Attic Insulation: The Complete Guide to R-Values, Costs & Materials

Quick Answer: Attic insulation is the single highest-ROI energy upgrade for most homes. The 2021 IECC requires R-49 to R-60 in the attic depending on your climate zone. Blown-in cellulose or fiberglass on the attic floor delivers the best value at $0.60–$2.30/sq ft installed. The most common mistake: insulating without air sealing first — which leaves 15–25% of potential energy savings on the table.

Table of Contents

• Air Seal First — The Step Most People Skip
• R-Value Requirements by Climate Zone
• How to Check Your Existing Insulation
• Material Comparison for Attics
• Vented vs Unvented (Conditioned) Attic
• Depth Chart: How Much Insulation Do You Need?
• Attic Insulation Cost
• DIY Blown-In Attic Insulation
• R-30 vs R-38 vs R-49: Which Target?
• Common Mistakes
• Key Takeaways
• FAQ

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Air Seal First — The Step Most People Skip

We cannot overstate this: air sealing the attic floor before insulating is the single most important step in any attic insulation project. Skip it, and you leave 15–25% of potential energy savings on the table — according to DOE research on insulation and air sealing. Building Science Corporation's field research confirms that convective air leakage through the ceiling plane is the primary heat loss path in most attics — far more significant than conductive losses through insulation alone.

Insulation resists conductive heat transfer — heat moving through solid materials. It does almost nothing to stop convective losses — warm air physically rising through gaps, cracks, and penetrations in your ceiling into the cold attic above. In a typical pre-1990 home, the attic floor is riddled with air leaks: unsealed top plates, wire and pipe penetrations, recessed light housings, duct boots, bathroom exhaust fan housings, and the attic hatch itself.

We've measured homes where air sealing the attic floor reduced heating costs by $200–$400/year before a single batt of insulation was added. On a 1,500 sq ft ranch in climate zone 5, professional air sealing runs $500–$1,500 — a 1–3 year payback.

Priority Air Sealing Points

Seal these areas before any insulation goes down:

1. Top plates — where interior and exterior wall framing meets the attic floor. Often the largest cumulative air leak.
2. Electrical and plumbing penetrations — every wire, pipe, and duct that passes through the ceiling.
3. Recessed (can) lights — use IC-rated fixtures or install code-approved airtight covers over them.
4. Duct boots and HVAC penetrations — where supply and return ducts connect to ceiling registers.
5. Attic hatch or pull-down stairs — weatherstrip the frame, add rigid foam to the back of the hatch, and install a latch to compress the gasket.
6. Chimney and flue chases — use fire-rated sealant and sheet metal flashing (never spray foam near a flue).
7. Bathroom and kitchen exhaust fan housings — seal around the housing with caulk or foam.
8. Whole-house fan openings — insulated covers required when not in use.

Use caulk for gaps under ¼ inch, expanding foam for ¼ to 3 inches, and rigid foam or sheet metal backed by foam for large openings. For the complete process, read our guides on how to air seal an attic and air sealing vs. insulation.

Pro Tip: The best time to air seal an attic is when the existing insulation is minimal or has been removed. Once you've blown in 14+ inches of cellulose, accessing the ceiling plane to seal penetrations is nearly impossible. If your existing insulation is thin enough to work around, seal first. If it's already deep, consider having old insulation vacuumed out, air sealing the floor, then re-insulating. The energy savings usually justify the extra cost.

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R-Value Requirements by Climate Zone

The 2021 IECC (Table R402.1.3) sets attic insulation minimums by climate zone. The 2021 code increased ceiling requirements from R-49 to R-60 in zones 4–8 — a significant change from the 2018 code.

The R-49 exception: Where R-60 is required, R-49 over 100% of the ceiling area satisfies code if the full, uncompressed R-49 extends over the wall top plates at the eaves. This is a practical concession — it's hard to get full depth at the eaves where the roof slope meets the attic floor.

Energy Star Retrofit Recommendations

If you're adding to existing insulation, Energy Star's retrofit guidance provides separate targets:

Use our climate zone map to find your zone. For state-specific code amendments (some states lag 1–2 code cycles behind), check our insulation code requirements page or the requirements by climate zone breakdown.

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How to Check Your Existing Insulation

Before adding insulation, identify what you already have and how much.

Step 1: Identify the material.

• Pink, yellow, or white fluffy material = fiberglass (batts or blown-in)
• Gray or brown shredded paper-like material = cellulose (blown-in)
• Dense brown or greenish batts = mineral wool
• Gray/silver granules (like gravel) = vermiculite — STOP. This may contain asbestos (Zonolite brand). Do not disturb it. Have it tested by a certified lab before any work.

