Introduction
Hypothetical case study — illustrative example, not a real project. The numbers below are worked through a realistic scenario to show how the calculator’s outputs turn into decisions, a material order, and a budget. No real house, client, or contractor is involved; the value is in watching the full planning sequence run end to end.
The scenario: a homeowner in a humid climate with a 30 ft × 40 ft (9.144 m × 12.192 m) vented crawl space, 3 ft (0.914 m) clearance, musty odors in summer, and visible condensation on ductwork. The goal: convert to a sealed, encapsulated crawl space. The Crawl Space Calculator provides every quantity used below.
The Starting Point
The homeowner’s observations, mapped to causes:
| Symptom | Likely cause | Implication for the plan |
|---|---|---|
| Musty odor in summer | Humid outside air condensing on cool ducts and framing | Vented strategy failing; sealing warranted |
| Condensation on ductwork | Warm moist air meeting cool metal | Dehumidification needed after sealing |
| Damp soil, no standing water | Soil moisture evaporating, no bulk water entry | Vapor barrier sufficient; no sump required |
| Downspouts discharging at foundation | Roof water feeding the damp soil | Exterior fix first — cheapest line item |
| Sagging floor batts | Old insulation, moisture-softened supports | Remove; insulate walls instead after sealing |
Key judgment call: no standing water means no sump or interior drain — the moisture is vapor and condensation, solvable with barrier + sealing + dehumidification. Had there been standing water, drainage would come first, per the pro tips guide.
The Quantities
From the calculator, for 30 × 40 ft × 3 ft:
- Floor area: 1,200 ft² (111.5 m²)
- Perimeter: 140 linear ft (42.7 m)
- Vapor barrier with 15% allowance (piers present): 1,200 × 1.15 = 1,380 ft² (128.2 m²)
- Wall area for foam: 140 × 3 = 420 ft² (39.0 m²)
- Volume: 3,600 ft³ (101.9 m³) → 70–90 pint/day dehumidifier class
- Vent area (for reference, being eliminated): 8 ft² (0.74 m²) — sealed instead
The derivation of each figure is in the formula guide; measuring technique in the estimating guide.
The Plan and Sequence
- Exterior water work (DIY weekend): downspout extensions to 6+ ft (1.8 m), regrade one low side. Cost: ~$250.
- Prep (DIY weekend): remove old batts and debris, seal rim joists and penetrations with caulk and foam. Cost: ~$200 in sealants.
- Vapor barrier (DIY, 2 people, one long weekend): 20-mil reinforced poly, 12 in (300 mm) laps, taped seams, 12 in up the walls, piers wrapped. One 20×100 ft roll covers the 1,380 ft² (128.2 m²) need. Cost: ~$2,400 + $150 tape.
- Wall insulation (DIY weekend): 2 in XPS on 420 ft² (39 m²) of wall ≈ 14 sheets, mechanically fastened, leaving the termite inspection gap. Cost: ~$1,300.
- Vent sealing + dehumidifier (pro half-day for electrical): vents sealed with foam board and sealant; 90-pint dehumidifier with condensate to daylight; hygrometer installed. Cost: ~$1,800 installed + $40 monitor.
The Budget
| Phase | DIY/Pro | Cost |
|---|---|---|
| Exterior water management | DIY | $250 |
| Prep and air sealing | DIY | $200 |
| 20-mil barrier + tape | DIY | $2,550 |
| Wall foam board | DIY | $1,300 |
| Vent sealing + dehumidifier | Pro (electrical) | $1,840 |
| Total | Hybrid | ~$6,140 |
Against the cost guide brackets: a full-pro encapsulation of this space would run $9,500–$12,000+. The hybrid path saves roughly $3,500–$6,000 by DIY-ing the labor-heavy but low-skill phases — the exact split the DIY vs. pro guide recommends.
Verification and Outcome
Before close-up, the homeowner runs the pre-close-up checklist: seams taped, piers wrapped, vents sealed, foam fastened, dehumidifier draining, hygrometer reading. Over the first summer, the monitor holds 50–55% relative humidity — under the 60% target — and the musty odor doesn’t return. The condensation problem is gone because the humid air source is gone, not because the ducts changed.
What would have changed the plan: standing water (add sump + drain, +$1,000–$3,000), clearance under 18 in (reconsider DIY phases), or a radon-prone area (add sub-membrane rough-in before the liner — see the code compliance guide).
Related Guides
- Complete Guide — the vented vs. sealed decision behind this scenario.
- Cost Guide — the brackets this budget sits inside.
- DIY vs. Pro — the hybrid split used here.
- Pre-Close-Up Checklist — the verification pass.
- Pro Tips — the sequencing logic.
Frequently Asked Questions
Is this case study a real project?
No — it’s a hypothetical, illustrative example built from typical conditions and planning-range costs. Real projects vary with climate, soil, foundation condition, and local pricing. Use it as a planning template, not a quote.
How long does a project like this take?
The hybrid sequence above spans about 4 weekends of DIY plus a pro half-day — roughly a month of weekends. A full pro crew does the same scope in 3–5 working days.
Why was no sump pump needed in this scenario?
Damp soil with no standing water indicates vapor and condensation, not bulk water entry — a vapor barrier plus exterior water management addresses it. Standing water after rain would change the diagnosis and the budget.
What was the single highest-value step?
The exterior water work — $250 of downspout extensions and regrading — removed the moisture source feeding the damp soil. Everything downstream (barrier, dehumidifier) then handles residual vapor instead of fighting a continuous supply.
How was success verified?
A hygrometer left in the space holding 50–55% relative humidity through the first humid summer, plus the disappearance of the musty odor and duct condensation. Monitoring, not assumptions.
What would make this a pro-only job?
Standing water requiring drainage design, clearance too low to work in, structural issues (sagging or rotted framing), or significant mold — any of which moves the project into professional assessment before improvement work begins.
Get Your Number
Use this scenario as a template: document your symptoms, run your dimensions through the Crawl Space Calculator, sequence water → sealing → barrier → insulation → drying, and verify with the checklist before close-up. Your quantities, costs, and timeline will differ — the method won’t.
Disclaimer: This is a hypothetical illustrative example, not a real project. Calculator results are planning estimates based on the dimensions you enter. Actual costs and requirements vary widely — verify with local suppliers and contractors, and consult a licensed professional or your local building authority for moisture, structural, or code-related decisions.
