Blown-In Attic Insulation Calculator
Adding loose-fill insulation to an attic floor comes down to two numbers: how many bags to buy, and how deep to blow them. Buy too few bags and you fall short of the rated R-value; blow to the settled depth instead of the deeper installed depth and the layer under-performs once it settles. Both mistakes cost money or comfort.
This calculator turns an attic-floor area and a target R-value (or a target depth) into the bags of blown cellulose or fiberglass to buy, read straight off a manufacturer-style coverage chart. Its signature output is the pair of depths: the installed “blow-to” depth you pile at the machine, and the settled depth that actually delivers the rated R. It also shows the weight the finished layer puts on your ceiling framing and a running cost. Everything runs in your browser.
Switch between imperial and metric at any time: imperial shows R-values, inches, square feet and pounds; metric shows RSI, centimetres, square metres and kilograms. The coverage chart and the physics are the same either way — only the units and labels change.
How bags come off a coverage chart
Bagged loose-fill is sold against a published coverage chart, not by pure volume. Each row of the chart maps a target R-value to a settled thickness, a net coverage in square feet per bag, and a minimum number of bags per 1,000 square feet. The higher the target R, the fewer square feet one bag covers and the more bags each 1,000 square feet needs.
The calculator honours both limits with a dual-constraint bag count: it takes the larger of the bags your area needs at the chart coverage (plus a waste allowance) and the chart's own minimum bags per 1,000 square feet. That minimum is the floor that keeps the installed density high enough to actually reach the rated R — spreading fewer bags thinner hits the depth but not the R.
The bar figure below compares bags per 1,000 square feet for cellulose and fiberglass across the common attic targets. Cellulose and fiberglass sit on different curves because they pack at different densities, so the same target R buys a different number of bags of each.
Installed (blow-to) depth vs settled depth
Loose-fill goes in fluffed and then settles. The installed or “blow-to” depth is what you pile up at the machine; the settled depth is what remains after the material compacts, and it is the settled depth that delivers the rated R-value. Blowing only to the settled number leaves you short once it settles, so the calculator reports both and marks the settled line as the depth that reaches the target.
Fiberglass loose-fill is charted at its settled density and barely settles further, so its installed and settled depths are effectively the same. Cellulose is denser and settles roughly a further 10% built into this chart, so its blow-to depth sits above its settled line — the gap between the two is the settling allowance.
The cross-section below is the same schematic the live tool draws: joists across the attic floor, a solid settled fill up to the depth that delivers the rated R, and a hatched blow-to overlay above it with the settling gap called out.
Cellulose vs fiberglass
For the same target R-value the two materials behave differently. Cellulose reaches a given R in a thinner settled layer because it has a higher R-per-inch (about R-3.5 per inch versus about R-2.9 for blown fiberglass), but each bag covers fewer square feet, so a cellulose job usually needs more bags and lands a heavier layer on the ceiling. Fiberglass is lighter and covers more area per bag but has to be piled deeper.
There is no bespoke comparison screen: add two attic sections with the same area — one cellulose, one fiberglass — and read the bags, depths, settling and weight side by side. The figures below show the same R-49 target met both ways: cellulose settling below its blow-to line, and fiberglass as a single fill with no settling gap.
Which to choose is usually about weight, air-sealing and budget rather than the count alone. Use the readouts to compare, then set the material per section to match what you are actually buying.
Cellulose coverage and settled depth here are a derived estimate, not a manufacturer chart — verify against your bag label and override the coverage to match your product before buying.
Climate-zone attic R-value
How much R-value an attic should reach depends on climate. Colder zones lose more heat, so they call for higher attic R-values. The calculator uses the US IECC climate zones (1 warmest, 8 coldest) to fill in a recommended attic target R, which you can override per section.
As a simplified single-number guide, attics run from about R-30 in the warmest zone up to R-49 from Zone 4 and colder. The reference figure below lists the recommended attic R-value for each zone.
This is recommended guidance, not a code-compliance calculation. Requirements vary by jurisdiction and code year, so treat the zone value as a sensible starting point and verify against your local building code.
Weight on the ceiling
A deep loose-fill layer adds real weight to the ceiling framing below it, and the calculator reports both the total pounds and the load per square foot. The weight is the bag count times the net bag weight, so it climbs with the target R and differs between materials — cellulose is denser than fiberglass, so the same area lands more weight.
Bag weight is a visible, editable assumption because real products vary widely (roughly 25–28 lb per bag is typical). Override it to match the bag you are actually buying for an accurate ceiling load. On older or lightly framed ceilings the load per square foot is worth checking before piling on a very high R.
The formulas
Every output comes from the attic-floor area, the target R-value (or target depth) and the material's coverage-chart row. Bags are always rounded up to whole bags.