Step 2: Measure depth. Push a ruler or tape measure straight down through the insulation to the attic floor (drywall or plaster). Take measurements in 6–8 locations — depth is rarely uniform.

Step 3: Estimate current R-value. Multiply average depth by R-value per inch:

• Fiberglass batts: ~R-3.2 per inch
• Blown-in cellulose: ~R-3.5 per inch
• Blown-in fiberglass: ~R-2.5 per inch
• Mineral wool batts: ~R-4.2 per inch

Example: 6 inches of blown-in cellulose = 6 × 3.5 = approximately R-21. You need R-28 to R-39 more to reach R-49 to R-60.

For additional diagnostics — uneven temperatures between rooms, ice dams, high energy bills — our signs of poor insulation guide covers the full checklist.

Consider a professional energy audit. For $150–$350, an auditor performs a blower door test (measures total air leakage), infrared thermal imaging (shows exactly where insulation is missing or thin), and provides a prioritized list of improvements with estimated savings. Many utility companies offer free or heavily subsidized energy audits through their residential efficiency programs. The blower door test is particularly valuable because it quantifies your home's air leakage rate — the number one factor that insulation alone can't fix. If your audit reveals air leakage above 0.35 ACH50 (air changes per hour at 50 pascals), air sealing should be your very first priority project, before adding any new insulation.

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Material Comparison for Attics

Four materials are used in attic insulation. Here's how they compare for this specific application.

Our take: For most attic floors, blown-in cellulose is the best overall value. It delivers higher R-value per inch than blown fiberglass, flows beautifully around wiring, pipes, and junction boxes, and costs only marginally more. The 20% settling is manageable — install 20–25% extra depth. Blown-in fiberglass is a strong second choice if you prefer minimal settling and non-combustibility.

Fiberglass batts work in new construction attics with uniform joist spacing and no obstructions — but in practice, attics are full of wiring, plumbing, ductwork, and cross-bracing. Batts leave gaps around every obstruction. We rarely recommend batts for attic work anymore.

Spray foam on the roof deck makes sense only when creating a conditioned attic (bringing the attic into the thermal envelope) — typically when HVAC equipment or ductwork is in the attic. For standard unconditioned attics, spray foam on the floor is 2–5× more expensive than blown-in for the same R-value.

Pro Tip: If you're topping off existing insulation, R-values are additive. Existing R-19 plus new R-30 of blown cellulose on top = R-49 total. Just make sure to remove or slash any existing vapor retarder (kraft facing) on the old insulation — a vapor barrier trapped in the middle of an assembly traps moisture.

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Vented vs Unvented (Conditioned) Attic

These are two fundamentally different strategies, and choosing the wrong one leads to moisture problems.

Vented attic (standard approach): Insulate the attic floor. The attic itself stays unconditioned — outdoor temperature in winter, oven-hot in summer. Ventilation through soffit vents and ridge vents keeps moisture from accumulating and extends roof life. This is the approach for 90%+ of homes. Blown-in insulation on the floor is the standard.

Unvented / conditioned attic: Insulate the roof deck (underside of rafters or above the sheathing). The attic becomes part of the conditioned space — same temperature as the house. No attic ventilation is needed or wanted.

Use a conditioned attic when HVAC equipment, ductwork, or significant plumbing runs through the attic. In a vented attic, ducts in 140°F summer attic air lose 10–25% of cooling capacity. Bringing the attic inside the thermal envelope eliminates this problem.

Code requirements for unvented attics: IRC Section R806.5 requires air-impermeable insulation (spray foam) on the underside of the roof deck, OR rigid foam above the roof deck. You cannot use fiberglass batts or blown-in cellulose between rafters in an unvented attic — the moisture dynamics will cause condensation and rot on the roof sheathing.

Cost difference: A 1,000 sq ft conditioned attic (spray foam on roof deck) costs $3,000–$7,000 versus $1,200–$3,500 for blown-in on the floor. The conditioned approach costs 2–3× more but can improve overall HVAC efficiency by 15–20% if ductwork is in the attic.

Quick Decision Framework

In our experience, the conditioned attic approach pays for itself within 5–8 years in homes where HVAC ductwork runs through the attic — the efficiency gains from bringing those ducts into conditioned space are substantial. For homes with all ductwork in conditioned space already, the vented approach with blown-in insulation on the floor delivers better ROI.

Pro Tip: If you're replacing your roof, that's the ideal time to consider adding rigid foam insulation above the roof deck. You can add 2–4 inches of polyiso or XPS on top of the sheathing, then a new layer of sheathing and roofing over that. The marginal cost is much lower when the roof is already off, and you get continuous insulation with zero thermal bridging through the rafters.

