Ridge Vent Added, Soffit Intake Lost: Diagnosing Condensation and Curling
A one-story ranch in Slidell has a 2,100-square-foot attic floor and a 5:12 roof, re-roofed 14 months ago with architectural shingles over 30-pound felt. The crew installed a 34-foot ridge vent rated at 18 square inches of net free area per foot and left the original 8x16-inch soffit vents in place. The homeowner now reports a musty odor, rust on sheathing nails, and shingle edges curling along the ridge. Inspection finds: soffit vents covered by blown insulation pushed to the eaves with no baffles installed; two bath fans terminating about four feet above the insulation inside the attic; several shingle nails placed below the manufacturer's nail line, plus shiners. The contractor blames the shingle manufacturer; the manufacturer's warranty requires attic ventilation meeting code.
1. Quantify the ventilation and decide whether this is a product failure, an installation failure, or a building-envelope problem.
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Run the code arithmetic first, because it reframes the whole dispute.
- Required net free area at 1/150 of attic floor area (the 1/300 exception requires a vapor retarder on the warm side, which a vented attic in this climate does not have): 2,100 / 150 = 14.0 sq ft = 2,016 sq in.
- Balanced intake and exhaust at 50/50: roughly 1,008 sq in each.
- Actual exhaust: 34 ft x 18 sq in/ft = 612 sq in - about 60 percent of the required exhaust and 30 percent of the total.
- Actual intake: effectively zero, because the baffles were never installed and the insulation was pushed to the eaves.
A ridge vent with no low intake cannot develop convective flow. The attic air is close to static, so the deck cooks from below in summer and, in the shoulder months, warm moist air from the two bath fans and ceiling bypasses condenses on the cold nail points and stays there. That is a moisture and heat problem, not an asphalt chemistry problem.
Verdict: primarily a building-envelope and ventilation failure, a large part of which the reroof crew created when it disturbed the insulation and failed to restore an intake path. Separately, nails below the nail line and shiners mean the field is not fastened to the manufacturer's pattern - an independent workmanship failure that the manufacturer will use to deny any claim regardless of ventilation.
2. Define the corrective scope and the order in which it must be done.
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Sequence matters more than the parts list. If you add ventilation first, you simply move more humid air through a building that is still producing it.
- Stop the moisture sources. Air-seal the ceiling plane: can lights, top plates, and chases. Duct both bath fans to the exterior with insulated duct, backdraft dampers, and short, pitched runs.
- Restore the intake path. Install baffles at every bay, pull insulation back from the eave line, and verify the soffit panels are actually perforated where the vents are cut.
- Rebalance net free area. With 612 sq in of exhaust, the job needs at least a matching intake plus margin - target the full 2,016 sq in total. Practically: add intake to at least 1,000 sq in and consider supplementing exhaust with gable or additional ridge vent so both sides are adequate, not just one.
- Then fix the field work. Replace curled and unbonded shingles, and correct nails below the nail line and shiners, since those are not repairable in place.
- Verify. Take attic temperature and moisture readings on a hot afternoon and a cool morning, and photograph the baffles before the insulation is blown back.
Do not promise the owner that ventilation alone will stop the curling; the shingles already affected will not recover.
3. Who pays, and how does the nail placement affect your allocation?
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Split the claim by cause, and say so in writing.
- Contractor: the disturbed insulation, missing baffles, and non-conforming nailing are yours. Those are visible, documented conditions within your scope, and they convert what would have been an owner-side moisture complaint into your liability.
- Owner: the two bath fans discharging into the attic are a pre-existing building defect that no roofing scope would fix, and their CFM capacity is the dominant moisture source. Get the fan discharge documented on the first inspection; it is the strongest piece of evidence you have.
- Manufacturer: no exposure. Curling at the ridge with documented ventilation shortfall and nailing defects is a textbook warranty denial, and pressing the claim wastes the relationship you need for a future material discount.
Strategic advice: offer a written repair scope that prices the ventilation correction (yours) separately from the fan ducting and air sealing (owner's), with a release. If you fold everything into one number you will be negotiating the owner's fan ducts for the next six months. Also check your own contract - if it excluded ventilation correction in writing and the owner signed it, the split is already defined.