BuildMeter guide · Energy & Electrical
Attic Ventilation Baffle Planning Guide
Count vented rafter bays, choose a chute run that protects the eave insulation and separate baffle quantity from ventilation sizing.
Direct answer
What to know first
Count the rafter or truss bays connected to actual soffit or eave intake vents, then determine how far each chute must extend upslope to maintain a clear air path above the insulation. Baffles preserve the ventilation channel; they do not by themselves establish the required intake and exhaust area.
Open Attic Ventilation Baffle CalculatorCount vented rafter bays, not total roof area
Baffles are installed in the bays that carry intake air from soffit or eave vents. Measure the vented eave length, divide by the actual rafter or truss spacing and count unusual bays separately. A hip, dormer or blocked bay may not match the regular module.
| Input | Planning question |
|---|---|
| Vented eave length | Which roof edges actually contain intake vents? |
| Framing spacing | How many separate bays receive chutes? |
| Required chute run | How far upslope must the air channel extend beyond deep insulation? |
| Stock length | Does each bay need one piece or connected pieces? |
Worked example
Two vented eaves are each 48 ft long with framing at 24 in. on center. The planning count is 24 bays per side, or 48 bays. A 4 ft chute run using 4 ft pieces requires 48 base pieces. With an 8% spare allowance, order 52 pieces and round to full packs.
Baffles and wind dams perform different details
The chute preserves an air channel above insulation. A wind dam at the eave helps prevent loose insulation from entering the soffit and keeps ventilation air from washing through the insulation. Some products combine these functions; others require separate blocking.
Common mistakes
- Installing chutes in bays with no intake opening while missing actual vented bays.
- Stopping the chute where deep insulation still blocks the air path.
- Using baffle count as a substitute for net-free ventilation-area calculations.
- Failing to air-seal the ceiling plane before adding insulation.
- Applying vented-attic details to an unvented roof assembly.
Use the calculators
Use the Attic Ventilation Baffle Calculator for pieces and packs, the Roof Vent Calculator for preliminary vent-area planning and the Attic Insulation Calculator for insulation quantity.