Attic ventilation & insulation · Calculator

Rafter Baffles Calculator

Enter the length of eave with vented soffit, the rafter or truss spacing, the finished insulation depth at the eaves, the pitch and the baffle length, and the calculator returns the bays, the baffles per bay, the total, the bundles and the staples.

A baffle that stops below the top of the insulation does nothing, and on a shallow roof with deep insulation one standard baffle often does stop short; the calculator works out how far up the rafter the baffle must reach and how many it takes.

Updated · See the formula

Rafter Baffles Calculator

Units
ft

Both sides of a gable roof, all four of a hip

in

The finished depth after the new insulation; from the attic insulation calculator

Rise per 12 of run (6 means 6/12), or degrees if chosen below

in

Most are 48 in long; some 22 in

per bundle

Example result for the starting values. Enter your own and press Calculate.

Rafter baffles

40

40 bays × 1; 4 bundles of 10

Bays
40
Per bay
1
Bundles
4
How the baffles are counted
Rafter bays along the eaves80 ft ÷ 24 in = 40
Baffle must reach16 in insulation + 2 in clear = 18 in above the plate
Length along the rafter18 in ÷ sin 26.6° = 40.2 in
Baffles per bay1 of 48 in
Baffles40 × 1 = 40
Bundles4 of 10
Staplesabout 240
Show the arithmetic
  1. 80 ft ÷ 24 in = 40
  2. 18 in ÷ sin 26.6° = 40.2 in
  3. 40 × 1 = 40
  • Install a baffle in every rafter bay that has a soffit vent below it, from the top plate up past the insulation, so air can flow from the soffit to the attic and the insulation does not spill into the soffit.
  • Block the eave between rafters at the wall (a wind wash barrier or cut rigid foam) so the insulation does not blow back into the bay.
  • One baffle per bay reaches past the insulation at this depth and pitch.

What to order

  • 4 bundles rafter vent baffles for 24 in spacing 40 baffles

Understanding your result

The headline is the number of baffles, with the bays and baffles per bay. The breakdown shows the bays along the eaves, how high the baffle must reach, that height measured along the rafter and the count per bay.

The insulation depth comes from the attic insulation calculator, and the vented eave length from the soffit vent calculator. The point of the baffles is to keep the intake the attic ventilation calculator asked for actually open.

How we calculate this

bays = round up (vented eave length ÷ rafter spacing) reach height = insulation depth + 2 in along rafter = reach height ÷ sin(pitch angle) per bay = round up (along rafter ÷ baffle length) baffles = bays × per bay

The sine turns the vertical height the baffle must clear into a length along the sloping rafter: on a 6/12 roof the angle is 26.6 degrees and a vertical 18 inches is 40 inches along the rafter; at 3/12 it is 14 degrees and the same height is 74 inches.

The assumptions behind the numbers

Assumption Default Where it comes from
Clear air space above insulation 2 in Common practice (the code asks for 1 in of air space at vents, IRC R806.3)
Baffle length 48 in Common product
Staples 6 per baffle Installer practice
Spacing 24 in Common truss spacing

Assumptions last reviewed October 7, 2026.

The calculator does not account for raised-heel trusses, which leave more room at the eaves. The insulation guide covers the eave detail.

Two worked examples

A 6/12 roof at 24 inches

80 feet of vented eave, 16 inches of insulation, 48-inch baffles.

  • Bays: 40; reach: 18 in; along the rafter: 40.2 in
  • One per bay: 40 baffles, 4 bundles

A shallow roof at 16 inches with deep insulation

140 feet of eave, 3/12, 20 inches of insulation.

  • Bays: 105; reach: 22 in; along the rafter: 90.7 in
  • Two per bay: 210 baffles, 21 bundles

Where to find your inputs

Vented eave length. Where the soffit has vents, both sides of a gable roof.

Spacing. Measure between rafters or trusses in the attic.

Insulation depth. The finished depth from the insulation calculator.

Common mistakes

  • Baffles below the insulation top. Reach 2 inches above it.
  • Missing bays. Every vented bay.
  • Blowing insulation first. Baffles first.
  • Crushing the baffle. Keep the channel open.
  • No eave blocking. Wind blows insulation back.
  • Skipping bays without vents. Fine, but check where the vents actually are.

Questions people ask

How many rafter baffles do I need?
One for every rafter bay with a soffit vent below it, more if one baffle does not reach above the insulation. Eighty feet of vented eave at 24-inch spacing is 40 bays and 40 baffles; at 16-inch spacing, 60.
How far up should a baffle go?
From the top plate to at least 2 inches above the finished insulation, so air from the soffit has a clear path. On a 6/12 roof with 16 inches of insulation that is 18 inches of height, about 40 inches along the rafter, which one 48-inch baffle covers.
When do I need two baffles per bay?
On shallow pitches with deep insulation: at 3/12 with 20 inches of insulation the baffle must run about 91 inches along the rafter, two 48-inch baffles overlapped.
Do I need baffles if I have ridge vent?
Yes, if the soffits are the intake: the ridge vent draws air from the soffits, and without baffles the insulation blocks them.
What are baffles made of?
Polystyrene, cardboard or plastic chutes stapled to the underside of the roof deck between rafters. Some installers also block the eave with rigid foam as a wind wash barrier.
Do baffles matter with blown insulation?
Even more: loose insulation drifts into the soffits without them. Fit baffles before blowing.
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