Insulation & Energy
Attic Baffle / Rafter Vent Calculator
Calculates the number of rafter-vent baffles needed along the eaves
Updated June 26, 2026 · Live
What this tool does
Calculates the number of attic rafter-vent baffles (insulation stops) needed along the eaves, one per rafter bay, from the eaves length and rafter spacing.
Formula Used
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How the attic baffle calculator works
This calculator divides the total eaves length by the rafter spacing to get the number of rafter bays, and rounds up — you need one vent baffle in each bay where insulation could otherwise slump into the soffit and choke the air intake.
Why baffles matter
Baffles (also called insulation stops or rafter vents) hold attic insulation back from the soffit so outside air can flow up from the soffit intake, across the underside of the roof deck, and out at the ridge or roof vents. Without them, blown or batt insulation blocks the intake and the attic can’t breathe — leading to moisture, frost and ice damming in a cold climate.
Rafter spacing
Most Canadian framing is at 406 mm (16") or 610 mm (24") on centre. If your trusses are at an irregular spacing, measure a representative bay and enter that figure. Baffles only go in the bays along the vented eaves, not across the whole attic.
What this tool does not do
It estimates the baffle count only. It does not size the total intake and exhaust ventilation area (NBC 9.19 calls for roughly 1/300 of the insulated ceiling area, split between high and low vents) — use a roof ventilation calculator for that.
Adjusting the defaults
Every input is editable. Enter your own eaves length and rafter spacing and the baffle count recalculates instantly.
On-site considerations
Install baffles before the insulation goes in, stapled to the roof deck so they hold a clear air channel from the soffit up past the top plate. In a deep-heel truss you have more room; in a low heel, baffles are essential to keep the required insulation depth without choking the vent. Pair them with a continuous wind-wash dam at the top plate.
Using this alongside other BuildMetricLab tools
Pair this with the attic insulation, roof vent and vapour barrier calculators when finishing an attic, and with the material cost calculator to price the baffles. Every BuildMetricLab tool runs in your browser — no sign-up, no data sent anywhere, and every formula is shown so you can audit the math.
Sources & methodology
Calculates the baffle count as the total eaves length divided by the rafter spacing, rounded up — one baffle per rafter bay. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are roof ventilation calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical Canadian trade allowance for roofing. Increase it for complex cuts, awkward shapes, or first-time DIY. The default wastage allowance reflects common trade practice; values lower than the default may underestimate offcuts.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, the National Building Code (NBC), property-line or shared-wall, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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