How Attic Ventilation Actually Works
A vented attic is meant to stay close to outdoor temperature. Cold outside air enters low, at continuous soffit intake, rises as it picks up heat and moisture, and leaves high, at the ridge. Intake and exhaust must be balanced; exhaust without intake simply pulls conditioned air out of the house through every ceiling gap.
When it works, the roof deck stays cold, the snow pack does not melt from underneath, and meltwater does not refreeze at the eave. When it does not, you get ice dams every February no matter how good the shingles are.
What Usually Goes Wrong Up Here
- Soffit vents painted shut, or blocked by insulation blown in without baffles
- Ridge vent installed with no matching intake, so it pulls house air instead of attic air
- Mixed exhaust types — box vents plus ridge vent — short-circuiting airflow
- Bath and kitchen fans dumping moist air into the attic instead of outdoors
- Vaulted ceilings with no vent channel above the insulation
- Recessed lights and attic hatches leaking warm air straight into the assembly
Air sealing comes first
Ventilation removes moisture that reaches the attic. Air sealing keeps it from getting there. Doing only the first is treating a symptom.
What a Correct System Looks Like
- Continuous soffit intake, protected with baffles so insulation cannot block it
- Ridge exhaust sized to match the intake, as one system
- One exhaust type, not several competing
- Attic floor air-sealed at penetrations, hatches and top plates
- Insulation brought up to depth after sealing, not before
- Bath and kitchen fans ducted through the roof or wall with insulated duct
Nearly every major manufacturer conditions its warranty on adequate ventilation, so this is also the difference between having warranty coverage and thinking you do.
Common questions
- Will more roof vents stop my ice dams?
- Only if intake is added to match. Adding exhaust alone often makes things worse by drawing warm, moist house air into the attic.
- Can ventilation be fixed without replacing the roof?
- Sometimes — intake, baffles and air sealing can often be improved independently. Ridge exhaust work is usually done during a reroof.
- What about a vaulted ceiling with no attic?
- Those assemblies need a continuous vent channel above the insulation, or a properly designed unvented assembly. Both are solvable; the wrong hybrid is what rots sheathing.