Attic Ducts, 4 Tons on 2,700 Square Feet, and No Ventilation
A 2,700 ft² house in Climate Zone 3A gets a 4-ton split system with the air handler and all supply and return ducts in a vented attic. The plans specify R-8 duct wrap. Field report: R-6 wrap installed on roughly 60 percent of the runs; a post-construction leakage-to-outside test of 6.2 cfm per 100 ft² of conditioned floor area; a blower door of 2.8 ACH50; no whole-house ventilation device installed; and no load calculation submitted - 'we always use a ton per 675 square feet here.' The owner, who has a newborn, complains of stuffiness, long cooling runtimes, and high humidity. The general contractor wants to close this week and asks what the inspector can 'live with.'
1. Evaluate the duct insulation and the duct leakage result. What is allowed, what fails, and what is the cheapest credible path to compliance?
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Reasoning: Two independent failures, and neither is curable with a conversation.
Insulation: ducts in an unconditioned attic must carry the duct-insulation R-value the table assigns to that location - R-8 in current editions. R-6 on 60% of the runs is an unapproved substitution; the table has no 'close enough' clause and there is no component trade available under the prescriptive path.
Leakage: the post-construction test limit is expressed per 100 ft² of conditioned floor area (4 cfm/100 ft² leakage to outside in current residential provisions).
- Allowance: 2,700 × 4 / 100 = 108 cfm.
- Measured: 2,700 × 6.2 / 100 = 167 cfm → fail.
- The exemption for systems entirely inside the conditioned envelope does not apply: the air handler and all ducts are in the attic.
Cheapest credible fix, in order: mastic plus mesh at the plenum, wyes, takeoffs, boot-to-subfloor junctions, and the return pan; listed UL 181 tape only (not cloth duct tape); then re-test with the same protocol and retain the report. If it still fails, encapsulate the duct system or bring the attic within the envelope. The R-6 must be corrected to R-8 regardless.
2. At 2.8 ACH50 with no ventilation device, is mechanical ventilation actually required? What would you require, and why can't infiltration be counted?
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Reasoning: Yes - required. The mechanical ventilation provision requires the dwelling to be provided with whole-house ventilation meeting ASHRAE 62.2, and it is not conditioned on a leakage threshold. The code does not permit counting natural infiltration as the ventilation strategy, and at 2.8 ACH50 the house is tight enough that relying on leakage would be unreliable in any case (and weather-dependent by a factor of several).
What to require:
- A system sized to the 62.2 dwelling-unit ventilation rate (bedroom count and conditioned floor area), with documented design airflow.
- Controls appropriate to the strategy (continuous low-speed, or intermittent with the required control interlock and capacity multiplier).
- Compliance with the fan-efficacy limit for the installed fan, verified from the fan curve or a measured airflow.
Judgment on system type: exhaust-only is cheapest but drives the house negative, which is a combustion-appliance and backdrafting concern depending on where the furnace and water heater sit. A balanced ERV/HRV avoids that and is also the better answer for the humidity complaint; note that ventilation does not fix an oversized, short-cycling air conditioner - that is the next task.
3. No load calculation was submitted and the equipment is 'a ton per 675 ft².' How should the inspector handle equipment sizing, and what does oversizing have to do with the humidity complaint?
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Reasoning: Equipment must be sized from a calculated load (Manual J or an approved heating/cooling load method), with the selection made per Manual S. A rule of thumb is not a calculation and cannot be accepted as the basis for compliance.
Sizing check: 4 tons for 2,700 ft² = 675 ft²/ton. In a mixed-humid zone that is very likely oversized - typical calculated loads in reasonably tight new construction run well above 800–1,000 ft²/ton.
Consequences the inspector should tie to the complaint:
- Oversized equipment short-cycles: it satisfies sensible temperature before it removes latent moisture, so the house feels clammy and the runtimes look strange (short, repeated cycles - not "long runtimes" caused by capacity).
- Combined with leaky attic ducts (167 cfm to outside) and no ventilation, the humidity complaint has three contributing causes, not one.
Action: require the load calculation and equipment selection documentation; if the calc supports substantially less capacity, the condenser/air handler must be corrected or an approved justification produced. Also verify the air handler airflow is set to the installed capacity - an oversized unit with a low fan setting creates a coil-icing and latent-control problem.