What attic ventilation actually has to do
A vented attic is an assembly, not a collection of roof vents. It needs a continuous air barrier at the ceiling, insulation that does not choke the eaves, low intake openings, and high exhaust openings. PNNL describes a vented attic as a large airspace ventilated with low intake vents and high exhaust vents. (PNNL condensation-control guide)
The short answer: size the system from net free ventilation area, split the required area between low and high openings, and check the locally adopted code before ordering. Then verify that every intake path is actually open. A nominally large vent does little when screening, insulation, or framing blocks its free area.
Why air sealing and ventilation work together
Ventilation is only one control in a moisture-safe attic. PNNL says warm, moist indoor air can escape into a cold attic and condense on roof framing or sheathing. (PNNL condensation-control guide) That is why water staining, frost, corrosion, or mold should trigger an air-leakage and moisture investigation, not an automatic order for a larger fan.
PNNL says the primary controls in a ventilated attic are limiting air leakage from the house and providing proper intake and exhaust ventilation. (PNNL condensation-control guide) Air-seal the ceiling plane and duct penetrations first. Ventilation then helps the roof deck dry when incidental moisture gets into the assembly; it should not be asked to carry a continuous stream of indoor air.
Build a continuous low-intake-to-high-exhaust path
A passive system needs openings at both ends of the path. PNNL says lower openings should be at soffits or eaves, with a lower vent in each rafter bay. It also calls for at least 1 inch between insulation and roof sheathing at vented paths, with a barrier or baffle keeping insulation away from openings. (PNNL passive-ventilation guide)
At the eave, baffles preserve the channel while soffit dams hold insulation back. PNNL instructs installers to place baffles and soffit dams at each rafter bay to provide a clear ventilation path and stop insulation from falling into soffit vents. (PNNL attic-eave guide) The related soffit and fascia guide shows why the finished eave and the attic-side opening must be considered together.
High vents are the exhaust side. PNNL says 40% to 50% of the required area should be high, at least 3 feet above the low openings and no more than 3 feet below the roof’s highest point. (PNNL passive-ventilation guide) On short roofs or unusual geometry, use the governing project detail rather than forcing that layout.
Calculate NFA, then choose vent types
Net free ventilation area, or NFA, is usable opening area, not the face size of a grille. PNNL defines net free ventilation area as clear open area after accounting for screens and vent slots. (PNNL passive-ventilation guide)
The PNNL guide calculates total NFA by dividing the attic floor area by 300 for its balanced high/low example. It uses 1/150 where low eave or soffit openings cannot be provided. (PNNL passive-ventilation guide) These are reference calculations, not permission to skip the adopted code, amendments, product instructions, or design documents controlling a job.
Worked 1/300 example
For a 1,200-square-foot flat attic:
- Total NFA: 1,200 ÷ 300 = 4 square feet.
- Convert to square inches: 4 × 144 = 576 square inches.
- At a 50/50 split: plan 288 square inches low and 288 square inches high.
- Divide each target by the selected product's labeled NFA. Always round the vent quantity up.
AirVent lists continuous soffit intake at 9 square inches of NFA per linear foot. One current AirVent rolled ridge product lists 11 square inches of NFA per linear foot. (AirVent product catalog) In this example, those specific labels produce 32 linear feet of soffit intake and 27 linear feet of rolled ridge exhaust after rounding up. The unequal lengths are not an imbalance; the NFA totals are the comparison.
| Vent type | Typical role | What to record |
|---|---|---|
| Continuous soffit or eave vent | Low intake | NFA per linear foot and clear baffle path |
| Individual undereave vent | Low intake | NFA per vent and spacing across the eave |
| Continuous ridge vent | High exhaust | NFA per linear foot and usable ridge length |
| Static roof vent | High exhaust | NFA per unit and roof location |
| Gable louver | Wall opening, commonly sold as exhaust | NFA per unit and whether the whole system still has a low-to-high path |
| Turbine or powered unit | Mechanical or wind-assisted exhaust | Rated capacity, required intake, controls, and pressure effects |
Choose by the assembly's available low and high openings and by labeled capacity, not by a generic ranking of vent products.
AirVent’s current catalog classifies continuous soffit products as intake and ridge, static, gable, turbine, and powered attic products as exhaust. (AirVent product catalog)
Vented and unvented attics use different boundaries
PNNL frames the decision around the thermal boundary: it can remain at the ceiling plane with a vented attic, or move to the roof line for a sealed, insulated, conditioned or semi-conditioned attic. (PNNL vented-versus-unvented guide)
| Assembly | Thermal and air-control location | Vent strategy | Main design consequence |
|---|---|---|---|
| Vented attic | Ceiling plane | Low intake plus high exhaust | Eaves need clear baffles; ceiling penetrations need air sealing |
| Unvented attic | Roof line | No passive attic vent path | Roof-line insulation, airtightness, climate, and condensation control must be designed as one assembly |
Do not add soffit or ridge vents to an unvented roof-line design, and do not convert a vented attic by spraying an isolated area of roof deck. If the strategy changes, the insulation, air barrier, moisture control, and mechanical-system implications change with it. For material choices at the ceiling or roof line, use the insulation types comparison; for the weather-control layers above the deck, see the roof underlayment guide.
Field checklist before ordering vents
Use this order on a real job:
- Identify the thermal and air boundary: ceiling plane for a vented attic or roof line for an unvented design.
- Measure only the attic floor or sloped ceiling area served by the proposed vent system.
- Record the adopted code provision and any project-specific detail.
- Calculate total NFA, then allocate the required low and high portions.
- Select products by their labeled NFA, not rough opening or grille dimensions.
- Confirm that soffit openings, baffles, and rafter bays form a continuous intake path.
- Locate and count the high exhaust openings; do not assume a ridge product covers hips, valleys, or short ridge segments.
- Air-seal ceiling penetrations before relying on ventilation to manage attic moisture.
- Photograph the paths and retain the NFA worksheet with the job file.
Powered attic fans need a separate pressure check. PNNL says their negative pressure can draw conditioned air through ceiling gaps. PNNL also says non-sealed combustion appliances may draft poorly or potentially backdraft. (PNNL attic-fan guide) Do not treat a powered fan as a drop-in substitute for passive exhaust or for blocked intake.
Attic ventilation FAQ
What is net free ventilation area?
NFA is the clear open vent area after accounting for screening and slots. Use the labeled NFA, not the vent's outside dimensions.
Is attic ventilation sized at 1/300 or 1/150?
PNNL demonstrates 1/300 for a balanced high/low layout and 1/150 where low openings cannot be provided. Use the adopted code and project documents that govern the job.
How should intake and exhaust be split?
The PNNL guide places 40% to 50% of required NFA high and the remainder low. Its upper openings are at least 3 feet above low vents and no more than 3 feet below the highest roof point.
Can a ridge vent work with blocked soffits?
Not as the documented low-to-high system. Clear the eave path and protect it with baffles before adding high exhaust.
Should insulation touch a soffit vent?
No. Hold insulation back with a dam and preserve the channel with a baffle.
Are powered attic fans a direct replacement for passive exhaust?
No. They create negative pressure and require enough intake plus a tight ceiling; PNNL also flags potential combustion-drafting risks.
Can the same vent calculation design an unvented attic?
No. An unvented attic moves the thermal and air boundary to the roof line and needs an assembly-specific design.