The one place where the two jobs fight
Everywhere else in an attic, insulation depth and ventilation are unrelated problems. At the eave they are the same problem, because both of them want the space between the top plate and the roof deck, and at that point there is almost none. The roof deck meets the wall at the heel height, which on a conventionally framed roof is set by the depth of the rafter and the seat cut, and is commonly somewhere between four and eight inches. Take an airway off the top of that and what is left for insulation is often two or three inches.
Moving inward, the deck climbs at the pitch, so the space grows. At 6/12 you gain half an inch of headroom for every inch you move in from the wall, which is six inches per foot. That is the number the calculator turns into a distance: how far in you have to go before the space finally holds the depth your target R-value needs.
Worked through with the defaults
A 6.5 inch heel with a 2 inch airway leaves 4.5 inches at the wall. At 2.6 R per inch that is about R-11.7 directly over the top plate. A target of R-49 at the same 2.6 per inch needs 18.85 inches of depth, so the space is 14.35 inches short at the wall. At 6/12 the deck rises 0.5 inches per inch of run, so it takes 28.7 inches — a little under two and a half feet — before the full depth fits. Over 160 feet of eave that is 383 square feet of ceiling in the shallow band, which on a 1,120 square foot attic is about 34 percent of it, averaging around R-30 rather than R-49.
None of that is a defect. It is what a conventionally framed eave does, and it is why raised heel framing exists. To carry the full 18.85 inches right out to the wall you would need a heel of 18.85 plus the 2 inch airway, which is 20.85 inches, and that is a framing decision made before the roof went on, not something you fix with insulation.
What people do instead, and what it costs
The common field response to a shallow eave is to push insulation into it anyway until the bay is full to the deck. This does two things. It raises the R-value over the top plate by a couple of points, and it closes the intake for that bay completely. Once the intake is closed, the vented soffit panel below is decorative, and any exhaust at the top of the roof makes up its air from somewhere else — which, as the attic vent net free area audit lays out, generally means from the house through the ceiling. Trading a small gain in ceiling R-value for a total loss of intake in that bay is a bad trade, and it is easy to make by accident with a blower hose.
The other thing worth knowing is that the shallow band is not just lower R-value, it is lower R-value in the coldest, most exposed part of the ceiling, sitting over the top plate where the wall and ceiling air barriers meet and where air leakage concentrates. It shows up as an ice dam at the eave in snow country, which the ice dam heat loss calculator quantifies from the heat side, and as a frost stripe on the sheathing in a cold attic.
Counting the baffles
One baffle per rafter bay is the arithmetic, and the bay count is the eave run divided by the spacing. What the count does not tell you is whether the baffle you buy actually reaches. A short baffle that stops at the top plate holds the airway open exactly where the insulation is shallowest and does nothing above that, so on a deep insulation job the baffle needs to carry the airway up past the point where the insulation reaches full depth — which is the distance this page computes. That is what the pieces-per-bay field is for.
Measure the spacing rather than assuming it. Trusses at 24 inches and rafters at 16 are both common, roofs that have been altered can have both, and a bay count that is out by a third is a second trip to the store. The insulation side of the job, depth and bag count over the flat part of the ceiling, is on the attic insulation top-up calculator, and the whole-assembly effect of framing running through the insulation is on the assembly R-value calculator.
Questions people ask
How do I measure the heel height without getting to the eave?
From inside the attic, lie a straightedge on the ceiling joists near the outside wall and measure up to the underside of the deck directly over the wall. That is awkward and it is also the only honest way, because the number cannot be inferred from the rafter size — the seat cut takes an unpredictable bite out of the depth. If you genuinely cannot reach it, measuring at a known distance in from the wall and working back down the pitch gets you close: subtract the rise over that distance from the height you measured.
What airway clearance should I use?
Whatever your adopted code requires and whatever the baffle you buy is built to hold, and this page will not name a figure because both of those vary. What it will do is show you what the choice costs: at 6/12 with a 6.5 inch heel, changing the airway from one inch to two moves the point where full depth starts two inches further in, and at 2.6 R per inch it takes about two and a half points of R-value off the ceiling directly over the plate. That trade is worth seeing in numbers before deciding it does not matter.
Does a raised heel truss solve this completely?
It solves the depth problem at the wall, which is the whole of the geometry on this page. A heel deep enough to hold the target insulation plus the airway means the ceiling reaches full R-value right out to the exterior wall, with no wedge at all. What it does not solve is anything else: air sealing at the top plate still has to be done, the intake still has to be open, and the heel has to be detailed at the eave so the extra height does not just become a new path for wind-washing through the insulation. It is a framing decision, made before the trusses are ordered.
Why does the R per inch matter so much here?
Because it sets the target depth, and the target depth sets everything downstream. R-49 at 2.2 per inch needs 22.3 inches; the same R-49 at 3.8 per inch needs 12.9. That is a nine inch difference in the depth you are chasing, which at 6/12 moves the start of full depth by about eighteen inches on every eave. The figure on the bag is for a stated installed depth and settled density, so it is worth taking from the bag you actually bought rather than from a general table.
Can I just use spray foam at the eave instead?
Foam at the eave changes the assembly rather than filling the wedge, and once insulation goes against the underside of the deck the attic may no longer be a vented attic at all. Whether that is permitted, what condensation control it needs and how it interacts with the rest of the roof depend on your climate zone and your adopted code, and it is not a decision this page is equipped to make. If you are considering it, the question to bring to the building department is what the assembly becomes, not whether the foam fits.