Attic Insulation Calculator: Blown-In & Cellulose Material Estimator
Our Attic Insulation Calculator helps you estimate how much blown-in or cellulose insulation you need for your attic. Simply enter your attic dimensions to calculate the required material, improve energy efficiency, and plan your insulation project with confidence. So, let’s start!
Table of Contents
How many bags of blown-in insulation do I need for my attic?
For a 1,000 sq ft attic blown to 12 inches of cellulose (about R-42) with bare joists, expect roughly 37–40 bags, already built in a 15–20% settling allowance. Switch to Owens Corning AttiCat blown-in fiberglass at the same depth and you’ll land closer to R-30, needing a different bag count since fiberglass covers less R-value per inch. Both Home Depot and Lowe’s loan the blowing machine free for 24 hours with a minimum bag purchase — enter your own square footage and depth below for an exact count.
BLOWN-IN INSULATION CALCULATOR
Bags of Cellulose or Blown Fiberglass Needed
Enter 0 for bare joists with no existing insulation.
Bags Needed
38 bags
blown cellulose, 12 in over bare joists
- R-value at target depth–
- Additional depth to blow–
- Estimated materials cost–
- Blower rental–
Buy insulation and the bag doesn’t ask for an R-value. It asks how deep you’re blowing it, because depth is the number the machine actually controls. This calculator works the way the job actually gets priced and bought — enter your square footage and depth, and it tells you exactly how many bags to load into the cart, plus whether you’ll clear the free blower-rental threshold.
In this guide
What Is Blown-In Insulation and How Is It Different from Batts?
Blown-in insulation is loose fill — shredded cellulose or fiberglass — that gets fed through a machine and blown through a hose into your attic, rather than laid down as pre-cut sheets. The machine breaks up compressed bags of material and pushes it through the hose with air pressure, so it settles into an even, fluffy layer that fills around joists, wiring, and vent boxes without the gaps you get from cutting batts around obstacles.
That’s also why it’s priced and bought by the bag instead of the square foot. Every bag has a coverage chart printed on it — how many square feet it covers at a given settled depth — and that chart, not a general R-value target, is what actually tells you how many bags to buy.
How Many Bags of Cellulose or Fiberglass Do You Need for Your Attic?
For a 1,000 sq ft attic with bare joists, blowing to 12 inches of cellulose (about R-38 to R-42) typically takes 37 to 40 bags. Switch to blown fiberglass at the same 12 inches and you’ll only reach about R-30, since fiberglass has a lower R-value per inch — but you’ll also use fewer bags per square foot because it’s a lighter fill. Going deeper to 15 inches for an R-49 to R-52 target pushes cellulose up to roughly 48 to 52 bags on the same 1,000 sq ft.
These numbers already build in a settling allowance, because loose-fill material compresses under its own weight over the following months. Manufacturer coverage charts account for this by listing a slightly higher initial fill depth than the final settled depth you’re targeting.
How to Use This Blown-In Insulation Calculator
Enter your attic’s square footage, then either type a custom depth or click one of the two preset buttons — 12 inches or 15 inches, the two most common attic targets. If you already have some insulation up there, enter its depth so the calculator only counts the additional material you actually need to add. Pick cellulose or AttiCat-style blown fiberglass, and it converts everything into a bag count, an estimated cost, and a note on whether you’re likely to clear the free blower-rental threshold most stores set at 10 bags.
Cellulose vs. AttiCat Blown Fiberglass: Which Should You Choose?
Both materials get blown in the same way and use the same rented machine, but they behave differently once they’re in your attic.
| Blown Cellulose | Blown Fiberglass (AttiCat-style) | |
|---|---|---|
| R-value per inch | ~3.5 | ~2.5 |
| Made from | Recycled paper, treated with fire retardant | Spun glass fibers |
| Settling over time | 15–20%, needs a deeper initial fill | Much less, holds its installed depth |
| Moisture behavior | Absorbs and holds moisture | Doesn’t absorb moisture |
| Dust during install | Noticeably dusty, N95 recommended | Itchy fibers, gloves and long sleeves recommended |
| Typical price per bag | $16–$20 | $15–$19 |
Owens Corning’s AttiCat system is the best-known blown fiberglass option and comes with its own applicator gun that clips onto the hose, which some installers find easier to control precisely than a bare hose end. Cellulose remains the more common DIY choice because it fills irregular gaps slightly better and is usually the cheaper way to hit a high R-value target.
How to Rent a Blowing Machine at Home Depot or Lowe’s
Neither store charges a separate rental fee if you buy enough bags — the machine effectively comes free with the material.
- Check the bag minimum. Most locations waive the rental fee for a 24-hour loan once you buy 10 or more bags of that store’s insulation brand. Buying fewer bags usually means paying a rental fee instead.
- Call ahead to reserve. Blower availability varies by store and season — insulation season (fall and early winter) is when machines get booked up fastest.