For a full analysis of roof deck insulation approaches, check our cathedral ceiling insulation guide.

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Depth Chart: How Much Insulation Do You Need?

These charts tell you exactly how deep each material needs to be to hit standard R-value targets.

Blown-In Cellulose Attic Depths (Settled Thickness)

Blown-In Fiberglass Attic Depths

Blown fiberglass requires 30–50% more depth than cellulose for the same R-value due to its lower R-per-inch (R-2.2–2.7 vs R-3.2–3.8). However, it settles far less, so the installed depth remains closer to the target over time.

For a cross-material comparison, our insulation thickness chart covers every material at every R-value target.

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Attic Insulation Cost

Total Project Costs (1,000 sq ft attic, 2025–2026)

The numbers speak clearly: blown-in insulation on the attic floor delivers R-49 for $1,200–$3,500 including professional air sealing. Spray foam on the roof deck costs $3,000–$7,000 for a conditioned attic approach — justified only when HVAC equipment is in the attic.

The DOE estimates that upgrading from R-19 to R-49 in a 1,500 sq ft attic saves $200–$400 per year on heating and cooling. At $1,200–$3,500 installed, that's a payback period of 2–5 years — one of the fastest returns on any home improvement.

For detailed pricing by material and region, our attic insulation cost page has the full breakdown, and the insulation cost calculator generates personalized estimates.

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DIY Blown-In Attic Insulation

Blowing loose-fill insulation into an open attic is one of the most realistic and rewarding DIY insulation projects. It doesn't require specialized skills — just preparation, patience, and proper safety gear.

Equipment: A blowing machine is essential. Home Depot and Lowe's typically offer free machine rental with purchase of 20+ bags of insulation (~$12–$15/bag for cellulose). Third-party rental runs about $100/day.

Time estimate: Two people can insulate a 1,000 sq ft attic in 4–8 hours, not counting air sealing time (add 4–6 hours for thorough air sealing).

Basic process: Air seal all penetrations → install rafter baffles at each soffit vent → set up the blowing machine at the attic access or below → start blowing from the far end, working toward the access → use depth markers to verify consistent coverage → confirm installed depth is 20–25% above settled target for cellulose.

Safety gear (non-negotiable): N95 respirator, safety glasses, headlamp, long sleeves and pants, gloves, sturdy shoes. Use kneeling boards on joists — never step between joists onto drywall.

When DIY works: Open, accessible attic with standard joist framing, no knob-and-tube wiring (fire risk with cellulose covering it), no vermiculite (potential asbestos), and adequate headroom to move around safely.

When to hire a pro: Complex attic layouts with multiple levels, significant air sealing needed around recessed lights and ductwork, existing moisture or mold issues, knob-and-tube wiring present, or if you simply don't feel comfortable working in a hot, cramped attic.

For the complete step-by-step, read how to insulate an attic and how to install blown-in insulation.

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R-30 vs R-38 vs R-49: Which Target?

The cost difference between R-30 and R-49 in blown-in material is surprisingly small — roughly 30% more material for 63% more thermal resistance. Here's the math for a 1,000 sq ft attic with blown cellulose:

Going from R-30 to R-49 costs approximately $140–$210 more in material for a 1,000 sq ft attic. That extra ~$175 in cellulose saves an additional $50–$100/year in energy costs — a 2-year payback on the upgrade alone.

Our recommendation: always go to at least R-49 in zones 3+. The marginal cost of extra material is trivial compared to the labor and setup cost of the project itself. If you're already up in the attic with a blowing machine, adding 4–6 more inches costs almost nothing in additional effort. For the full analysis, read our R-30 vs R-38 vs R-49 comparison.

Pro Tip: If you're in zones 4–8 where code requires R-60, remember the exception: R-49 over 100% of the ceiling area satisfies R-60 if the full uncompressed R-49 extends over the wall top plates at the eaves. This is easier to achieve than true R-60 (which would need 17+ inches of cellulose everywhere, including at tight eave areas). Use rafter baffles and intentionally ensure full depth extends right to the exterior wall plane.

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Common Mistakes

1. Insulating without air sealing. We've said it three times in this article because it's the #1 mistake. Air leaks through the attic floor account for a massive share of heat loss in most homes. Seal first, insulate second. The DOE estimates air sealing alone saves 15–25% on heating and cooling. Read our air sealing vs. insulation guide for the complete priority order.

2. Blocking soffit vents with insulation. Blown-in insulation piled against the eaves blocks airflow from soffit vents, which causes moisture buildup and potential roof rot in vented attics. Install rigid foam or cardboard rafter baffles in every bay between rafters at the eaves before blowing insulation. The baffles maintain a 1–2 inch ventilation channel from soffit to ridge.

