How much extra can a tight envelope cost before it stops paying back on a 480 sq ft backyard cottage?

What it found

Inverts the question house-energy asks: instead of the lowest-cost package, the largest extra first cost each upgrade can carry before its 30-year energy saving no longer covers it, on the house-open-design ADU (24 × 20 ft). The whole tight package (advanced 2x6 wall with R-5 foam, R-49 attic, U-0.27 windows, 3 ACH50, R-30 floor) saves $72 to $158 a year with a heat pump, so at a 3% real rate it can carry $1,409 (Houston) to $3,104 (Minneapolis) of extra cost, and $892 to $1,965 at 7%. Energy is linear in heat loss, so the one-at-a-time savings add up exactly to the package. At house-energy's unsourced unit prices the attic ($319 extra) only breaks even in Minneapolis, air sealing ($500) needs energy 1.3 to 2.6 times as dear, and the windows ($1,392) 2.7 to 5.5 times; the wall switch is the best of the five upgrades. Stylised: degree-day climates from memory, every price assumed, no first-cost data for the wall or floor, not a design anyone should build from without a licensed review.

A figure from its own output

Paper, code and results

• README and method
• Experiments and results
• Code
• Tests

More in Open-source residential construction

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Produced with AI assistance (Claude). Results are from the code in this repository and are simulated or stylised unless the abstract says otherwise; nothing here implies affiliation with or endorsement by any lab, researcher or company cited. MIT licensed.