Prioritizing an $8,000 Retrofit Budget on a Leaky 1978 House
A 2,200-square-foot single-story house built in 1978 in a hot-humid climate has a 3-ton, 10 SEER split system with an 80% AFUE gas furnace in a vented attic. A blower door test reads 3,200 CFM50 with 8-foot ceilings. Duct testing with registers taped reads 220 CFM25 total leakage, of which 190 CFM25 is leakage to the attic; the supply trunk and return are largely in the attic, covered by deteriorated R-6 duct wrap under about 4 inches of loose-fill ceiling insulation (roughly R-13). Attic temperature at 3 p.m. is 138°F. The homeowner wants to replace the air conditioner first because a contractor quoted a 16 SEER2 system. The budget is about $8,000. Summer electric bills run around $260 per month with the thermostat at 78°F. The house has a natural-draft gas water heater in a closet opening to the hall and a bath fan that discharges into the attic.
1. Calculate ACH50 and the duct leakage percentages, then explain why these numbers change the priority order the homeowner has in mind.
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Calculations:
- Conditioned volume = 2,200 × 8 = 17,600 ft³.
- ACH50 = (3,200 × 60) ÷ 17,600 = 10.9 air changes per hour at 50 Pa. That is a leaky envelope; modern tight construction targets roughly 3 ACH50 or lower, and many programs use 5 ACH50 as a threshold for significant air-sealing savings. Estimated natural infiltration is on the order of 10.9 ÷ 17–20 ≈ 0.55–0.65 ACH.
- Total duct leakage = 220 ÷ 1,200 = 18.3% of system airflow - well above the 10% figure most utility and code-based programs treat as the target.
- Leakage to the attic = 190 ÷ 1,200 = 15.8%, meaning nearly all the leakage is in the worst possible location.
Why this reorders priorities: Equipment efficiency multiplies the duct and envelope losses. In a 138°F attic, return leakage pulls 138°F air into the airstream, directly adding sensible load and lowering delivered capacity, while supply leakage dumps cooled air into the attic. A 16 SEER2 condenser attached to 18% leaky ducts in a 138°F attic may deliver less than the existing system's rated capacity. Dollars spent on ducts and envelope are not glamorous, but they reduce the load the new equipment must meet and improve comfort immediately - and they are cheaper per unit of savings here.
2. Recommend a sequencing plan for the $8,000 that maximizes savings and durability, and justify why equipment replacement should come last.
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Recommended sequence:
- Air-seal the attic ceiling plane first (top plates, penetrations, chases, can lights rated for insulation contact, dropped soffits). This cuts stack effect and reduces the pressure difference that drives both infiltration and duct leakage. Doing it before adding insulation means you can still find the holes.
- Seal and then bury or replace the ducts. Mastic-seal every joint, boot, and plenum connection, then enclose the ducts in insulation to a high R-value rather than leaving R-6 wrap exposed to 138°F attic air. Duct sealing must precede insulation so the joints remain accessible.
- Upgrade ceiling insulation to the level recommended for the climate zone, after air sealing and duct work.
- Vent the bath fan to the outdoors, not into the attic - moisture in the attic undermines both the insulation and the duct wrap.
- Equipment last, with a load calculation. After load reduction, a smaller and/or right-sized system may be appropriate; replacing first locks in an oversized unit sized for a leaky house.
Justification: Duct and envelope measures reduce load permanently and cost less per unit of savings here. Equipment efficiency is a multiplier applied to whatever the load and distribution system are. Also caution the homeowner that a 16 SEER2 quote based on rules of thumb rather than a room-by-room Manual J is likely oversized.
3. Identify the safety testing that must accompany this work, and describe how you would verify performance after the upgrades.
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Combustion safety (non-negotiable): Tightening a house that contains a natural-draft gas water heater can cause backdrafting and spillage of combustion products. Before and after air sealing:
- Perform a worst-case depressurization test on the combustion appliance zone (CAZ) - all exhaust fans running, interior doors positioned per protocol - and check draft in the water heater flue.
- Test for spillage at the draft hood, check for flue blockage, and confirm the water heater has a proper source of combustion air after sealing. If it fails, options include a dedicated combustion air opening, an induced-draft or sealed-combustion replacement, or relocating the appliance - this must be resolved before the house is tightened.
- Verify the bath fan is ducted outdoors with a tight, insulated, correctly sloped run and confirm measured exhaust airflow at the grille.
Verification after the work:
- Repeat the blower door - expect meaningful reduction from 10.9 ACH50; document before/after.
- Repeat the duct blaster - target total leakage at or below about 10% of airflow, with leakage to outside minimized.
- Inspect all ducts for mastic coverage and insulation contact.
- Measure the system: external static near rating, airflow at roughly 400 CFM/ton, delta-T, superheat/subcooling per the chart, and room temperatures on a design day.
- Explain to the homeowner what was measured and what to expect on the next bill - the credibility of the whole job rests on documented before/after numbers.