- Ask for the fill tube. A flexible 8–10 ft extension hose lets you reach corners and tight eaves without repositioning the whole machine — most rentals include one, but confirm when you reserve.
- Bring a truck or trailer. The blower unit itself is bulky — plan for a full-size pickup bed or a rented truck to get it home along with your bags.
- Return it on time. The free period is usually a strict 24 hours from checkout, so plan your install day around picking it up the morning you start.
How to Operate an Insulation Blower: A Beginner’s Walkthrough
Running the machine is simple, but it works best with two people — one feeding bags, one directing the hose.
- Air-seal first. Before any material goes in, seal top-plate gaps, wire penetrations, and recessed light housings with low-expansion spray foam. Insulation doesn’t stop air leaks — it just sits on top of them.
- Install baffles at the eaves. Rigid baffles at every rafter bay keep the loose fill from blocking soffit vents, preserving a clear air channel to the roof.
- Set up outside if possible. Most rental machines are designed to sit outside near a window or attic hatch, with the hose fed inside — this keeps the dust and noise out of the living space.
- Feed bags steadily. Break up any compressed clumps before dropping a bag in, and feed roughly one bag every 45–60 seconds — too fast and the machine chokes; too slow and the hose sputters.
- Keep the hose moving. Work in slow, overlapping passes rather than pointing the hose at one spot, and check your depth against a ruler or marked stakes as you go.
- Wear real protection. An N95 respirator, sealed goggles, gloves, and long sleeves aren’t optional — cellulose dust irritates airways, and fiberglass fibers irritate skin.
A two-person crew blowing a 1,000–1,500 sq ft attic with no major obstructions typically finishes in 2 to 3 hours, not counting setup and air-sealing time. Tight attics with lots of HVAC ducting or low headroom can add 30–60 minutes per 500 sq ft.
A Worked Example: Insulating a 1,200 Sq Ft Attic Yourself
Say you’ve got a 1,200 sq ft attic with 4 inches of old, thin fiberglass batts, and you’re topping it up with cellulose to hit 15 inches total depth.
Step-by-step
- Target depth: 15 inches. Existing: 4 inches. Additional depth to blow: 11 inches.
- R-value from the additional 11 inches of cellulose: 11 × 3.5 ≈ R-38.5 added on top of the existing batts.
- Bags needed: scaling the 50 sq ft/bag reference coverage to 1,200 sq ft and this R-value, plus an 18% settling allowance, lands around 55 bags.
- At $18/bag: 55 × $18 ≈ $990 in materials.
- At 55 bags, you’re well past the 10-bag threshold most stores use for a free 24-hour blower rental.
How This Calculator Differs from Our Other Attic Insulation Tools
If you’ve used our other calculators, here’s how they split up so you land on the right one for your question:
| This calculator | Attic Insulation Calculator | Blown-In Cost Calculator | |
|---|---|---|---|
| Best for | Bag count from a specific blow depth | Bag/roll count from a target R-value | DIY vs. pro cost comparison |
| Primary input | Depth (12″/15″/custom) | Climate zone R-value | Depth |
| Materials | Cellulose, blown fiberglass | Cellulose, rock wool | Cellulose, fiberglass |
| Extra content | Blower rental & operation how-to | Climate zone R-value tables | DIY vs. pro pricing breakdown |
Working from a target R-value instead? Our Attic Insulation Calculator starts there. Comparing DIY materials against a contractor’s installed quote? Try the Blown-In Attic Insulation Cost Calculator.
Not sure what R-value you actually need before picking a depth? Our Insulation Calculator covers attics, walls, floors, and crawl spaces by climate zone.
Common Mistakes That Waste Bags and Leave Cold Spots
- Blowing to the calculated depth exactly. Loose-fill settles 15–20% over the following months. If you stop right at your target depth, you’ll end up a couple inches short by next winter — blow slightly deeper to account for it.
- Skipping the baffles. Loose fill piled against the eaves chokes off soffit ventilation, which leads to moisture buildup and ice dams in colder climates.
- Not air-sealing first. Insulation slows heat conduction, not air leakage. Uncovered gaps around lights, plumbing stacks, and the attic hatch can undo a big chunk of the R-value gain.
- Underestimating bags to save money. Running short mid-job means a second trip and a second machine reservation — buy a couple of extra bags rather than cutting it exactly to the calculator’s number.
- Mixing materials without checking compatibility. Cellulose can go over old fiberglass batts if they’re dry and intact, but never blow insulation over wet or moldy material — fix the moisture source first.
Bag counts and R-value figures in this guide are estimates based on typical manufacturer coverage charts for cellulose and blown fiberglass insulation. Always confirm the coverage rate printed on the specific bag you buy, since it varies by brand and can shift the final count by several bags.
Frequently Asked Questions
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