Hypothetical case study — illustrative example, not a real project. Reading an insulation estimate on a screen is one thing; walking through a full project is what makes the method stick. This article follows a single hypothetical garage-to-workshop insulation job from measurements to material order, using the same math as the free Drywall Insulation Calculator. Every assumption is listed openly so you can swap in your own numbers — none of this happened, and no client is involved.
The Scenario (Hypothetical Example)
A homeowner converts a detached garage into a heated workshop: insulate all four walls with R-13 fiberglass batts before drywall. Building is 20 ft × 24 ft with 9-ft walls (6.10 m × 7.32 m × 2.74 m), 2×4 framing at 16 in o.c. Openings: two 9×7 ft garage doors, one 3×6.67 ft man door, four 3×4 ft windows. Assumptions, stated openly:
- Scope: walls only — the attic/ceiling is a separate future phase and is excluded.
- R-value target: R-13 (RSI 2.29) assumed adequate for illustration; a real project would confirm the target with the local AHJ first.
- Supply: big-box retail pricing assumed for illustration — check local suppliers.
- Labor: DIY for the base budget; a hired-install alternative is priced alongside.
- Permits/inspections: no complications assumed for illustration; verify locally.
Project Specs (Given Assumptions)
| Spec | Imperial | Metric |
|---|---|---|
| Building footprint | 20 ft × 24 ft | 6.10 m × 7.32 m |
| Wall height | 9 ft | 2.74 m |
| Framing | 2×4 at 16 in o.c. (89 mm cavity) | |
| Insulation | R-13 fiberglass batts, 15 in × 105 in | |
| Waste allowance | 10% | |
| Garage doors (2) | 2 × (9 × 7 ft) = 126 ft² | 11.71 m² |
| Man door (1) | 3 × 6.67 ft = 20 ft² | 1.86 m² |
| Windows (4) | 4 × (3 × 4 ft) = 48 ft² | 4.46 m² |
The Calculation: Inputs → Formula → Result
Step 1 — Gross wall area
Perimeter = 2 × (20 + 24) = 88 ft. Gross = 88 × 9 = 792 ft² (73.58 m²). Metric check: 2 × (6.096 + 7.315) = 26.82 m × 2.743 m = 73.58 m². ✓
Step 2 — Deduct openings
126 + 20 + 48 = 194 ft² (18.02 m²). Net = 792 − 194 = 598 ft² (55.56 m²). The estimating guide covers the deduction method used here.
Step 3 — Batt coverage and count
15 in × 105 in batts (sized for 9-ft walls): (15 ÷ 12) × (105 ÷ 12) = 1.25 × 8.75 = 10.9375 ft² per batt (0.381 m × 2.667 m = 1.016 m²).
598 ÷ 10.9375 = 54.67; × 1.10 waste = 60.14 → round up to 61 batts. Metric: 55.56 × 1.10 = 61.11 m² ÷ 1.016 = 60.14 → 61 batts. ✓
Step 4 — Convert to bags
At an illustrative 8 batts per bag: 61 ÷ 8 = 7.625 → 8 bags. (Check the actual bag label — counts vary by manufacturer.)
Step 5 — R-value confirmation
R-13 in a 3.5-in cavity → RSI 13 × 0.176 = 2.29. Assumed adequate here; a real project confirms against the local energy code per the code compliance guide.
Material Order List (Illustrative Quantities)
| Material | Quantity to order | Notes |
|---|---|---|
| R-13 fiberglass batts (15×105) | 61 batts (8 bags) | 10% waste included; rounded up |
| 6-mil poly vapor barrier + tape | 1 roll (~800 ft² coverage) | if required by local code |
| Caulk + canned spray foam | 6 tubes + 4 cans | air sealing plates/penetrations |
| PPE (masks, gloves, goggles) | 1 set per worker | fiberglass protection |
Cost Estimate (Ranges — Check Local Suppliers)
Disclaimer: prices vary widely by region, season, and market conditions. Treat these as rough US-style ranges for budgeting only — confirm every figure with local suppliers. Line items sum exactly as shown.
| Line item | Illustrative range (USD) |
|---|---|
| R-13 batts, 61 count (598 ft² @ ~$0.60–$0.90/ft²) | $360–$540 |
| 6-mil poly vapor barrier + seam tape | $40–$70 |
| Air-sealing supplies (caulk, foam cans) | $60–$100 |
| PPE and miscellaneous | $30–$50 |
| Debris disposal | $0–$80 |
| DIY materials total | $490–$840 |
| Professional install alternative (598 ft² @ ~$1.00–$2.00/ft²) | $600–$1,200 |
| Hired-install project total | $1,090–$2,040 |
Check the sums: DIY low = 360 + 40 + 60 + 30 + 0 = $490; DIY high = 540 + 70 + 100 + 50 + 80 = $840. Hired low = 490 + 600 = $1,090; hired high = 840 + 1,200 = $2,040. Every line item adds exactly to its total. These ranges use the same method as the cost guide — ranges, not quotes, until a supplier is involved.
What This Scenario Teaches
- Openings are a quarter of the wall. 194 ft² of 792 ft² — nearly 25% — vanished in deductions. Gross-area estimating would have overbought by a full 15 batts.
- Waste is a multiplier applied last. The 10% went on the net 598 ft² via × 1.10, not on a rounded figure — 60.14, then up to 61.
- Rounding up is the whole game at the end. 60.14 batts became 61; 7.625 bags became 8. Fractions don’t insulate bays.
- DIY vs. hired roughly doubles the budget. $490–$840 in materials vs. $1,090–$2,040 hired — the DIY vs. pro guide breaks down when each makes sense.
- The R-value target is an input, not an output. The calculator sizes the order; the local energy code sets the R-13 assumption — confirm it before buying.
Frequently Asked Questions
How many batts does a 20×24 ft garage with 9-ft walls need?
Under this scenario’s specs: 792 ft² gross − 194 ft² of openings = 598 ft² net; with 10% waste, 61 × (15×105 in) R-13 batts, or about 8 bags. Enter your own dimensions in the Drywall Insulation Calculator to match your building.
Why 105-inch batts instead of 93-inch?
Batt length matches wall height minus plates: 9-ft (108-in) walls less ~3 in of plates = 105-in bays. Eight-foot walls use 93-in batts the same way.
Is the $490–$840 DIY range realistic?
As an illustrative planning range, yes — it’s 598 ft² of R-13 material at typical retail coverage pricing plus air-sealing supplies. Real totals move with local prices, which is why every line item says to confirm with suppliers.
Would spray foam change the math?
The area math stays identical — 598 ft² net. Only the material, depth formula (installed depth = target R ÷ R-per-inch), and cost change: closed-cell foam at ~$4–$9/ft² installed would put this project roughly in the $2,400–$5,400 range for the foam alone.
What about the ceiling?
Excluded from this scenario by design — it was scoped as walls-only. A ceiling/attic phase would estimate separately by target depth (R-38+), as covered in the material guide.
Ready to estimate your own walls? Swap the garage numbers for your dimensions in the free Drywall Insulation Calculator — same formulas, same waste logic, your project.
