TryBuildCalcAttic Insulation Bags Calculator (Fiberglass & Cellulose Bags, Top-Up & Cost)
Estimate loose-fill bags, top-up, and cost from real 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 26 bag(s)
Covering 1,200.0 sq ft of attic / ceiling.
Bag Summary
Weight Needed (before wastage): 628 lb
Wastage Added: 10%
Final Weight (incl. wastage): 690 lb
Bags to Buy (all materials): 26 bag(s)
Coverage & Depth
Total Area: 1,200.0 sq ft (111.5 m²)
Target Thickness: 13.5 in
Bags Needed by Material
| Material | Area | Weight Needed (incl. wastage) | Bag Size | Bags to Buy |
|---|---|---|---|---|
| Fiberglass Loose-Fill | 1,200.0 sq ft | 690 lb | 27.5 lb/bag | 26 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 63 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: 26 bag(s) + recommended tools
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Bill of Materials
Main material: 26 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)
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Blown-In Insulation Calculator.
Attic insulation bag count
This page answers exactly how many bags a 40 ft x 30 ft (1,200 sq ft) attic needs at a target R-38 using fiberglass loose-fill — the calculator reads the real chart and adds 10% wastage automatically.
Enter your own attic area and target R-value below to get your exact bag count instead of a rough rule-of-thumb estimate.
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-38 | 13.5 in |
| Weight needed per sq ft | Chart lookup at 13.5 in | 0.523 lb/sq ft |
| Total weight needed (before wastage) | Sum of the above | 628 lb |
Step 3 & 4 — Final Weight & Bags
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Final weight (incl. wastage) | 628 lb x (1 + 10/100) | 690 lb |
| Fiberglass Loose-Fill bags | CEIL(690 lb / 27.5 lb per bag) | 26 bag(s) |
| Total bags (all materials) | Sum of every material's own whole-bag count | 26 bag(s) |
Therefore, for this job you need 26 bag(s).
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Blown-In Insulation Calculator.
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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.