The Attic With No Ventilation Path
You inspect a 1975 bungalow with a vented attic. The roof is asphalt shingle over plywood sheathing. In the attic you find 6 in of loose-fill fibreglass between the ceiling joists, soffit vents along both eaves, and a ridge vent. However, the insulation has been pushed tight against the roof deck at the eaves, blocking the soffit openings. You also see dark staining on the roof sheathing near the ridge and along the north slope, and frost is visible on the sheathing nails in several locations. The bathroom exhaust fan terminates in the attic, discharging about 4 ft from the ridge vent. The outdoor temperature is -5°C and the indoor relative humidity is 45%. The seller says the attic 'has always been dry in summer.'
1. Explain the interaction between the blocked soffit vents, the exhaust fan, and the staining you observed.
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The three findings form a coherent picture. The ridge vent needs a supply of low air to draw from; with the soffit path blocked by insulation, the ridge vent has little or no make-up air and the attic ventilation system is effectively disabled. Meanwhile the bathroom fan is dumping warm, moist air directly into the attic. At -5°C outdoor temperature, that moisture condenses on cold roof sheathing, which explains the frost on the nails and, over time, the dark staining - likely a combination of moisture staining and possible mould growth on the sheathing. The staining pattern near the ridge and on the north slope is consistent with this: the north slope stays colder longer and condenses first. The seller's 'dry in summer' comment is not reassuring, because the problem is a winter phenomenon driven by the stack effect and cold surfaces. I would report the blocked ventilation path, the improperly terminated exhaust, and the staining as related findings rather than three separate line items.
2. What can you conclude and what must you stop short of concluding about the staining?
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I can conclude that the attic has a moisture problem with a plausible and observable mechanism: blocked intake ventilation plus an exhaust fan discharging into the attic. I can conclude that the staining is consistent with moisture exposure and possible biological growth. I cannot conclude that the sheathing is structurally compromised, that the staining is definitely mould, or that the roof covering has been damaged - those require testing, probing, or a specialist's evaluation. I should not scrape or sample the staining myself unless my scope and client agreement include it, and I should not describe it as mould with certainty. The defensible report language is: dark staining observed on the roof sheathing, consistent with moisture exposure; recommend evaluation by a qualified contractor, and if biological growth is a concern, testing by a qualified indoor air or mould professional. I would also recommend that the exhaust fan be terminated to the exterior and that baffles be installed to maintain the soffit air path.
3. The buyer asks whether the attic problem makes the house a bad purchase. How do you respond within your professional role?
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I would be clear that this is not my call and that I am not going to make it for them. My role is to describe conditions and their implications, not to advise on whether to buy. What I can do is help them understand the scope: the ventilation defects are correctable - baffles at the eaves and re-routing the fan are relatively modest repairs - but the extent of any sheathing deterioration or biological growth is unknown until someone evaluates it, and that uncertainty is what should drive their decision and any negotiation. I would suggest they get a quote from a qualified contractor and, if mould is a concern, an assessment from a specialist before removing conditions. I would also remind them that the inspection is a snapshot and does not predict future performance. Staying in that lane is both ethically required and practically protective: the moment I say 'don't buy it' or 'it's fine,' I have stepped outside my role and created liability for myself.