13 to R-15 costs roughly $0.05–$0.15 more per square foot. Over a 1,500 sq ft home with approximately 1,000 sq ft of exterior wall area, that's an extra $50–$150 for a 15% improvement in wall insulation — one of the cheapest performance upgrades available.

We recommend high-density fiberglass in exterior walls for every project where budget allows. Interior partition walls don't need thermal R-value, so standard density is fine there (or mineral wool if you want soundproofing).

Pro Tip: In climate zones 3–4 where code requires R-20 walls (per 2021 IECC Table R402.1.3), a 2×6 wall with R-21 high-density fiberglass gets you to R-21 in the cavity — meeting the R-20 wall requirement without needing continuous exterior insulation. That alone can save $1,500–$3,000 on a typical home build versus adding rigid foam sheathing.

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Kraft-Faced vs Unfaced

Fiberglass batts come in two configurations: kraft-faced (brown paper backing on one side) and unfaced (no backing).

Kraft-faced batts have a kraft paper vapor retarder laminated to one side, rated at approximately 1 perm — making it a Class II vapor retarder. The facing should always be oriented toward the warm side of the assembly (interior in cold climates). Use kraft-faced in exterior walls and cathedral ceilings where you need a vapor retarder.

Unfaced batts have no vapor retarder. Use them when: adding insulation on top of existing insulation (a vapor retarder in the middle of the stack traps moisture), insulating interior walls for sound, or when a separate vapor retarder is specified (like polyethylene sheeting in cold climates or a smart membrane like CertainTeed MemBrain).

Common mistake: installing kraft-faced batts with the facing toward the cold side. In cold climates, this traps interior moisture in the wall cavity. In hot-humid climates (zones 1–2), the warm side is actually the exterior — some builders skip the kraft facing entirely and use unfaced batts with a separate interior vapor strategy.

For the complete breakdown, see faced vs. unfaced insulation and our vapor barrier guide.

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Installation Quality — The Make-or-Break Factor

This section matters more than anything else in this article. Fiberglass insulation's real-world performance is entirely dependent on installation quality. No other insulation type has this large a gap between lab-tested R-value and field performance.

RESNET (Residential Energy Services Network) grades insulation installation on three tiers:

Grade I — Perfect installation. Complete cavity fill, uniform thickness, no gaps, no voids, no compression around obstructions. Insulation contacts all six sides of the cavity. Material is split and fitted around wiring, plumbing, and electrical boxes.

Grade II — Minor imperfections. Small gaps or compression in spots, up to 2% void area. Acceptable but not ideal.

Grade III — Visible defects. Missing insulation in areas, significant compression around wiring and plumbing, gaps between batts, batts cut too short or not fitted to cavity width. Insulation stuffed rather than installed.

The performance gap between Grade I and Grade III is 30% or more. A Grade III R-19 wall performs like an R-13 wall — or worse. We've inspected hundreds of homes and can say confidently that the majority of fiberglass batt installations we see in existing homes are Grade II or Grade III.

Specific failures we encounter regularly:

• Batts stuffed behind electrical wiring instead of split around it, leaving a void on the warm side
• Batts compressed to fit around plumbing, reducing R-value in that section by 25–40%
• Batts cut too narrow, leaving ½-inch gaps along studs (heat flows through the path of least resistance — a ½-inch gap devastates performance far more than its area would suggest)
• Batts installed with kraft facing folded over instead of stapled flat to framing faces
• R-19 batts compressed into 2×4 cavities (delivers R-13, not R-19 — see compression section below)

For a deeper explanation of why installation quality matters more than label R-value, see What Is R-Value? and our discussion of thermal bridging. Building Science Corporation's field research on insulation performance consistently shows that real-world fiberglass performance depends more on installation grade than on the product's labeled R-value.

Pro Tip: If you're hiring a builder or insulation contractor, ask them specifically: "Do you install to RESNET Grade I standards?" If they don't know what you're talking about, that tells you something. Better yet, hire a HERS rater to inspect the insulation before drywall goes up — it costs $150–$300 and is the single best quality-control step you can add to a new build.

