TryBuildCalcBlown-In Insulation Calculator (Fiberglass & Cellulose Bags, Top-Up & Cost)
Calculate loose-fill insulation bags needed from real manufacturer coverage charts.
🕒 Last updated: October 3, 2026
Inputs
Section 1: Area & Material
ℹ️Set to more than 1 if several identical areas share this same size.
Section 2: Wastage
ℹ️Use 5-10% for a simple, open attic/floor and 10-15% for a complex roof, many obstructions, or a harder-to-reach cavity.
Section 3: Cost
Buy at least 28 bag(s)
Covering 1,000.0 sq ft of attic / ceiling.
Bag Summary
Weight Needed (before wastage): 688 lb
Wastage Added: 10%
Final Weight (incl. wastage): 757 lb
Bags to Buy (all materials): 28 bag(s)
Coverage & Depth
Total Area: 1,000.0 sq ft (92.9 m²)
Target Thickness: 17.0 in
Bags Needed by Material
| Material | Area | Weight Needed (incl. wastage) | Bag Size | Bags to Buy |
|---|---|---|---|---|
| Fiberglass Loose-Fill | 1,000.0 sq ft | 757 lb | 27.5 lb/bag | 28 bag(s) |
Quick Insight
The real manufacturer coverage chart was used to find the weight of material needed, then 10% wastage was added on top — that's 69 lb extra beyond what the chart alone requires.
Calculated from the additional weight of material needed to reach your target R-value (accounting for any existing insulation), with 10% wastage added on top.
Bill of Materials
Main material: 28 bag(s) + recommended tools
+−
Bill of Materials
Main material: 28 bag(s) + recommended tools
Computed items reflect your entered dimensions and material choice; consumables below are general recommendations — actual needs vary by product and site.
For Your Job
Fiberglass Loose-Fill Bags
General Tools & Consumables
Blowing machine rental (or supplier loaner)
Blower hose (typically 3 in or larger)
Depth markers / rulers
Attic baffles (eave vent chutes)
Dust mask / respirator, gloves, eye protection, long sleeves
Approximate results for planning only. Verify with a professional.
Loose-Fill Depth (One Area)
What Is a Blown-In Insulation Calculator?
If you're asking "how many bags of blown-in insulation do I need" for an attic top-up or a brand-new floor cavity fill, this calculator answers it from your area, material choice, and target R-value. It reads each material's real published manufacturer coverage chart — the minimum weight of material per square foot required at that R-value — rather than a single invented R-per-inch number, and returns whole bags to buy, adjusted for wastage.
It is built for homeowners, insulation contractors, and energy auditors pricing an attic/ceiling or dense-pack floor loose-fill job, whether starting from bare framing or topping up existing insulation to a higher R-value, in fiberglass or cellulose.
What makes this calculator different:
Most blown-in insulation calculators online use one flat R-value-per-inch figure to convert a target R-value straight to a depth, then divide area by a single "coverage per bag" number. That misses a real, documented effect: a manufacturer's own coverage chart shows the required density per inch actually INCREASES somewhat at greater installed depths (loose-fill settles and compresses more the deeper it's blown), so R-value-per-inch is not quite constant. This calculator instead reads each material's real chart directly — the same minimum-weight-per-sq-ft figure the FTC's R-Value Rule requires every bag to publish — and interpolates it for your exact target R-value, so the bag count reflects that real nonlinearity instead of a flat shortcut.
Applicable standards:
- Local energy codes (commonly based on the IECC/ASHRAE 90.1 climate zone tables, or regional equivalents) set minimum R-values by application and climate zone, which should always be confirmed against this calculator's editable R-Value input
- The FTC R-Value Rule requires every loose-fill insulation bag to print its own coverage chart (minimum thickness, maximum net coverage per bag, minimum weight per sq ft) specific to that exact product
- This calculator estimates material quantity and cost only, not energy-code compliance or thermal-performance verification
How the blown-in insulation calculation works
Each area is converted to a required WEIGHT of material, read from the real manufacturer coverage chart for the chosen material — not a flat area-divided-by-coverage% shortcut.
