TryBuildCalcAttic Insulation Calculator (Batts, Rolls, Vapor Barrier & Cost)
Estimate batts/rolls, vapor barrier, and cost by counting real bays.
🕒 Last updated: October 3, 2026
Inputs
Section 1: Area Dimensions
ℹ️Set to more than 1 if several identical runs share this same size.
Section 2: Product
ℹ️Check your product's actual total roll length — commonly 20-50 ft depending on R-value and width.
Section 3: Wastage & Vapor Barrier
ℹ️Use 5-10% for straightforward runs and 10-15% where there are more obstructions or odd bays to cut around.
Section 4: Cost
Buy at least 9 roll(s)
Covering 575.0 sq ft of net insulated area (Ceiling / Attic).
Roll Summary
Linear Feet Needed (before wastage): 300.0 ft
Wastage Added: 10%
Final Linear Feet: 330.0 ft
Rolls to Buy: 9 roll(s) (40 ft each)
Coverage & Vapor Barrier
Net Insulated Area: 575.0 sq ft (53.4 m²)
Quick Insight
Real stud/joist bays were counted across the run, then 10% wastage was added on top — that's 30.0 extra ft beyond what the bay count alone requires.
Calculated by counting real stud/joist bays across the run and pooling the total product length needed (10% wastage added on top).
Bill of Materials
Main material: 9 roll(s) + recommended tools
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Bill of Materials
Main material: 9 roll(s) + recommended tools
Computed items reflect your entered dimensions and product choice; consumables below are general recommendations — actual needs vary by product and site.
For Your Job
Insulation Roll
General Tools & Consumables
Insulation support wire / netting
Staples (for faced batts)
Vapor barrier tape
Utility knife & straightedge
Dust mask / respirator, gloves, eye protection, long sleeves
Approximate results for planning only. Verify with a professional.
Insulation Bay Layout (One Run)
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Batt & Roll Insulation Calculator.
Attic insulation estimate
This page defaults to a 30 ft × 20 ft attic plane at 24 in on-center joist spacing, using continuous roll insulation cut to each joist bay's span — a common attic setup for R-38.
Switch to batts instead of rolls, or change the run, span, spacing, or R-value below for your own attic.
How the batt/roll insulation calculation works
Each run is counted as real stud/joist bays, rather than dividing one combined area by one package's coverage.
Step 1 - Deduct openings as an equivalent run length
Openings Run = Openings Area / Height-or-Span
Effective Run = Run Length - Openings Run
Window/door openings are entered as a total area and converted to an equivalent linear run they displace — a simplification, not an exact per-bay map (see Limitations).
Step 2 - Count real bays
Number of Bays = CEIL(Effective Run x 12 / On-Center Spacing)
A run that doesn't divide evenly into whole bays still rounds up to a real extra bay, the same way framing itself is placed at fixed intervals.
Step 3 - Pool the linear footage needed within each matching product
Linear Footage for a Run = Number of Bays x Height-or-Span
Product Group Total = Sum of every run/area using the SAME product width and R-value
Both batt and roll track the same linear footage of product consumed. A bay whose span exceeds one precut batt length needs more than one piece, but the leftover offcut from cutting a batt to length is normal site practice to reuse as the filler piece in a DIFFERENT bay — provided that bay takes the same product. Pooling is grouped by framing spacing (product width) AND R-value: a 16 in o.c. run and a 24 in o.c. run are always totaled separately, since an offcut from a 15 in batt cannot fill a 23 in bay.
Step 4 - Add wastage, then convert to whole batts or rolls exactly once per product group
Final Linear Footage = Product Group Total x (1 + Wastage / 100)
Batt: Final Batts (per group) = CEIL(That Group's Final Linear Footage / Precut Batt Length)
Roll: Final Rolls (per group) = CEIL(That Group's Final Linear Footage / Roll Length)
Job Total = Sum of every product group's own whole-unit count
Wastage is applied to each product group's own pooled linear footage directly — this step itself is NOT rounded, only scaled. The whole-unit (batt or roll) conversion is the only rounding, and it happens exactly once PER GROUP — not once per bay, and not once across a combined multi-group total — so real offcut reuse is reflected in the final count without pooling across physically incompatible products. Every group's own whole-unit count is then summed for the job total.
Step 5 - Bundle into packages per product group, add vapor barrier and cost
Packages (per product group) = CEIL(That Group's Final Batts / Batts per Package)
Total Packages = Sum of every product group's own package count
Vapor Barrier Area = Gross Surface Area x (1 + Wastage / 100)
Total Cost = Material Cost + Labor Cost + Vapor Barrier Cost
Packages are bundled WITHIN each product group before summing — one batt of a 15 in R-13 product and one batt of a 23 in R-19 product still need 2 separate packages even at 8 batts per package, never 1 combined package. Vapor barrier sheeting (where enabled) covers the whole assembly face — studs/joists included — so it is sized off the gross surface area (full on-center module width), not the narrower net insulated area the batt/roll product itself occupies. Material cost assumes the same price per package across every product group — if your products have different real prices, estimate each group's cost separately.
Real-World Batt/Roll 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 bay counts are computed |
| Product | roll (40 ft roll) | Sets the piece length used per bay |
| Wastage | 10% | Adds allowance on top of the bay-counted quantity |
Step 1 & 2 — Bay Counting & Linear Footage
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Bay count & linear footage | CEIL(Effective Run x 12 / Spacing) bays x span x 1 area(s) | 15 bay(s) per area |
| Total linear footage needed (before wastage) | Sum of the above | 300 ft |
Step 4 — Final Quantity & Wastage
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Final linear footage (incl. wastage) | 300 ft x (1 + 10/100) — scaled directly, not rounded to a whole foot | 330 ft |
| Final linear footage | Same as above | 330 ft |
Step 5 — Packages/Rolls
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Rolls | 330 ft / 40 ft | 9 unit(s) |
| Net insulated area | Bays x product width x height/span | 575 sq ft |
Therefore, for this job you need 9 roll(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 Batt & Roll 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.