"Insulate your house" is $500–$15,000 of advice depending on which surface it lands on. The payback per project differs by an order of magnitude, and the ranking surprises people. Here is the ladder, with the arithmetic under each rung.
The payback ladder
| Rank | Project | Typical cost | Simple payback | Why |
|---|---|---|---|---|
| 1 | Air sealing (attic leaks, hatch) | $150–600 DIY | <1–2 years | Cheapest dollars per Btu saved, ever |
| 2 | Attic floor to zone target | $1,500–3,000 | 2–5 years | Big surface, cheap R, no walls to open |
| 3 | Rim joist foam board | $100–300 DIY | 2–4 years | Small area, framing-heavy, easy access |
| 4 | Basement/crawlspace walls | $1,000–4,000 | 5–10 years | Ground-coupled loss, damp-prone — needs the right material |
| 5 | Wall cavities (drilled and blown) | $2,000–6,000 | 10–20 years | Real but slow; often a comfort play |
| 6 | Window replacement for energy | $8,000–20,000 | 20–40+ years | Comfort wins, payback rarely does |
Why the ranking looks like this
Payback = cost ÷ annual savings. Savings scale with surface area × temperature difference × (1/R before − 1/R after). The attic wins because it has the biggest area, the biggest temperature difference (attic to house), and the cheapest R. Windows lose because they are expensive per square foot and start with R-2-to-R-3 units — tripling a small number saves a small number, however nice the frames look.
The exceptions that reshuffle
- HVAC in the attic: duct losses can exceed window losses; sealing and insulating ducts jumps the queue.
- Hot climates: attic radiant gain and duct location still favor the attic — but cooling savings are smaller than heating savings, stretching paybacks.
- Rebates: a 30% utility rebate rescales everything; run the cost comparison with your rebate before ranking.
- Comfort: cold upstairs bedrooms and drafty rim joists have real dollar value the payback table ignores — spending there is rational beyond the bill.
Run the ladder top-down until the payback passes your horizon, then stop. The house will still be standing when the windows' turn comes — the attic will have paid for itself twice by then.