Step 1 - Look up target thickness from the real coverage chart
Target Thickness (in) = chart lookup, interpolated at your Target R-Value
Chart used = the chosen material's OWN chart for the chosen area type (attic/open-blow or dense-pack floor — never shared between the two)
Figures come from the chosen material's own published coverage chart, interpolated between the chart's real data points rather than assumed from one flat R-per-inch constant. Attic (open-blow) and dense-pack floor are genuinely different applications with different density requirements, so each has its own separate chart per material — never one shared table.
Step 2 - Subtract any existing insulation to get the ADDITIONAL depth (top-up mode)
Additional Depth (in) = Target Thickness - Existing Depth (floored at 0)
Additional Weight (lb/sq ft) = chart lookup, interpolated DIRECTLY at the Additional Depth
This matches real manufacturer top-up instructions: measure the existing depth, subtract it from the target chart thickness to get the remaining depth to add, then read the material weight for THAT additional depth directly off the chart — not a weight-difference calculation between the full target and the existing layer, which would overstate the real top-up requirement. Assumes the existing layer is the same material being added (see Limitations for mixed-material top-ups).
Step 3 - Pool the weight needed within each matching material
Required Weight for an Area = Additional Weight Needed x Area
Material Group Total = Sum of every area using the SAME material
Unlike precut batts or rolls, loose-fill is a bulk bagged product with no discrete piece size — a bag opened for one area can finish filling a different area of the SAME material with no "offcut" restriction. Every area using the same material pools its required weight together; different materials are always totaled separately, since a fiberglass bag and a cellulose bag are different products.
Step 4 - Add wastage, then convert to whole bags exactly once per material group
Final Weight = Material Group Total x (1 + Wastage / 100)
Final Bags (per material) = CEIL(That Material's Final Weight / Bag Weight)
Job Total = Sum of every material's own whole-bag count
Wastage is applied to each material group's own pooled weight directly — this step itself is NOT rounded, only scaled. The whole-bag conversion is the only rounding, and it happens exactly once PER MATERIAL — not once per area, and not once across a combined multi-material total.
Step 5 - Cost
Material Cost = Total Bags (all materials) x Cost per Bag
Labor Cost = Total Area x Labor Rate per sq ft
Total Cost = Material Cost + Labor Cost
Material cost assumes the same price per bag across every material in the job — if your job spans materials with genuinely different real prices, estimate each material's cost separately.
Real-World Blown-In Insulation Calculation Example
This example uses the active calculator inputs above and follows the same steps from the formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Areas | 1 area(s) | Determines how many weight calculations are combined |
| Material | Fiberglass Loose-Fill | Sets the coverage chart used for the weight lookup |
| Wastage | 10% | Adds allowance on top of the chart-based weight |
Step 1 & 2 — Chart Lookup & Required Weight
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Target thickness | Chart lookup at R-49 | 17 in |
| Weight needed per sq ft | Chart lookup at 17 in | 0.688 lb/sq ft |
| Total weight needed (before wastage) | Sum of the above | 688 lb |
Step 3 & 4 — Final Weight & Bags
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Final weight (incl. wastage) | 688 lb x (1 + 10/100) | 757 lb |
| Fiberglass Loose-Fill bags | CEIL(757 lb / 27.5 lb per bag) | 28 bag(s) |
| Total bags (all materials) | Sum of every material's own whole-bag count | 28 bag(s) |
Therefore, for this job you need 28 bag(s).
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Measurement & Planning+-
- Area (length x width) for every attic/ceiling plane and floor plane was measured on site, not taken from a floor plan alone.
- Where insulation already exists, its current depth was measured at several points (not just one spot) and the lowest representative depth was used for the top-up calculation.
- Dimensions were entered in consistent units — all feet or all meters, not mixed.
✓Product Selection+-
- Target R-value meets or exceeds the local energy code minimum for the climate zone and application (attic/ceiling or floor), not just a generic default.
- Material choice (fiberglass or cellulose) matches local code/fire-rating requirements and supplier availability.