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Compression Effects

Compressing fiberglass batts is one of the most common and costly insulation mistakes. The R-value on the label assumes the product is installed at its designed thickness.

The physics: fiberglass insulates by trapping still air in tiny pockets between glass fibers. Compression forces the fibers closer together, reducing the trapped air volume. The R-value per inch actually increases slightly with compression, but the total R-value drops because you've eliminated thickness. The net result is always lower total R-value.

The rule is simple: use the product designed for your cavity depth. R-13 or R-15 for 2×4 walls. R-19 or R-21 for 2×6 walls. R-25 for 2×8. Never compress a higher-R product into a shallower cavity and expect to retain the label R-value. Check our insulation thickness chart for correct pairings.

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Cost Analysis

Fiberglass wins on cost — no other insulation material comes close on a per-square-foot basis.

For comparison, mineral wool runs $1.00–$2.10/sq ft (40–70% more), cellulose runs $0.60–$2.30/sq ft (blown-in), and closed-cell spray foam runs $1.50–$5.00/sq ft (3–5× more).

Project Cost Examples (2025–2026)

A 1,500 sq ft ranch in climate zone 5:

• Attic to R-49 (blown-in fiberglass, 1,200 sq ft): $600–$2,400 installed
• Exterior walls, new construction (R-21 high-density batts, ~1,000 sq ft): $400–$1,700 installed
• Basement walls (R-15 batts + separate vapor barrier, 600 sq ft): $300–$1,100 installed

DIY fiberglass batts save 40–60% on labor. Machine rental for blown-in runs about $100/day — and many home centers offer free rental with a 20+ bag purchase. Energy Star's insulation recommendations can help you determine the right R-value target before you buy. See our insulation cost calculator for personalized pricing and insulation cost per square foot for regional data.

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Fiberglass vs Other Materials

We publish detailed head-to-head comparison guides for each matchup. Here's the quick summary:

Fiberglass vs Cellulose: Cellulose provides better coverage around obstructions, offers modest air sealing (dense-pack), and has higher recycled content (80% vs 40–60%). Fiberglass costs less, doesn't settle, and is non-combustible. For attic blow-in, cellulose is marginally better; for new construction walls with good installers, fiberglass is fine. → Fiberglass vs Cellulose

Fiberglass vs Mineral Wool: Mineral wool delivers higher R-value per inch (R-4.2 vs R-3.1 typical), dramatically better fire resistance (2,150°F vs 1,300°F), superior sound (NRC 1.0+ vs 0.85–0.95), and won't absorb moisture. Fiberglass costs 40–70% less. If fire safety, soundproofing, or moisture resistance matters, mineral wool wins. If pure budget matters, fiberglass wins. → Fiberglass vs Mineral Wool

Fiberglass vs Spray Foam: Spray foam provides air sealing that fiberglass cannot, plus higher R-per-inch (closed-cell). But spray foam costs 3–5× more and requires professional installation. For most walls in new construction, fiberglass + separate air sealing is the practical choice. → Spray Foam vs Fiberglass

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Health & Safety

Fiberglass insulation causes skin, eye, and respiratory irritation during handling. The glass fibers are mechanically irritating — wear long sleeves, gloves, safety glasses, and an N95 respirator when installing.

Cancer risk: IARC (International Agency for Research on Cancer) classified fiberglass as Group 2B (possibly carcinogenic) in 1988, but this was based on older formulations. In 2001, IARC reclassified biosoluble glass wool fibers (which includes modern fiberglass insulation) as "not classifiable as to carcinogenicity" (Group 3). Modern fiberglass insulation is not considered a cancer risk by current scientific consensus.

Formaldehyde: Traditional fiberglass uses a phenol-formaldehyde binder that can off-gas at low levels. Formaldehyde-free options exist — Knauf EcoBatt uses an acrylic binder derived from bio-based materials and has GREENGUARD Gold certification. Johns Manville and other manufacturers also offer formaldehyde-free lines.