3. Compressing batts at the eaves. If using fiberglass batts, the temptation is to fold or cram them where the roof slope meets the attic floor. Compressed R-30 batts at the eaves might deliver R-12 — exactly where you need full R-value the most (directly above the exterior wall, the coldest spot on the ceiling). Use high-density batts cut to fit the eave triangle, or better yet, switch to blown-in.

4. Placing a vapor barrier in the wrong location. In cold climates (zones 4+), any vapor retarder should be on the warm side of the insulation — meaning on the ceiling, not on top of the insulation in the attic. Placing poly sheeting on top of attic insulation traps moisture in the ceiling assembly. When adding blown-in over existing kraft-faced batts, slash the old kraft facing so moisture can pass through. Our vapor barrier guide has the full details.

5. Ignoring old vermiculite insulation. Vermiculite — small gray/brown/silver granules that look like gravel — was used from the 1920s through 1990s. A significant percentage of vermiculite insulation (particularly the Zonolite brand) contains tremolite asbestos. Do not disturb it. Have it tested by a certified lab before any insulation work. If asbestos is present, professional abatement is required before adding new insulation.

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Key Takeaways

• Air seal the attic floor before insulating. This step alone saves 15–25% on heating and cooling costs — skipping it is the #1 attic insulation mistake.
• The 2021 IECC requires R-49 (zones 2–3) to R-60 (zones 4–8) for attic ceilings. R-49 over 100% of the area can satisfy the R-60 requirement if it extends to the eaves.
• Blown-in cellulose is the best value for attic floors: R-3.2–3.8/inch at $0.60–$2.30/sq ft installed. Blown-in fiberglass is a close second with less settling.
• A 1,000 sq ft attic insulated to R-49 costs $1,200–$3,500 with blown-in material — payback is typically 2–5 years.
• DIY blown-in insulation is feasible for open attics — free machine rental with 20+ bag purchase at major home centers.
• Conditioned (unvented) attics use spray foam on the roof deck and cost $3,000–$7,000 for 1,000 sq ft — justified when HVAC equipment is in the attic.
• Always target at least R-49 in zones 3+. The marginal cost of going from R-30 to R-49 in blown-in material is only $140–$210 for 1,000 sq ft.
• Install rafter baffles at the eaves to prevent blocking soffit vents.
• Check for vermiculite before disturbing any existing insulation — it may contain asbestos.

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FAQ

How much does attic insulation cost?

For a 1,000 sq ft attic insulated to R-49: blown-in cellulose runs $1,200–$3,500 installed, blown-in fiberglass $1,200–$2,800, and fiberglass batts (DIY) $800–$2,000. Add $500–$1,500 for professional air sealing. Spray foam on the roof deck (conditioned attic) costs $3,000–$7,000. Removing old insulation adds $1.00–$2.00/sq ft. DIY blown-in reduces cost to $300–$550 in material with a free machine rental. Full pricing at our attic insulation cost page.

What R-value do I need in my attic?

Per the 2021 IECC: R-30 for zone 1, R-49 for zones 2–3, and R-60 for zones 4–8. Energy Star's retrofit guidance aligns with these targets. Use our climate zone map to find your zone. Our recommendation: aim for at least R-49 in any zone above zone 1 — the cost difference between R-30 and R-49 in blown-in material is small relative to the labor cost.

Can I add new insulation over existing insulation?

Yes — R-values are additive. Blow new insulation directly over old material as long as the existing insulation is dry, free of mold and pest damage, and not vermiculite (potential asbestos). If the existing insulation has a vapor retarder (kraft facing), slash it before adding blown-in on top — you don't want a vapor barrier in the middle of the assembly. No need to remove old fiberglass or cellulose that's in decent condition. Our R-value chart shows what target you need.

Should I remove old insulation before adding new?

Usually not — unless the old insulation is wet, moldy, contaminated by pests, or contains vermiculite/asbestos. Removing old insulation adds $1.00–$2.00/sq ft to the project. The main reason to remove: if you need to access the ceiling plane for comprehensive air sealing. Some contractors vacuum out old insulation, do thorough air sealing, then re-insulate. This is the gold-standard approach but adds $1,500–$3,000 to a typical job.

Is attic insulation worth the investment?

Attic insulation is the most cost-effective energy upgrade for most homes. The DOE estimates that upgrading from R-19 to R-49 saves $200–$400/year on heating and cooling for a 1,500 sq ft home. At $1,200–$3,500 installed (with air sealing), the payback period is 2–5 years — after which you save money every year for the life of the insulation. Heating and cooling account for 50–70% of the average US energy bill (~$2,000/year), so even modest percentage improvements translate to real dollars. Check tax credits and rebates that may further reduce your cost.