attic-insulation-payback

Use when considering attic insulation upgrade, before accepting an insulation contractor quote, or when comparing home energy upgrades — R-value stack math, HDD/CDD heating-energy model from fuel bills, real payback in years with degradation and air-sealing priority doctrine (seal before you stack), and quote forensics that catches overblown R-value claims and missing prep charges.

Overview

Attic insulation is the single highest-return home energy upgrade for most houses — when it's actually thin. The physics has brutal diminishing returns: going from R-2 to R-13 cuts heat loss through the attic by ~85%; going from R-38 to R-60 cuts it by ~37% more of a small remainder. Contractors quoting "double your R, halve your bill" are exploiting the fact that homeowners don't know which side of the curve their attic is on.

This skill computes it honestly. You enter attic area, current R-value, target R-value, fuel type, local climate (heating degree-days), and system efficiency; the tool runs the degree-day heat-loss model against your actual fuel bills (calibration), then outputs annual savings, simple payback, and the verdict: DO IT / OVERSOLD — SEAL FIRST / NOT WORTH IT, with the air-sealing-first doctrine (moving air through fiberglass carries heat straight through the batt — stacking R over unsealed bypasses buys almost nothing).

When to Use

• Contractor quote for attic top-up (blown cellulose/fiberglass) — sanity-check the savings claim
• Deciding between competing home energy upgrades (insulation vs windows vs heat pump)
• House feels cold upstairs / ice dams recur — is more R actually the fix?
• Comparing DIY blown-in vs contractor price
• Verifying a "R-60" quote will actually deliver R-60 (settlement, density, existing R debt)

Don't use for: wall insulation (different economics — cavity fill cost dominates), radiant barriers (not conductive insulation; separate physics), or mobile homes. Flat-roof/hot-climate cooling dominance: use the CDD branch but expect wider error bars.

Quick Start

bash

# Core payback: 1200 sqft attic, R-19 current -> R-60 target, gas heat, 5200 HDD65
python3 scripts/attic_payback.py --area 1200 --r-current 19 --r-target 60 \
    --fuel gas --hdd 5200 --price 3.20 --system-eff 0.80 \
    --quote 3400

# Calibration against reality: your gas bill says 90 therms/mo avg in winter
python3 scripts/attic_payback.py --area 1200 --r-current 19 --r-target 60 \
    --fuel gas --hdd 5200 --price 3.20 --system-eff 0.80 --quote 3400 \
    --calibrate-winter-therms 90

# Cheap-test mode: is the quoted R even real? (12 inches settled cellulose)
python3 scripts/attic_payback.py --r-check --inches 12 --material cellulose

The Physics (what the script computes)

1. Attic heat loss share: U = 1/R (Btu/h·ft²·°F). Q = U × area × ΔT. Degree-day integration over the heating season gives annual Btu through the ceiling.
2. Savings = Q(current R) − Q(target R), converted to fuel units (therms/gallons/kWh) at your price and system efficiency.
3. Bill calibration: predicted whole-house gas use from attic loss alone can't exceed the meter. If model output > ~40% of winter fuel, either the R estimate or the area is wrong — the script flags it and rescales.
4. Diminishing-returns table: prints savings from R-current→R-30/38/49/60 so you can see the flattening curve and pick the knee, not the brochure number.

Doctrine: Seal Before You Stack

• Air first, then insulation. Fiberglass/cellulose do almost nothing to moving air. Attic bypasses — top-plate gaps, can lights, chase ways, bath fans venting into the attic — can waste more energy than a missing R-20. Seal (caulk/foam) before or WITH the top-up; most utility rebates require it.
• Compressed insulation loses R. A closed soffit-baffle-free install that blocks ventilation or a floor decked over leaves dead zones; and R-value labels assume full loft — settled 12-yr-old cellulose delivers ~80% of label.
• Vented attic stays cold in winter. The insulation goes on the attic floor, not the underside of the roof deck (unless it's a conditioned attic — different project entirely). Warm roofs = ice dams.
• The last inch is the cheapest per R but the least valuable per dollar of energy. The knee of the savings curve is usually R-38–49 in cold climates; R-60+ pays mainly in very cold zones or cheap material.

Verdict Bands

Result	Verdict
Payback ≤ 7 yr, attic R < 30	DO IT — textbook case
Payback ≤ 7 yr but R ≥ 38 already	Check calibration; savings likely oversold
Payback 7–15 yr	BORDERLINE — do only with rebate or DIY cost
Payback > 15 yr	NOT WORTH IT — the curve has flattened; spend on air sealing instead
No air sealing in quote	OVERSOLD — SEAL FIRST; get sealing priced separately

Common Pitfalls

1. Trusting the brochure R-value on old existing insulation — measure depth × material, apply settlement (cellulose ~20% over 12+ yrs), subtract compressed/kitty-litter-squared zones.
2. Skipping bill calibration. A model that says the attic is 70% of your bill in a house with single-pane windows is lying; rescale or re-measure.
3. Paying for R-60 when the knee is R-38. The table output exists so you can quote the contractor the flat part of the curve.
4. Unsealed can lights / chases under new insulation — the money walks out through the bypasses you just buried.
5. Venting bath fans into the attic because the insulation crew "was up there anyway" — moisture destroys R and sheathing; demand exterior venting.
6. DIY without a blower (hand-spreading bags) — density is wrong, R claim void; machine-blown only.

Verification Checklist

• Attic depth measured at ≥5 spots; material identified; settlement applied
• Area from house footprint (outer walls), not room interior
• HDD from the nearest weather station (climate zone), HDD65 base
• Fuel price from a real recent bill (per-therm delivered, all charges)
• Savings checked against winter bills (model attic share ≤ ~40%)
• Air sealing included in quote or priced separately BEFORE the top-up
• Diminishing-returns table read; knee chosen over brochure R
• Payback computed at honest quote price including prep/deck removal

Example Session

text

$ python3 scripts/attic_payback.py --area 1200 --r-current 19 --r-target 60 \
    --fuel gas --hdd 5200 --price 3.20 --system-eff 0.80 --quote 3400
ATTIC INSULATION PAYBACK — 1200 ft², R-19 -> R-60, gas @ $3.20/therm
  season heat loss now:   98.5 therms (R-19)
  season heat loss after: 31.2 therms (R-60)
  annual savings:         ~67.3 therms = $215/yr
  payback on $3,400 quote: 15.8 yr   [NOT WORTH IT at this price]
  diminishing returns (savings vs staying at R-19):
    R-30  $116/yr   (~$912 pro-rated)
    R-38  $158/yr   (~$1,576 pro-rated)
    R-49  $193/yr   (~$2,488 pro-rated)
    R-60  $215/yr   (~$3,400 pro-rated)
  -> the knee is R-38: R-49/60 add <$60/yr over R-38. Ask for the R-38 price.
VERDICT: NOT WORTH IT at this price — the savings curve has flattened;
  with a utility rebate or DIY blow machine (~$900) it becomes a win;
  air sealing at $200-600 returns faster than any depth here.