Attic Insulation Calculator
Enter your attic footprint and what's already up there. This works out the extra depth needed to reach your target R-value, how many bags or batts that takes, and what it should cost.
Your project
Old blown fiberglass ≈ 2.5, cellulose ≈ 3.2, old batts ≈ 3.0
Covers cuts, compression and odd bays. 10% is the usual default.
Your results
- Target R-value
- R-60
- Climate zone 5
- Existing R-value
- R-10
- Additional depth to add
- 20 in
- Bags required
- 34 bags
- 39 sq ft per bag at this depth
- Estimated material cost
- $1,428
- Installed cost (pro)
- $2,448
- Material plus typical contractor labor
Estimates use typical US material and labor pricing. Confirm quantities against the coverage chart on the bag before you buy.
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Smart Insulation Calculator keeps your insulation jobs on your phone, offline. On iPhone? It's inside Roofing Calculator & Estimator.
Gear for this job
Blown In Cellulose Insulation
Aimee’s Workshop Attic Anchor - Insulation Ruler Depth Markers for Blown-in Insulation 30 Pack, 24 Inch Heavy Cardstock
$29.98
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Attic Measuring Rulers - Attic Insulation R-Value Ruler,Thick Card Stock,25 inches,for Ceiling Insulators Insulation (100 per Pack)
$42.99
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Attic Insulation R-Value Ruler - Attic Measuring Rulers, 25 Inch Colorful Card Stock Attic Depth Ruler for Home,(100 per Pack (100)
$42.99
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Attic Ventilation Baffles
SmartBaffle 24 Insulation Baffle - Durable Polyethylene Attic Baffles for 24" On-Center Rafter Bays with 2" Air Channel - Rafter Vents Improve Roof Ventilation & Prevent Insulation Blockage- 30 pieces
$172.50
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Sunmuliiiy Cardboard Attic Baffles 25"x24" 75 Pcs Corrugated Cardboard- Bundle Attic Insulation Baffle Rafter Vents Improve Roof Ventilation and Prevent Insulation Blockage
$84.99
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SmartBaffle 16 Attic Insulation Baffle - Polyethylene Attic Baffles for 14.5" Rafter Bays with 2" Air Channel - Rafter Vents Improve Roof Ventilation and Prevent Insulation Blockage - Black- 36 pieces
$153.00
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Prices as of Oct 11, 2026, 4:41 AM UTC. Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
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How this calculator works
- Existing R = existing depth × R-per-inch of what's up there
- R needed = target R − existing R
- Depth to add = R needed ÷ R-per-inch of new material
- Bags = (R needed × area) ÷ bag coverage rating, then add waste
Keep going
- Ducts in the attic: duct insulation calculator (R-8 wrap)
- Doing it yourself? how to blow in attic insulation
- How deep to blow it: insulation thickness calculator
- Don't forget the access: attic hatch insulation calculator
- Laying batts in the attic: R-30 insulation calculator
- Or the thicker batt: R-38 insulation calculator
- Hot upstairs rooms? Check the AC size too: BTU calculator
- Rolling out unfaced rolls across the joists? roll insulation calculator
- Venting the attic for a whole house fan: whole house fan calculator
- Before heating season: attic insulation payback by climate zone
- Leaky hatch? best attic hatch and stair covers
Frequently asked questions
Can I add new insulation over old insulation?
Yes, as long as the old material is dry and not mouldy. Never lay faced (paper or foil backed) batts over existing insulation — the facing traps moisture. Use unfaced batts or blown-in.
How many inches is R-49?
About 19–20 inches of blown fiberglass (at R-2.5 per inch), about 14 inches of cellulose, or 13 inches of high-density batts. Loose-fill bags list a coverage chart on the side — follow it.
Can I add new insulation on top of old?
Usually yes, and it is the cheapest upgrade available. Blown-in or unfaced batts go straight over existing material. The one thing not to do is lay faced batts on top of existing insulation with the facing down, because that puts a vapour retarder in the middle of the assembly where moisture can collect against it. Use unfaced material when topping up.
How deep should attic insulation be?
Divide your target R-value by the material's R per inch. At R-49 that is roughly 13.5–14 inches of cellulose or about 19.5 inches of blown fibreglass — the difference is why the same target produces very different depths. Mark the depth on a rafter before you start so you can see when you have reached it.
Should I cover the attic hatch and recessed lights?
The hatch yes, with a rigid box or a purpose-made cover, because an uninsulated hatch is a large hole in the middle of the ceiling. Recessed lights need care: only IC-rated fixtures may be covered, and anything not rated for contact needs a clearance kept around it. Burying a non-IC fixture is a genuine fire risk, not a code technicality.
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Additional depth to add
20 in