Duct Leakage to Outside in a Vented Attic: Seal It or Replace It?
Single-story 1,500 sq ft ranch, slab-on-grade, 8-ft ceilings, built 1986, mixed-humid climate. The 2.5-ton split heat pump has its air handler on a panned platform in a vented attic; eight R-4 flex supply runs and one central return cross 128°F attic air. Conditioned volume: 12,000 cu ft. Blower door: 3,200 CFM50. Duct Blaster: 320 CFM25 total leakage, 265 CFM25 leakage to outside. Pressure pan with the house at -50 Pa (register referenced to room): -2 to -4 Pa at seven supplies, -9 Pa at the master bedroom supply. The return platform has visible open seams to the attic and a 2-in. gap where the filter grille frame meets the drywall. Homeowner complaints: dust, bedrooms that never reach setpoint in July, and $260 July power bills. Static pressure at the air handler: 0.92 in. w.c. on a 0.5-in.-rated coil.
1. Compute the ACH50 and the leakage-to-outside ratio, then interpret the pressure pan and static pressure data. Where is the leakage actually coming from?
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Run the numbers before drawing conclusions.
- Envelope: ACH50 = 3,200 × 60 ÷ 12,000 = 16 ACH50. Leaky, but not the first priority here.
- Duct leakage: 265 ÷ 320 = 83% of the leakage is to outside - the worst kind, because that is conditioned air the owner pays for and then loses, or attic air drawn in to replace it.
- 265 CFM25 against a nominal ~1,000 CFM (2.5 tons × 400 CFM/ton) is about 26% leakage to outside. The usual target for ducts in unconditioned space is roughly 6% or less.
- Pressure pans: readings of -2 to -4 Pa against a -50 Pa house mean the supply boots are comparatively tight. A wide-open register would read far closer to the house-to-outside difference. The -9 Pa at the master is a modest outlier - worth a look, not the main story.
- Where is the rest? The return. A panned platform with open seams and a 2-in. gap at the filter grille sits under negative pressure whenever the fan runs, so it pulls 128°F attic air straight into the return. That explains both the dust complaint and the high leakage-to-outside figure.
- Static pressure of 0.92 in. w.c. on a 0.5-in.-rated coil is nearly double the rating. The blower is fighting a restrictive return and probably crushed flex, so airflow and capacity are down, rooms do not reach setpoint, and the owner runs the thermostat lower.
Every complaint in the file traces back to the return.
2. Recommend seal-and-repair versus full duct replacement, with a sequence, a cost frame, and verification criteria. Flag any second-order risks introduced by the fix.
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Seal and repair rather than replace, because the diagnosis identifies specific, fixable defects rather than a failed system.
Sequence:
- Seal the return platform with rigid foil-faced board and mastic; close the filter-grille gap and convert to a sealed filter-back grille or relocate the filter to the air handler.
- Mastic all flex collars and boot-to-floor joints; support and straighten the runs; replace crushed sections with R-8.
- Reduce the static pressure - add return capacity (second return or properly sized transfer grilles and door undercuts) and re-measure.
- Verify: leakage to outside at or below 6% of airflow, static pressure at or below 0.5 in. w.c., room-to-room pressures within about 3 Pa, and flow-hood readings at each register.
Cost frame: sealing plus partial flex replacement runs roughly $1,200–2,000 versus $4,000–6,000 for a full replacement - and a replacement installed over the same leaky platform would fail the same way.
Second-order risks: the ducts sit in 128°F attic air, so even sealed R-4 loses capacity; quote R-8 for replaced runs and discuss attic encapsulation as a later step. And because sealing changes the pressure map of the house, re-check room pressures and any combustion appliance before you leave - the return fix can shift the pressure field in ways the original system never saw.
3. The homeowner asks why she should not just replace the whole system and the ducts since she is “already spending money.” How do you answer?
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Answer with her own measurements, not with general advice.
“Replacement is on the table, but it is not the first move, and here is why. Your equipment is not the main problem right now - the duct system is feeding it attic air and choking it at the same time. If we put a new air handler and new ducts on the same platform, we would still have a leak and a restriction unless we fix that platform, and we would pay twice.”
Then make the case concrete:
- The leakage-to-outside number (26% of airflow) and the static pressure (0.92 in. w.c.) are both far outside normal, and both are repairable.
- Sealing and rebalancing is a fraction of the cost of replacement, and it is the prerequisite for any new equipment actually delivering its rated efficiency.
- After sealing, we retest. If airflow and static pressure come into range and the rooms still do not hold temperature, then equipment replacement is a justified next step, and I will have the data to size it correctly.
Also be honest about what sealing will not fix: R-4 ducts in a 128°F attic will still lose capacity, and if the compressor is failing, sealing will not save it. Offer the phased plan, put the numbers in the report, and let the homeowner decide with the trade-offs in front of her.