Every quantity comes from the attic-floor area, the target R-value and the material's coverage-chart row. Bags are rounded up to whole bags.
Dual-constraint bag count
bags = max( ceil(area ÷ 1000 × bagsPer1000), ceil(area ÷ coverage × (1 + waste)) )- coverage (sq ft/bag) and bagsPer1000 come from the material's coverage-chart row at your target R.
- The bagsPer1000 term is the floor that keeps the density high enough to actually reach the rated R.
- coverage is a visible, editable default — override it to match your product's chart.
Installed vs settled depth
settled = targetR ÷ rPerInch; installed = settled ÷ (1 − settling)- rPerInch ≈ 3.5 for cellulose, ≈ 2.88 for blown fiberglass (shared with the insulation calculator).
- settling ≈ 0.10 for cellulose and 0 for fiberglass, so fiberglass installed = settled.
- The settled depth delivers the rated R; the installed depth is what you blow to.
Weight on the ceiling
weight = bags × bagWeight; perArea = weight ÷ area- bagWeight ≈ 25 lb cellulose, ≈ 27.5 lb fiberglass — a visible default, override to match your bag.
- perArea is the load per square foot (metric: kg/m²) on the ceiling framing.
- Cellulose is denser, so the same area usually lands more weight.
Upgrade and depth mode
additionalR = max(0, targetR − existingR); depthMode: R = installed × (1 − settling) × rPerInch- Upgrade bags only the top-up: the R the new layer must add over what is already there.
- Enter the existing layer as a settled depth or an R-value directly.
- In depth mode the entered blow-to depth settles, then converts to the R it delivers.
How to use the blown-in calculator
Choose imperial or metric units and your climate zone at the top; the calculator remembers your choices in your browser and uses the zone to fill in a recommended attic target R.
Add an attic section for each area you are insulating, enter its dimensions or a known area, pick cellulose or fiberglass, and set a target R-value or a blow-to depth. Turn on Upgrade existing layer to bag only the top-up over insulation that is already there.
Each section shows its bags, blow-to depth, settled depth, achieved R-value and weight; the Results panel totals every section into one shopping list, the weight on your ceiling and a cost once you add a price per bag. Open Assumptions & prices to override the coverage or bag weight to match your product.
How to estimate blown-in insulation in 5 steps
- Choose units and climate zoneSelect imperial (R, in, ft², lb) or metric (RSI, cm, m², kg) and pick your IECC climate zone. The zone fills in a recommended attic target R-value.
- Add attic sectionsAdd a section for each attic area, then enter its length and width or a known area. Add more sections to total a whole attic into one shopping list.
- Pick cellulose or fiberglassChoose the loose-fill material per section. Cellulose reaches an R in a thinner but heavier layer; fiberglass is lighter but piled deeper.
- Set the target R-value or depthUse the zone's recommended R-value, type your own, or switch to depth mode and enter a blow-to depth. Turn on Upgrade to bag only the top-up over an existing layer.
- Read the bags and depthsEach section shows bags, blow-to depth, settled depth, achieved R and weight; the Results panel totals the bags, the ceiling load and a cost once you add a price.
Worked examples
Three quick estimates at the default coverage and bag weights. Your own numbers will differ once you set the zone, target and material to your spec, but these show how the bags and the two depths come together. Cellulose figures are a derived estimate — verify against your bag label.
Worked at the default coverage and bag weights. Bags are rounded up to whole bags. Cellulose figures are a derived estimate — verify against your bag label.
Attic — cellulose (imperial)
Attic · cellulose · Zone 5- Given
- Attic 1,000 ft², Zone 5 attic target R-49
- Cellulose R-3.5/inch, settles ~10%
- Coverage ≈ 25.7 ft²/bag, bag weight 25 lb
- Formula
bags = ceil( 1000 ÷ 25.7 ); settled = 49 ÷ 3.5; installed = settled ÷ 0.9- Working
- Settled depth: 49 ÷ 3.5 = 14.0 in; blow-to: 14.0 ÷ 0.9 ≈ 15.6 in
- Bags: ceil(1000 ÷ 25.7) = ceil(38.9) = 39 bags
- Weight: 39 × 25 = 975 lb → 0.98 lb/ft²
- Result
- 39 bags · blow-to 15.6 in · settles to 14 in · R-49 · 975 lb
Attic — fiberglass (imperial)
Attic · fiberglass · Zone 5- Given
- Same attic 1,000 ft², target R-49
- Blown fiberglass, negligible settling
- Coverage ≈ 39.5 ft²/bag, bag weight 27.5 lb
- Formula
bags = ceil( 1000 ÷ 39.5 ); settled = installed = 16.5 in- Working
- Depth: 16.5 in blow-to and settled (no settling gap)
- Bags: ceil(1000 ÷ 39.5) = ceil(25.3) = 26 bags
- Weight: 26 × 27.5 = 715 lb → 0.72 lb/ft²
- Result
- 26 bags · 16.5 in · R-49 · 715 lb — lighter but deeper than cellulose
Top-up over an existing layer (imperial)
Upgrade · cellulose · R-19 → R-49- Given
- Attic 800 ft² with an existing R-19 layer
- Target R-49 in cellulose → top-up R-30
- Coverage at R-30 ≈ 42.0 ft²/bag
- Formula
additionalR = 49 − 19; bags = ceil( 800 ÷ 42.0 )- Working
- Top-up needed: 49 − 19 = R-30 of new cellulose
- New settled depth: 30 ÷ 3.5 ≈ 8.6 in
- Bags: ceil(800 ÷ 42.0) = ceil(19.0) = 20 bags
- Result
- 20 bags for the R-30 top-up · adds ~8.6 in settled · reaches R-49
Common mistakes to avoid
Blowing only to the settled depth. The settled depth is what delivers the rated R after the material compacts; you have to pile to the deeper installed (blow-to) depth so it settles to the right number. The calculator shows both — mark the blow-to depth on the joists.