- For a floor cavity, a dense-pack-rated material and installation method is being used — not open-blow fiberglass, which the manufacturer does not recommend for closed-cavity application.
- The specific product's own printed FTC coverage chart (on the bag) was checked and compared against this calculator's estimate before finalizing quantity.
✓Quantity & Purchasing+-
- Wastage allowance (typically 5-15%) was applied on top of the real weight-based total, not used to replace it.
- The final bag count including wastage — not the raw bag count — was used for purchase.
✓Installation+-
- Insulation is blown to at least the manufacturer's minimum settled thickness for the target R-value across the ENTIRE area, not just the easiest-to-reach sections.
- Depth markers/rulers are placed across the attic before blowing so installed depth can be checked and compared against the target as work proceeds.
- Baffles are installed at the eaves before blowing attic insulation so soffit vents stay clear — blocking them traps heat and moisture.
- Insulation is kept clear of recessed lighting and other heat-producing fixtures unless they're rated IC (insulation contact), per the fixture's own clearance requirement.
✓Safety & Final Check+-
- Skin, eye, and respiratory protection (long sleeves, gloves, eye protection, a dust mask or respirator) was used when handling and blowing insulation.
- Installed depth was spot-checked against the depth markers at multiple points across the area before considering the job complete.
Full QC Checklist+−
Verify measurement, material selection, quantity, installation, and safety before closing up an attic or floor cavity.
✓Measurement & Planning+-
- Area (length x width) for every attic/ceiling plane and floor plane was measured on site, not taken from a floor plan alone.
- Obstructions that reduce the real blow-able area (chimneys, large ductwork, skylights wells, structural elements) were accounted for in the entered area.
- Where insulation already exists, its current depth was measured at several points (not just one spot) and the lowest representative depth was used for the top-up calculation.
- Dimensions were entered in consistent units — all feet or all meters, not mixed.
- Separate area rows were used for sections with genuinely different material, target R-value, or application (attic vs. floor) rather than averaging them into one row.
✓Product Selection+-
- Target R-value meets or exceeds the local energy code minimum for the climate zone and application (attic/ceiling or floor), not just a generic default.
- Material choice (fiberglass or cellulose) matches local code/fire-rating requirements and supplier availability.
- For a floor cavity, a dense-pack-rated material and installation method is being used — not open-blow fiberglass, which the manufacturer does not recommend for closed-cavity application.
- The specific product's own printed FTC coverage chart (on the bag) was checked and compared against this calculator's estimate before finalizing quantity.
- A compatible blowing machine (rented or supplied with purchase) was confirmed available for the chosen material before the installation date.
- Baffles for soffit/ridge vent clearance are on hand before blowing attic insulation, if not already installed.
✓Quantity & Purchasing+-
- Wastage allowance (typically 5-15%) was applied on top of the real weight-based total, not used to replace it.
- The final bag count including wastage — not the raw bag count — was used for purchase.
- Where multiple materials are used (e.g. attic and floor use different products), each material's own bag count was purchased separately, not a single combined total.
- Product was ordered from the same production batch/lot where visual or performance consistency matters.
✓Installation+-
- Insulation is blown to at least the manufacturer's minimum settled thickness for the target R-value across the ENTIRE area, not just the easiest-to-reach sections.
- Depth markers/rulers are placed across the attic before blowing so installed depth can be checked and compared against the target as work proceeds.
- Baffles are installed at the eaves before blowing attic insulation so soffit vents stay clear — blocking them traps heat and moisture.
- Insulation is kept clear of recessed lighting and other heat-producing fixtures unless they're rated IC (insulation contact), per the fixture's own clearance requirement.
- Combustion appliance flues, chimneys, and electrical junction boxes retain their required clearance from loose-fill insulation per the manufacturer and local code.
✓Safety & Final Check+-
- Skin, eye, and respiratory protection (long sleeves, gloves, eye protection, a dust mask or respirator) was used when handling and blowing insulation.
- Adequate ventilation and lighting were maintained in the attic or floor cavity work area throughout installation.