Asbestos: Modern fiberglass insulation contains zero asbestos. Fiberglass was developed as an asbestos replacement. If you're working in a pre-1980 home and encounter insulation, have it tested before disturbing it — but any fiberglass manufactured after 1980 is asbestos-free.

For the full safety analysis, see Is Fiberglass Insulation Safe?.

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Major Brands

Owens Corning — The market leader. Their pink NEXT GEN fiberglass batts are the most widely available product line. ProPink L77 blown-in fiberglass is a top attic product. Comprehensive range from R-11 to R-49 batts.

Johns Manville — Full batt range plus Climate Pro blown-in fiberglass and Spider spray-applied fiberglass (a machine-applied product that adheres to cavity walls, reducing gaps and voids versus traditional blown-in). Strong commercial and residential presence.

CertainTeed (Saint-Gobain) — Sustainable Insulation batts, InsulSafe blown-in, and the MemBrain smart vapor retarder (a variable-permeance membrane that adapts to humidity — a significant advancement over kraft-paper facing). Good product innovation.

Knauf — Known for EcoBatt (formaldehyde-free, brown-colored, GREENGUARD Gold certified). EcoFill blown-in and JetSpray semi-rigid blown products. The go-to for homeowners concerned about indoor air quality.

All four brands deliver comparable R-values. The differentiation is in binder chemistry (formaldehyde-free vs traditional), product availability in your region, and contractor preference. We don't have a strong brand loyalty — we pick whichever is available and spec'd for the job.

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When Fiberglass Is the Right Choice

Choose fiberglass when:

• Budget is the primary constraint — nothing beats $0.30–$1.50/sq ft installed
• New construction walls with competent installers who will achieve Grade I installation
• Standard cavity depths (2×4, 2×6, 2×8) where pre-cut batts fit cleanly
• DIY projects — batts are the most accessible insulation for homeowners
• Attic floors (blown-in) where depth is unlimited and settling is a concern (fiberglass settles only 1–3% vs cellulose's 20%)
• Non-combustible requirement — unfaced fiberglass is Class A, FSI ≤25, SDI ≤50

Choose something else when:

• Retrofitting existing walls without removing drywall → dense-pack cellulose (blown through small holes) is the standard
• Soundproofing is a priority → mineral wool (NRC 1.00–1.05 vs fiberglass 0.85–0.95)
• Fire resistance is critical → mineral wool (2,150°F vs fiberglass 1,300°F melt point)
• Air sealing is needed → spray foam (fiberglass provides zero air sealing)
• Moisture-prone locations → closed-cell spray foam or mineral wool (fiberglass traps moisture and loses R-value when wet)
• Maximum R-value in minimum space → closed-cell spray foam at R-6.0–7.0/inch beats fiberglass's R-3.0–4.3/inch

See our types of insulation guide for a complete comparison and our R-value insulation chart for performance data across all materials.

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Common Mistakes

1. Accepting Grade III installation. The majority of fiberglass batt installations we inspect have visible gaps, voids, or compression. A Grade III R-19 wall performs like R-13 — you've paid for insulation you're not getting. Insist on Grade I or hire someone who knows the difference.

2. Compressing batts to fit. R-19 batts (6.25" thick) stuffed into a 2×4 cavity (3.5") deliver approximately R-13, not R-19. This is throwing money away. Use R-13 or R-15 for 2×4 walls, R-19 or R-21 for 2×6 walls. Match the product to the cavity.

3. Using fiberglass batts in crawl spaces and basements. We've removed hundreds of square feet of soggy, sagging, moldy fiberglass from crawl spaces. Fiberglass traps moisture, loses R-value when wet, and sags away from the subfloor over time. For below-grade and moisture-prone applications, use closed-cell spray foam, rigid foam, or mineral wool instead.

4. Not air sealing before insulating. Fiberglass does nothing to stop air leakage. If you insulate your attic without first sealing penetrations, top plates, recessed lights, and duct boots, you'll lose 15–25% of potential energy savings through air leakage. Always air seal first, insulate second.

