How to Estimate Insulation for Your Attic

To estimate attic insulation, measure your attic’s square footage, determine the R-value your climate zone requires, subtract any existing insulation you’re keeping, and multiply by the installed cost per square foot for your chosen material. Most attic jobs run between $1,500 and $4,500 for a 1,200 sq ft attic, depending on material and existing conditions.

Getting this wrong costs you money twice: once when you under-quote and eat the overrun, and again when the homeowner calls back because their heating bill didn’t drop. Let’s do this right.

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Quick Answer: Attic Insulation Estimate Formula

Here’s the core formula contractors use:

1. Net square footage = Attic floor area, minus obstructions (HVAC equipment, ductwork, walk boards)
2. Target R-value = Required by your climate zone (see table below)
3. Depth needed = Target R-value divided by the R-value per inch of your material
4. Material quantity = Net sq ft x depth (in feet) = cubic feet, then convert to bags or rolls
5. Installed cost = Net sq ft x price per square foot for labor + material

Use the Insulation Calculator to run these numbers automatically once you have your measurements.

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What R-Value Does Your Attic Actually Need?

R-value is the single most important variable in any insulation estimate. It determines how much material you need, which directly drives cost. The Department of Energy divides the country into eight climate zones, and the recommended attic R-values vary significantly.

Climate Zone	States (Examples)	Existing 0-3 in.	Existing 4-7 in.
Zone 1-2	FL, HI, TX Gulf Coast	R-38	R-25
Zone 3	GA, AL, AZ, NM	R-49	R-38
Zone 4	TN, OR, VA, NJ	R-49	R-38
Zone 5	CO, MN, MI, PA	R-60	R-49
Zone 6	MT, ND, ME, MN North	R-60	R-49
Zone 7-8	AK interior, northern MN	R-60	R-60

The two columns represent different recommendations based on how much insulation is already up there. A home in Nashville (Zone 4) with nothing in the attic needs R-49. That same home with 4 inches of old fiberglass already in place only needs to reach R-49 total, so you’re adding to what’s there.

Always verify local code requirements. Some municipalities have adopted more aggressive energy codes than the federal minimums shown above.

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How to Measure Your Attic Square Footage

You can’t estimate what you haven’t measured. Here’s how to get an accurate number.

Option 1: Measure from below. Pull the exterior dimensions of the house from the plans or measure the outside footprint. A 40 ft x 30 ft house has 1,200 sq ft of conditioned floor below the attic. If the attic covers the full footprint, that’s your starting number.

Option 2: Measure inside the attic. Useful for complex rooflines, additions, or partial attics. Walk the perimeter and sketch it out in sections. Add rectangles together. Subtract areas that are not accessible or not part of the thermal envelope (like a garage attic that’s already unconditioned).

Deductions to take:

• HVAC air handlers and ductwork chases
• Knee wall areas where you’ll insulate the wall instead of the floor
• Pull-down stair boxes (these get their own insulation covers)
• Water heater platforms if they’re thermally separated

Use the Square Footage Calculator to break irregular shapes into rectangles and add them up quickly.

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Material Options: R-Value Per Inch and What It Costs

Different insulation types give you different R-values per inch of thickness. This affects how deep you need to go, which affects labor time and material cost.

Material	R-Value Per Inch	Installed Cost/SF	Best For
Fiberglass batts	R-2.9 to R-3.8	$0.50 - $2.50	New construction, accessible attics
Blown-in fiberglass	R-2.2 to R-2.7	$1.00 - $3.00	Retrofits, irregular framing
Blown-in cellulose	R-3.2 to R-3.8	$1.00 - $2.50	Retrofits, eco-conscious clients
Open-cell spray foam	R-3.5 to R-3.8	$1.00 - $2.00	Air sealing + insulation combo
Closed-cell spray foam	R-6.0 to R-7.0	$2.00 - $5.00	Low-pitch roofs, moisture issues

Typical installed prices reflect material plus labor in most U.S. markets as of early 2026. Prices vary by region, access difficulty, and job size. Always verify current supplier pricing before finalizing a bid.

Blown-in materials are the go-to for most attic retrofits because they settle into irregular spaces around framing, wiring, and existing insulation. Spray foam makes sense when you’re dealing with a sealed attic (spray foam to the roof deck instead of the floor) or when moisture intrusion is a concern.

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Worked Example 1: Blown-In Upgrade, Zone 4 Home

Scenario: 1,400 sq ft ranch house in Virginia. Existing insulation is about 5 inches of old fiberglass batts (approximately R-15). Client wants to hit code minimum for Zone 4.

• Target R-value: R-49 (Zone 4 with existing 4-7 in.)
• Existing R-value: R-15 (keeping it)
• Additional R-value needed: R-49 minus R-15 = R-34
• Blown-in cellulose: R-3.5 per inch
• Depth to add: 34 divided by 3.5 = 9.7 inches, round to 10 inches
• Net attic sq ft: 1,400 minus 50 sq ft for HVAC = 1,350 sq ft

Material estimate (blown-in cellulose at 10 inches):

• Bags needed: At 10 inches, typical bag coverage is about 40 sq ft per bag (manufacturer coverage chart varies)
• 1,350 sq ft divided by 40 sq ft/bag = 34 bags

Cost estimate:

• Low end: 1,350 sq ft x $1.00 = $1,350
• Mid range: 1,350 sq ft x $1.75 = $2,363
• High end: 1,350 sq ft x $2.50 = $3,375

At the typical rate of $1.75/sf installed, this job quotes at $2,363 before any markup for access difficulty, disposal of debris, or air sealing work.