- A second person was stationed at the blowing machine (hopper-feeding) while the installer worked in the attic/floor cavity, for both safety and installation quality.
- Installed depth was spot-checked against the depth markers at multiple points across the area before considering the job complete.
- No insulation was left blocking attic access, electrical panels, or other areas that must remain accessible.
Reference Tables
Attic / open-blow: installed thickness by R-value and material
| R-Value | Fiberglass | Cellulose |
|---|---|---|
| R-19 | 7.0 in | 5.3 in |
| R-22 | 8.0 in | 6.1 in |
| R-30 | 10.75 in | 8.3 in |
| R-38 | 13.5 in | 10.4 in |
| R-49 | 17.0 in | 13.3 in |
| R-60 | 20.5 in | 16.1 in |
Source: Owens Corning AttiCat(R) PINK(R) fiberglass and SANCTUARY(R) by Greenfiber (current US chart, Rev K 05/26) cellulose published attic coverage charts. These figures are for OPEN ATTIC application only — a dense-pack floor needs meaningfully more material at a comparable R-value (see the separate table below) because it must be packed dense enough to resist settling against gravity in a closed cavity.
Dense-pack floor (cellulose only): installed thickness and minimum weight by R-value
| R-Value | Typical Joist Depth | Installed Thickness | Minimum Weight |
|---|---|---|---|
| R-13 | 2x4 | 3.5 in | 1.02 lb/sq ft |
| R-21 | 2x6 | 5.5 in | 1.60 lb/sq ft |
| R-28 | 2x8 | 7.5 in | 2.19 lb/sq ft |
Source: SANCTUARY(R) by Greenfiber (current US chart, Rev K 05/26) Sidewall & Floor Coverage Chart — Dense Pack Application (3.5 lb/cu ft minimum installed density). Fiberglass is not included here — the manufacturer does not recommend AttiCat for closed-cavity/dense-pack application. Only 3 points are published (tied to standard joist depths); a target R-value outside R-13 to R-28 is extrapolated from this table and less certain than the attic table above.
Typical R-value by application and climate
| Application | Typical R-Value Range |
|---|---|
| Attic/ceiling, moderate climate | R-30 to R-38 |
| Attic/ceiling, cold climate | R-49 to R-60 |
| Floor over unheated space | R-19 to R-30 |
Typical wastage by job complexity
| Job Complexity | Typical Wastage |
|---|---|
| Simple, open attic, easy access | 5-8% |
| Standard job with some obstructions | 10% |
| Complex roof (hips/valleys), many obstructions, or hard-to-reach floor cavity | 15% or more |
Confirm the minimum R-value required for your climate zone against the local energy code before finalizing an order — these figures are general planning guidance only.
How to Use the Blown-In Insulation Calculator
- Enter the length and width for one attic/ceiling plane or floor plane — or switch on Multiple Areas to combine several areas (even different materials) in one pass.
- Select the area type and loose-fill material, then choose a target R-value preset or enter a custom value.
- If insulation already exists, switch on "Existing Insulation Present?" and enter the measured depth — only the additional material needed is then counted.
- Set wastage to match your job's complexity, then add optional cost inputs for a material + labor estimate.
- Check the bag count against your actual product's own printed FTC coverage chart before ordering.
Practical Insulation Tips
- Tape depth markers (or pre-marked rulers) to joists/trusses across the whole area before blowing, so you can check installed depth against the target as you go, not just at the end.
- Blow in slow, even passes rather than dumping material in one spot — an uneven pour can look full by volume while being underweight (and under-R-value) in spots.
- Install baffles at the eaves before blowing attic insulation so soffit vents stay clear — blocking them traps heat and moisture.
- Keep insulation clear of recessed lighting and other heat-producing fixtures unless they're rated IC (insulation contact) — check the fixture's own clearance requirement.
- Confirm your blowing machine's hopper rate/gate setting matches the material — a setting tuned for fiberglass can underfill cellulose, and vice versa.
- For a dense-pack floor job specifically, confirm your installer or machine is set up for dense-pack technique (higher pressure, netting work) — it is a genuinely different process from an open attic pour, not just a different R-value target.