5. Installing kraft-facing toward the wrong side. In cold climates (zones 4+), the kraft vapor retarder goes toward the interior (warm-in-winter side). Installing it backward traps moisture in the wall cavity. In hot-humid climates, vapor management is more nuanced — consider unfaced batts with a smart vapor retarder instead. See faced vs. unfaced insulation.

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Key Takeaways

• Fiberglass is the most affordable insulation at $0.30–$1.50/sq ft (batts) and $0.50–$2.00/sq ft (blown-in), making it the default choice for budget-conscious projects.
• R-value ranges from R-3.0/inch (standard density) to R-4.3/inch (high-density), with batts available from R-11 through R-49.
• High-density batts (R-15 in 2×4, R-21 in 2×6) deliver 10–15% more R-value in the same cavity depth for ~20% more cost — almost always worth the upgrade in exterior walls.
• Installation quality is fiberglass's Achilles' heel: Grade III installation drops performance by 30%+. A Grade III R-19 wall performs like R-13.
• Never compress batts — R-19 in a 3.5-inch cavity delivers approximately R-13, not R-19.
• Fiberglass is non-combustible (Class A, FSI ≤25, SDI ≤50) but melts at 1,300–1,500°F.
• Fiberglass provides zero air sealing — always air seal before insulating, or choose spray foam for applications where air sealing is critical.
• Blown-in fiberglass settles only 1–3% (vs cellulose's ~20%), making it a durable attic option.
• For retrofits, soundproofing, fire safety, and moisture-prone locations, other materials outperform fiberglass. See our types of insulation comparison.

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FAQ

What R-value is fiberglass insulation?

Fiberglass insulation ranges from R-3.0 to R-4.3 per inch depending on density. Standard batts deliver R-11 to R-13 in a 2×4 wall (3.5") and R-19 in a 2×6 wall (6.25"). High-density batts achieve R-15 (2×4) and R-21 (2×6) in the same cavity depths. Blown-in fiberglass in attics delivers R-2.2–R-2.7 per inch but is installed thick (18–22 inches for R-49). See the complete product table above and our R-value insulation chart.

Is fiberglass insulation good enough?

For many applications, yes — fiberglass is a perfectly adequate insulation when installed properly. The emphasis is on "properly." A Grade I fiberglass installation in a well-sealed wall assembly performs comparably to more expensive alternatives. The problems arise with poor installation (Grade III), moisture exposure, or applications where air sealing is needed (fiberglass provides none). If your budget allows a competent installer and you pair fiberglass with thorough air sealing, it's absolutely good enough for walls and attic floors.

How long does fiberglass insulation last?

The fiberglass material itself lasts 80–100 years — the glass fibers don't degrade. However, effective performance life is closer to 15–30 years in many installations because of real-world factors: batts sag and create gaps, pest damage compresses or removes material, moisture reduces R-value, and blown-in fiberglass in attics can experience wind washing. A well-installed fiberglass system in a dry, sealed assembly can perform well for decades. An average installation in an older home often needs topping up or replacing after 15–20 years.

Can I install fiberglass insulation myself?

Yes — fiberglass batts are the most DIY-friendly insulation available. You need a utility knife, straight edge, tape measure, and proper safety gear (N95 respirator, safety glasses, gloves, long sleeves). Cut batts to fit the cavity precisely, split around wiring and plumbing rather than compressing, and ensure full contact with all cavity surfaces. Blown-in fiberglass for attics requires a blowing machine — available for ~$100/day rental, often free with 20+ bag purchases at home centers. Dense-pack wall applications require professional equipment. For step-by-step guidance, see how to insulate an attic.

Should I choose fiberglass or cellulose for my attic?

Both work well for attic floors. Blown-in cellulose (R-3.2–3.8/inch, $0.60–$2.30/sq ft) provides better coverage around obstructions and offers modest air-sealing properties. Blown-in fiberglass (R-2.2–2.7/inch, $0.50–$2.00/sq ft) settles much less (1–3% vs cellulose's ~20%) and is non-combustible. In our experience, cellulose has a slight edge in most attic applications due to its better conformity and air sealing. But fiberglass is a solid choice — especially if you prefer lower settling and non-combustibility. See fiberglass vs. cellulose for the full comparison.