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Worked Example 2: Full Attic Spray Foam, Sealed Attic

Scenario: 900 sq ft attic in Denver (Zone 5). Client is converting to a sealed conditioned attic to bring HVAC equipment inside the thermal envelope. Closed-cell spray foam to the roof deck.

• Target R-value for roof deck application in Zone 5: R-49 minimum (check local code)
• Closed-cell spray foam: R-6.5 per inch
• Depth needed: 49 divided by 6.5 = 7.5 inches
• Net roof deck sq ft: 900 sq ft floor area x 1.25 slope factor (4:12 pitch) = 1,125 sq ft of actual surface

Cost estimate:

• Low end: 1,125 sq ft x $2.00 = $2,250
• Mid range: 1,125 sq ft x $3.50 = $3,938
• High end: 1,125 sq ft x $5.00 = $5,625

Sealed attic conversions run higher because you’re covering sloped surfaces, often in tight spaces. Account for setup time, the slope multiplier, and any overspray protection in your labor estimate.

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How to Assess Existing Insulation Before You Bid

Walking the attic before quoting is non-negotiable. Here’s what to look for:

Depth: Bring a measuring stick. Poke it down to the top of the drywall or subfloor. Measure in multiple spots because old blown-in insulation settles unevenly. Take at least 5-6 readings and average them.

Material identification:

• Yellow or pink fluffy = fiberglass batts or blown-in fiberglass
• Gray fluffy = cellulose (may be original or retrofit)
• Silver foil = radiant barrier (separate product, not R-value rated the same way)
• Hard foam boards = polyiso or EPS, usually at the hatch or over a garage

Condition: Wet or compressed insulation loses R-value. Moisture stains mean you have a leak to fix before you add insulation. Document everything with photos.

Air sealing: Check around recessed lights, top plates, plumbing penetrations, and the attic hatch. Air sealing before adding insulation is standard practice and should be a line item in your estimate, not an afterthought.

For a broader look at how this fits into your overall bidding process, see How to Estimate Construction Jobs.

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Common Mistakes Contractors Make When Estimating Attic Insulation

Forgetting the slope multiplier. If you’re applying insulation to the roof deck (sealed attic), the actual surface area is larger than the floor area. A 4:12 pitch adds about 5 percent. A 6:12 pitch adds about 12 percent. Skipping this leaves money on the table.

Not measuring existing depth. Assuming the old insulation is at code is a fast way to under-quote. Measure it. Old homes from the 1970s and 1980s often have R-11 or R-19 when the client thinks they have “plenty.”

Ignoring access. A finished garage attic with a 24-inch hatch and a pile of stored furniture costs more to work in than an open attic with a pull-down stair. Price access difficulty into your labor rate.

Skipping air sealing. Most energy auditors will tell you that air sealing delivers more savings per dollar than added insulation in many climates. If you’re already up there, air sealing should be in your scope. It’s also a good upsell.

Using manufacturer bag coverage without checking. Bag coverage charts assume a specific density. Blown-in insulation installed too loosely will settle and lose R-value. Follow the manufacturer’s installed density specs, not just the depth number.

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Pro Tips for More Accurate Bids

• Pull the local energy code, not just DOE recommendations. Some jurisdictions require R-60 where the DOE only recommends R-49.
• Take photos of existing conditions before starting. This protects you if the client questions why you replaced material instead of adding to it.
• Add an air sealing allowance as a separate line. It makes your bid more competitive while giving you room to do the job right.
• Account for attic ventilation when quoting blown-in. You need to keep soffit baffles clear and maintain the ventilation channel. Some attics need new baffles installed, which is labor.
• Check for knob-and-tube wiring. In older homes, insulating over active knob-and-tube wiring can be a code violation and a fire hazard. Get an electrician involved before you quote.

The Construction Cost Estimator can help you build out a complete line-item estimate once you have your material quantities dialed in. And if you need to calculate your crew’s fully burdened labor rate for the bid, the Labor Cost Calculator builds in payroll taxes, insurance, and benefits so your labor line item is accurate. If the attic work is part of a larger interior scope that includes drywall, the drywall cost per square foot guide will help you price that piece accurately as well.

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Frequently Asked Questions

How many square feet does a bag of blown-in insulation cover? It depends on the depth and density. Most blown-in fiberglass bags cover 40-80 sq ft at R-30, but always use the manufacturer’s coverage chart for the specific product and target R-value. Installing to the chart is also the only way to validate the product warranty.

Do I need to remove old insulation before adding new? Usually not. In most cases, you can blow new material right over old insulation in good condition. Exceptions include wet or moldy insulation, asbestos-containing materials (test before disturbing), or heavily compressed material that needs leveling first.

What is the typical labor time for an attic insulation job? A one or two-person crew can blow-in insulation for a 1,200 sq ft attic in 3-5 hours on a straightforward job. Add time for air sealing, difficult access, moving stored items, and installing baffles. A full day per crew is a safe buffer for most residential jobs.

Does attic insulation help with cooling costs or only heating? Both. Proper attic insulation keeps conditioned air from escaping in winter and reduces heat gain through the ceiling in summer. In hot climates, a radiant barrier (separate from insulation) can further reduce summer cooling loads.

Should I insulate the attic floor or the roof deck? Attic floor insulation (the ceiling of the living space below) is standard for unconditioned attics. Roof deck insulation is used for sealed or conditioned attics where you want to bring HVAC equipment and ductwork inside the thermal envelope. The choice affects cost, R-value targets, and mechanical coordination.

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Pricing ranges reflect typical installed costs in U.S. markets as of early 2026. Material and labor costs vary significantly by region, local market conditions, job site access, and insulation depth. Always obtain current pricing from local suppliers and verify applicable building codes before submitting a bid.

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