Attic Insulation Calculator
Estimate the loose-fill insulation needed to top up an attic. Start with your climate target, subtract the insulation already in place, then plan bags and added depth.
Attic Top-Up
Ready to plan your attic top-up
Enter the attic size and the insulation already in place.
Last updated June 2, 2026 by our expert review team
Climate-Zone Starting Points
ENERGY STAR gives different retrofit targets for a bare attic and an attic that already has about 3 to 4 inches of insulation. Use these as planning ranges, then check local requirements.
| Climate zone | Bare attic | Existing 3 to 4 in |
|---|---|---|
| Zone 1 (Very warm) | R-30 | R-25 |
| Zone 2 (Warm) | R-49 | R-38 |
| Zone 3 (Warm / mixed) | R-49 | R-38 |
| Zone 4 (Mixed) | R-60 | R-49 |
| Zone 5 (Cool) | R-60 | R-49 |
| Zone 6 (Cold) | R-60 | R-49 |
| Zones 7-8 (Very cold) | R-60 | R-49 |
Expert Contributors
Hawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileEhsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileUpdated June 2026
How we verify our calculatorsAttic Depth Visualizer — will the fill clear your joists?
Loose fill should bury the joists in an unbroken blanket. If it stops at the joist tops, heat bridges straight through the wood. Set your numbers and see it.
Final depth ≈ 17.5″ of fiberglass
Sits ~8″ above your 2×10 joists — an unbroken blanket. Ideal.
Attic Top-Up Layer Guide
Treat the attic floor as one working assembly. Seal air leaks first, count the insulation that is already there, add the loose-fill depth you need, and keep the soffit path open.
Added depth belongs above the existing layer. Install baffles before blowing loose fill near the eaves.
Is Attic Insulation Worth It? Energy-Savings Estimator
Topping up attic insulation cuts heating & cooling loss. See your yearly savings and when it pays for itself.
Estimated yearly savings
$349/yr
Pays for itself in ~5 years, then it's money in your pocket.
Net ~$1,790 ahead in 10 years
Estimate: attic ≈ 25% of conditioning loss; savings scale with the R-value jump (1 − current/target). Install ~$1.70/sq ft blown-in. Actual savings vary with climate, fuel prices, and air sealing.
How the Calculator Works
The calculator uses the attic floor area, target R-value, existing R-value, and the selected product label chart. It estimates the added loose-fill layer, then rounds bags up so the order is not short.
Added R-value
target R - existing R
Bags
ceil(area / 1,000 x chart bags)
Cost
bags x product bag price
Use the insulated ceiling area, not the roof slope. Product labels still control the final minimum bags and installed depth for the R-value you choose.
Material Notes
- Blown fiberglass
- Low-settling AttiCat label baseline.
- Loose-fill cellulose
- SANCTUARY installed-depth label baseline.
- Batts and spray foam
- Useful in some assemblies, but not bag-based attic top-up materials.
Pro Tips
Air seal the ceiling plane before adding insulation. Pay attention to plumbing penetrations, wiring holes, attic hatches, and other bypasses.
Keep soffit vents open. Install baffles where loose fill could drift into the eave and block ventilation.
Use the calculator as an order-planning baseline. The bag label for the product in your cart is the final check for minimum bags and minimum installed depth.
Do not bury heat-producing fixtures or unsafe wiring. Stop and get professional guidance when the attic has vermiculite, knob-and-tube wiring, moisture damage, or mold.
Before You Add Loose Fill
Air leaks
Seal bypasses at the ceiling plane before the new insulation hides them.
Soffit vents
Install baffles so airflow from the eaves stays open.
Old insulation
Do not bury wet material, animal contamination, or an unresolved roof leak.
Electrical safety
Get professional guidance around vermiculite, unsafe wiring, and heat-producing fixtures.
Attic Insulation FAQ
Can I add insulation over the old layer?
Usually, yes. The existing layer should be dry and free of contamination. Do not compress it.
Why does the page ask for existing R-value?
Insulation layers add together. A top-up job only needs the difference between the target and the layer already in place.
Why should I still read the bag label?
Loose-fill products have specific coverage charts. The label gives the minimum bags and minimum installed depth required for the stated R-value.
Should I block attic ventilation in winter?
No. Keep soffit-to-roof airflow open where the assembly is designed for ventilation.
What if my attic has vermiculite?
Do not disturb it. The EPA recommends treating vermiculite insulation as potentially asbestos-contaminated and getting professional guidance.
Is cellulose the same as dense-pack wall insulation?
No. This page uses loose-fill attic coverage. Use the cellulose insulation calculator for wall and attic planning.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.