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 1970s ranch house from energy-audit findings to finished attic, using the same math engine as the free Insulation Calculator. Every assumption is listed openly so you can swap in your own numbers.
The Scenario (Hypothetical Example)
A homeowner in IECC climate zone 5 has a 1970s ranch: 1,400 ft² (130 m²) attic footprint, currently holding about 4 inches of settled fiberglass (estimated R-11 remaining), with visible air bypasses at the top plates and plumbing penetrations. The goal: bring the attic to R-49 with blown cellulose, with air sealing done first. To keep the exercise honest, its assumptions are listed openly:
- Scope: attic floor only — walls and basement are separate future projects (illustrative).
- Existing insulation: ~4 in settled fiberglass, estimated at R-11 remaining (illustrative derate).
- Access: pull-down stairs, adequate headroom at the eaves for baffle installation (illustrative).
- Hazards: no vermiculite, no knob-and-tube wiring assumed for illustration — both must be verified on a real job before disturbing old material.
- Permits: no permit complications assumed for illustration; verify locally — many jurisdictions treat attic top-ups as non-permit work, but not all.
Project Specs (Given Assumptions)
| Spec | Metric | Imperial |
|---|---|---|
| Attic footprint (net, after subtractions) | 130.0 m² | 1,400 ft² |
| Existing insulation | ~100 mm settled fiberglass, est. R-11 remaining | |
| Target R-value | R-49 (RSI-8.6) | |
| New material | blown cellulose, R-3.5/inch (RSI-0.62 per 25 mm) | |
| Additional R needed | R-49 − R-11 = R-38 | |
| Installed depth of new material | 279 mm (chart depth for R-38) | 11.0 in |
| Waste allowance | 5% (blown-in, few obstructions) | |
| Approach | DIY with rented blower (illustrative) | |
Why R-38 of new material rather than a full R-49? The existing R-11 counts toward the target — topping up means filling the gap, and the calculator’s depth formula applies to whatever R-value you still need. Estimating the gap instead of the total is the discipline behind accurate insulation estimating — most over-ordering comes from ignoring what’s already there.
The Calculation: Inputs → Formula → Result
Step 1 — Net area
Gross footprint 1,456 ft² minus a 6 × 8 ft HVAC platform (48 ft²) and a 2.5 × 3.5 ft chimney chase (8.75 ft², rounded to 9 ft²): 1,456 − 48 − 9 = 1,399 ft² → 1,400 ft² (130.0 m²; check: 1,400 × 0.092903 = 130.06 m² ✓).
Step 2 — Depth for the R-value gap
R-38 needed ÷ R-3.5/inch = 10.86 in → install to the manufacturer’s chart depth for R-38: 11.0 in (279 mm). The formula breakdown explains why the chart depth, not the raw division, is the number that matters for blown-in products.
Step 3 — Bag count
Chart coverage at R-38 for the assumed product: ~13.5 ft² per 25-lb bag. 1,400 ÷ 13.5 = 103.7 → round up to 104 bags.
Step 4 — Waste allowance
104 × 1.05 = 109.2 → round up to 110 bags.
Result: 110 bags of blown cellulose to take this attic from ~R-11 to R-49.
Material Order List (Illustrative Quantities)
Below is what an order based on this scenario would contain — waste applied, quantities rounded up. Items beyond the cellulose are illustrative estimates for an attic of this size:
| Material | Quantity to order | Notes |
|---|---|---|
| Blown cellulose, 25-lb bags | 110 bags | chart depth 11 in for R-38 over existing R-11 |
| Attic baffles (vent chutes) | 70–80 | every rafter bay at eaves (illustrative count) |
| Expanding foam sealant | 8–10 cans | top plates, penetrations, chase gaps |
| Caulk (fire-rated where needed) | 4–6 tubes | smaller gaps, fixture boxes |
| Recessed-light covers (if non-IC) | as needed | verify fixture ratings first |
| Depth rulers | 10–12 | stapled to joists for verification |
| Hatch insulation kit / foam-board box | 1 | insulated, weatherstripped hatch cover |
| Respirator (P100), goggles, coveralls | 2 sets | two-person crew |
Notice the sequencing discipline: air sealing and baffles go in before the blower starts — the order list is also the work order. The same structured approach is covered in the pro tips for attic work.
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 item | Illustrative range (USD) |
|---|---|
| Cellulose, 110 bags @ ~$9–$13 | $990–$1,430 |
| Blower rental | $0 (free with bulk purchase, typical) |
| Baffles, sealant, caulk, light covers | $250–$450 |
| Hatch kit, depth rulers, safety gear | $80–$150 |
| DIY total | $1,320–$2,030 |
| Pro-installed equivalent (for comparison) | $2,100–$4,200 |
So the DIY route lands roughly in the $1,300–$2,000 band, while hiring the same scope roughly doubles it. These ranges use the same method as the insulation cost guide — ranges, not quotes, until a supplier is involved. Whether the DIY savings are worth two weekends is the question the DIY vs. pro guide works through.
What This Scenario Teaches
- Estimate the gap, not the total. The existing R-11 counted toward R-49 — 110 bags instead of the ~150 a from-scratch R-49 would need. Measuring existing depth first is the highest-leverage step of the estimate.
- Chart depth beats raw division. 10.86 inches of arithmetic became 11 inches of chart depth — the chart already accounts for settling that raw math ignores.
- Air sealing is scope, not an accessory. The sealant and caulk lines in the order list are what make the R-49 perform as rated; blowing over open bypasses would have degraded the whole investment.
- Round up at every step. 103.7 bags became 104, then 109.2 became 110. A thin corner never gets topped up later.
Frequently Asked Questions
How many bags of cellulose for a 1,400 ft² attic going from R-11 to R-49?
Using this scenario’s specs: R-38 of new material at 11 inches chart depth, ~13.5 ft² per bag → 104 bags, 110 with 5% waste. Enter your own area and existing R-value in the Insulation Calculator to match your attic.
Should the old insulation be removed first?
Not in this scenario — dry, uncontaminated fiberglass can stay and counts toward the target. Removal is warranted for water damage, rodent contamination, or vermiculite (possible asbestos — test before disturbing).
How long does a DIY blow-in take at this scale?
Illustrative planning figure: air sealing and baffles one full day for two people; blowing 110 bags roughly half a day with a two-person crew (one feeding, one on the hose). Actual pace varies with attic access and experience.
Can one person do this alone?
Not practically for blown-in — the blower needs a feeder at the machine and an operator on the hose simultaneously. Batt work can be solo; blown-in is a two-person minimum.
Ready to price your own attic? Swap the ranch numbers for your dimensions in the free Insulation Calculator — same formula, same waste logic, your project. Hypothetical case study — illustrative example, not a real project.