Spreading too few bags to save money. Hitting the target depth with fewer bags than the chart calls for lowers the density and the layer never reaches the rated R. The dual-constraint count keeps you at or above the chart's minimum bags per 1,000 square feet.
Treating the cellulose numbers as a manufacturer chart. The cellulose coverage and settled depth here are derived from its R-per-inch and a typical bag yield, not read off a specific product's chart. Always verify against your bag label and override the coverage if it differs.
Ignoring the weight on the ceiling. A high-R loose-fill layer is heavy, especially in cellulose. Check the load per square foot on older or lightly framed ceilings before committing to a very deep fill.
Frequently asked questions
How many bags of blown-in insulation do I need?
The calculator reads a coverage chart for your target R-value and takes the larger of two limits: the bags your area needs at that coverage (plus waste), and the chart's minimum bags per 1,000 square feet. It rounds up to whole bags. For example, a 1,000 sq ft attic at R-49 needs about 39 bags of cellulose or 26 bags of fiberglass at the default coverage. Override the coverage in the assumptions card to match your product.
What is the difference between blow-to depth and settled depth?
The blow-to (installed) depth is what you pile up at the machine; the settled depth is what remains after the loose-fill compacts, and it is the settled depth that delivers the rated R-value. Cellulose settles about 10% built into this chart, so its blow-to depth sits above its settled line. Fiberglass is charted at settled density and barely settles further, so the two depths are the same. Always blow to the installed depth so it settles to the right number.
Should I use cellulose or fiberglass?
Cellulose reaches a target R in a thinner settled layer (about R-3.5 per inch versus R-2.9 for blown fiberglass) but usually needs more bags and lands a heavier layer on the ceiling. Fiberglass is lighter and covers more area per bag but is piled deeper. Add one section of each with the same area to compare the bags, depth, settling and ceiling weight, then choose on weight, air-sealing and budget.
How deep does blown-in insulation need to be?
The settled depth is the target R divided by the material's R-per-inch. Cellulose at about R-3.5 per inch needs roughly 14 inches for R-49; blown fiberglass at about R-2.9 per inch needs roughly 16.5 inches for the same R. Blow to the deeper installed depth so it settles to that number — the calculator shows both and marks the settled line as the target.
What R-value should my attic be?
As a guide, attics run from about R-30 in the warmest US climate zones up to R-49 from Zone 4 and colder. Pick your climate zone and the calculator fills in a recommended attic target, which you can override per section. This is consumer-style guidance, not a code-compliance figure — verify against your local building code.
How much weight does blown-in insulation add to the ceiling?
The weight is the bag count times the net bag weight, and the calculator shows both the total pounds and the load per square foot. A deep, high-R layer can be surprisingly heavy, especially in cellulose, which is denser than fiberglass. Bag weight is an editable assumption (roughly 25–28 lb is typical) — match it to your bag, and check the per-square-foot load on older or lightly framed ceilings.
Can I add blown-in over insulation that is already there?
Yes. Turn on Upgrade existing layer for a section and enter the existing insulation as a settled depth or an R-value. The calculator bags only the top-up — the R the new layer must add to reach your target — rather than the full target R. If the existing layer already meets the target, it reports that there is nothing to buy.
How accurate are the cellulose numbers?
The fiberglass rows are triangulated from a manufacturer data sheet. The cellulose coverage and settled depth are a derived estimate — computed from cellulose's R-per-inch and a typical bag yield rather than read off a specific product's chart — so they ship clearly tagged as derived. Verify against your bag label and override the coverage and bag weight in the assumptions card to match the product you are buying.
Is the calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to any server, and your project is saved only in your own browser's local storage.