Insulation payback for 600 ft² from R-30 to R-38 in Zone 3 heating with natural gas
| Annual saving4 therm of natural gas | $5 |
|---|---|
| Heat saved600 ft² × 2,800 HDD × 24 × 0.007 | 0.3MMBTU/yr |
| Cost — DIY material$0.030 per ft² per R | $144 |
| Cost — installed$0.060 per ft² per R | $288 |
| Payback — DIY | 27.1years |
| Payback — installed | 54.3years |
| 20-year return on DIY costinsulation does not wear out | 0.7× |
| First R-10 is worthgoing from R-30 up | $6/yr |
| Last R-10 is worthsame step — target is within R-10 of existing | $6/yr |
| ClimateAtlanta, Dallas, Los Angeles | Zone 3 — 2,800 HDD |
| Heating system80% efficient, $1.5/therm assumed | Natural gas |
- Area
- 600 ft²
- Existing R-value
- 30
- Target R-value
- 38
- Climate zone
- Zone 3
- Heating fuel
- natural gas
The saving
600 ft² going from R-30 to R-38 in Zone 3 (2,800 heating degree days — Atlanta, Dallas, Los Angeles) cuts about 0.3 MMBTU a year of heat loss. On natural gas that is 4 therm of fuel, worth $5 a year.
Payback
DIY material comes to about $144, which pays back in 27.1 years. Contractor-installed runs near $288 and pays back in 54.3. Over twenty years the DIY cost returns 0.7 times over — insulation has no moving parts and does not wear out.
Why the last inch is nearly worthless
This is the part almost nobody calculates, and it changes decisions.
Savings follow the difference between two reciprocals, not the difference between two R-values. The first R-10 of this upgrade is worth $6 a year. The last R-10 is worth $6.
That is why "more R is always better" is bad advice. Past a point you are buying insulation whose payback is longer than the roof above it.
Climate is half the answer
The same job saves $5 in Zone 3. In Zone 6 at 7,900 HDD it would save $15; in Zone 2 at 1,500 HDD, $3. Advice written for one climate is simply wrong in another, and most insulation advice on the internet does not say which climate it was written for.
What is assumed
Fuel priced at the US average, and the relationship is linear — scale it for your own bill. Utility rates vary by more than a factor of two across the country, which is enough to move a ten-year payback either side of five.
Notes
- **The first R-10 does most of the work.** Going from R-30 up buys $6 a year; the last R-10 of this upgrade buys $6. Savings track the difference between two *reciprocals*, not the difference between two R-values, so every additional inch returns less than the one before it. "More R is always better" is how people talk themselves into a 30-year payback.
- A 27.1-year payback is long. In Zone 3 with natural gas the heat being lost is not worth enough to justify this much insulation. Either stop at a lower R-value, or spend the money on air sealing and duct work instead.
- Climate is half the answer. The same job in Zone 3 (2,800 HDD) saves $5; in Zone 6 at 7,900 HDD it would save $15, and in Zone 2 at 1,500 HDD only $3. Advice written for one climate is wrong in another.
- Natural gas at 80% efficiency means 25% of what you buy goes up the flue before it reaches the room. Insulation saves the delivered heat, so the fuel saved is larger than the heat saved.
- Prices assumed: $1.5 per therm. Scale the saving straight through for your own rate — the relationship is linear, and utility rates vary by more than a factor of two across the US.
- Existing R-value is taken as installed and dry. Insulation that has been compressed, wetted or walked on performs below its label, so real savings on an old attic often run higher than this.
Nearby sizes
- Insulation payback for 400 ft² from R-30 to R-38 in Zone 3 heating with natural gas
- Insulation payback for 800 ft² from R-30 to R-38 in Zone 3 heating with natural gas
- Insulation payback for 1000 ft² from R-30 to R-38 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-19 to R-38 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-11 to R-38 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-30 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-49 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-60 in Zone 3 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-38 in Zone 2 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-38 in Zone 4 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-38 in Zone 5 heating with natural gas
- Insulation payback for 600 ft² from R-30 to R-38 in Zone 3 heating with electric resistance
- Insulation payback for 600 ft² from R-30 to R-38 in Zone 3 heating with a heat pump
Read more
- Finding Your IECC Climate Zone, and Why the Map Is Not the AnswerClimate zones are assigned by county, not by zip code. Where the two disagree, the county wins — and they disagree more often than you would expect.
- Zone 5 Attic Insulation: The Target, the Depth, and the Top-Up MathsZone 5 covers a large share of the country and sits right on the step where the code moved from R-49 to R-60. Which one applies to you is a local question.
- R-38 vs R-49 vs R-60: What Each Step Actually RemovesEach step up removes less than the one before it. Knowing how much less is the difference between a sensible upgrade and buying insulation you will never notice.
- R-Value by Climate Zone: Ceilings, Walls and Floors in One TableThe whole envelope in one table, plus the thing nobody says out loud — the edition your jurisdiction enforces may be two cycles behind the current one.