Common Mistakes
- Dividing total square footage by one bag's coverage at a DIFFERENT depth than your actual target — coverage per bag changes with R-value, so always use the figure for your specific target thickness.
- Stopping at the "fluffy" depth right after blowing instead of the SETTLED depth the chart specifies — fresh fiberglass and cellulose both settle somewhat over the following weeks/months.
- Skipping the wastage buffer and buying the exact chart-based bag count — an uneven pour, a miscounted attic obstruction, or a bag that doesn't fully empty leaves no spare material.
- Blocking soffit/ridge vents with attic insulation instead of installing baffles first — this traps moisture and defeats the roof assembly's intended ventilation path.
- Assuming a different manufacturer's bag matches this calculator's figures exactly — always check the specific product's own printed FTC coverage chart before finalizing an order.
- Measuring existing insulation depth at only one spot for a top-up job — real attics usually have uneven existing depth, so measure several points and use the lowest.
Blown-In Insulation Calculator Limitations
- This calculator provides estimation only and does not replace a manufacturer's installation guide or a local energy-code compliance check.
- Each material/application's coverage chart comes from one specific, currently published product (Owens Corning AttiCat for attic fiberglass; SANCTUARY by Greenfiber for both attic and dense-pack floor cellulose). Other brands' products are generally similar but not identical — always check your actual bag's own printed FTC coverage chart as the final word.
- Mineral (rock) wool loose-fill is deliberately not offered as a material option. Its only prior data source for this calculator (Thermafiber INSUL-FILL) was discontinued in 2023, and no other current North American manufacturer publishes a public coverage chart for a loose-fill mineral wool product — rather than ship unverifiable or discontinued-product numbers, the option was removed.
- Fiberglass loose-fill is attic/open-blow only in this calculator. The manufacturer does not recommend it for closed-cavity (dense-pack) application, so it is not offered as a Floor material option — only cellulose, which has its own real dense-pack chart.
- The dense-pack floor chart has only 3 published data points (R-13/R-21/R-28, tied to 2x4/2x6/2x8 joist depths). A target R-value outside that range is extrapolated and carries more uncertainty than the attic chart's published points.
- Target R-values beyond a material/application's own published chart range are extrapolated from the chart's last real data point, which is a reasonable approximation but not a chart-confirmed figure.
- The existing-insulation top-up mode assumes the existing layer is the SAME material as what you're adding — it looks up the ADDITIONAL depth (target thickness minus existing depth) directly on that material's own chart, the same way manufacturer top-up instructions work. Topping up with a different material is a reasonable approximation but not an exact physical blend of two different products' densities.
- This calculator does not map specific obstructions (chimneys, large ductwork, skylight wells) — reduce the entered area to account for any that take up meaningful space, or increase wastage for a complex layout.
- Materials using a different type are never pooled together, even in Multiple Areas mode — each is totaled to its own whole-bag count first, then summed. This avoids overstating savings between products that could never actually share material, but it also means a small, oddly-sized area of one material won't benefit from a nearby area of a DIFFERENT material's leftover.
- Material cost assumes one shared price per bag across every material in the job — if a Multiple Areas job spans materials with genuinely different real prices, estimate each material's cost separately rather than trusting the combined total here.
- This calculator assumes insulation is blown to the full minimum settled thickness and weight — it does not estimate or warn about R-value loss from an uneven or under-filled installation.
- Blowing machine rental, hose length requirements, and labor time are not quantified beyond the optional flat labor-rate-per-sq-ft input.
- Final R-value selection and installation method should follow the local energy code and the manufacturer specification for your actual product.
Related Construction Calculators
You may also find these calculators useful for the same job:
- Batt & Roll Insulation Calculator
Compare against precut batt or roll insulation for the same space.
- Attic Ventilation Calculator
Check soffit/ridge vent airflow so insulation baffles don't block it.
- Drywall Sheet Calculator
Estimate drywall sheets for a ceiling once insulation is in place.
- Unit Converter
Convert area dimensions before estimating.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.