What causes poor attic ventilation?
Short answer. Poor attic ventilation usually comes from too little intake, most often soffit vents buried by insulation or missing entirely, or from exhaust vents that work against each other, like a ridge vent combined with box vents, gable vents or a power fan. Painted-over vents, undersized exhaust, and roof shapes with short ridges or no overhangs make it worse.
The most common causes
- Blocked intake: blown insulation packed into the eaves without baffles. From outside the soffits look vented; inside, nothing flows.
- No intake at all: solid soffits, or houses with no overhang where a ridge vent was added with nothing to feed it.
- Mixed exhaust: a ridge vent plus box vents, a power fan, or open gable vents. The exhausts pull from each other instead of from the eaves.
- Paint and debris: perforated soffit filled with paint, screens clogged with pollen and dust.
- Too little exhaust: a couple of small box vents on a large attic.
- Roof design: hip roofs with short ridges, complex roofs with many separate attic pockets, and vaulted ceilings without vent channels.
- Bath fans into the attic: not a vent shortage, but it adds moisture faster than any vent system can remove it.
Symptoms to look for
- Attic temperatures far above outdoor temperatures on summer afternoons, and an attic air handler that struggles.
- Rusty nail tips, darkened decking or mildew, especially on north slopes and along the eaves.
- Shingles that blister or shed granules early.
- Frost in the attic on cold mornings.
Fixing it
Start at the eaves: open, baffle or add intake. Then choose one exhaust type and remove or close off the others. A roof replacement is the best moment, since the ridge is open and old vents can be decked over. See roof ventilation basics and attic heat and shingles.