The roof plane and the ceiling plane converge at the eaves, so somewhere short of the wall your target depth stops being possible. How far short is exact: at a 4:12 pitch aiming for 16 inches, with the ventilation clearance kept open above it, full depth is unreachable for four and a quarter feet in from every wall. On a 40 by 25 ft attic that is more than half the ceiling. It is the entire argument for a raised-heel truss and it is almost never made with a number.
Insulated area, the perimeter band, and the average that results
ft
ft
sq ft
sq ft
in
in
in
R/in
hatches
The ventilation clearance is an input here and is quoted on the roof ventilation page, which owns that section. Filling it turns a vented attic into an unvented one with no design behind it.
Specification summary
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Reopen that address to reproduce these figures exactly.
Entered
Result
The eave, and the band where full depth will not fit
Worked example
A 40 by 25 ft attic under a 4:12 roof, aiming for 16 in of insulation at R-3.7 per inch, 1 in of clearance kept open, no raised heel, one code-minimum hatch.
Ceiling. 1,000 sq ft, less 4.6 for the hatch = 995 sq ft.
The band. (16 + 1) ÷ tan(18.4°) = 51 in = 4.3 ft from every wall.
Split. 443 sq ft at full depth, 553 sq ft tapered.
In the band. Average usable depth 7.5 in, not 16.
Full depth R. 3.7 × 16 = R-59.2.
Whole ceiling.R-41.8, which is 70.6% of it.
Now put a 12 inch raised heel under the same roof and change nothing else. The band drops from 4.3 ft to 1.3, the tapered area from 553 sq ft to 163, and the whole-ceiling figure goes from R-41.8 to R-57.7, which is 97.4% of the full depth value. That is what a raised heel buys, on one house, in one number, and it is a decision that can only be made before the trusses are ordered.
The formula
Two straight lines and a division, then an area-weighted average:
w = (d + c − h) / tan θ R̄ = (AfieldRfull + AbandR̄band) / A
d
target depth of insulation
c
ventilation clearance kept open above it
h
heel height: how far the roof plane starts above the ceiling at the wall
θ
roof pitch, as an angle
w
how far in before full depth is possible at all
R̄band
mean of what actually fits across the band, walked in slices
The average across the band is taken by walking it rather than halving it, because the depth is clamped at zero where the roof is below the clearance line and clamped at the target where it is above. On a shallow roof with a tall heel both clamps bite, and a simple halving gets it wrong in opposite directions at each end.
Why no attic calculator mentions the eaves
Every attic insulation page multiplies an area by a depth. That describes a box, and an attic is a wedge at the edges.
The omission matters more than it sounds, because the band is not a trim at the corners. It runs the whole perimeter, its width is set by the pitch, and on a shallow roof it can be most of the ceiling. At 4:12 aiming for R-60 it is over four feet on every side.
It is also the part of the ceiling directly above the join between ceiling and wall, which is where the framing, the top plate, the wiring penetrations and the drywall edge all arrive at once. Shallow insulation and a lot of thermal bridging in the same place.
None of that needs a citation. Two planes, one division, checkable with a tape and a level in your own attic. What it does need is somebody to do it, which is what this page is for.
The perimeter band by pitch, with and without a raised heel
Pitch
From horizontal
Band, no raised heel
Band, 12 in raised heel
What the heel buys
2:12
9°
8.5 ft
2.5 ft
71% narrower
3:12
14°
5.7 ft
1.7 ft
71% narrower
4:12
18°
4.2 ft
1.2 ft
71% narrower
5:12
23°
3.4 ft
1.0 ft
71% narrower
6:12
27°
2.8 ft
0.8 ft
71% narrower
8:12
34°
2.1 ft
0.6 ft
71% narrower
10:12
40°
1.7 ft
0.5 ft
71% narrower
12:12
45°
1.4 ft
0.4 ft
71% narrower
Read 2026-09-10. This table is trigonometry rather than a source: two planes, one division, at a 16 in target and a 1 in clearance.
Full depth is geometrically impossible near the wall
An attic is not a box. The roof plane and the ceiling plane converge at the eaves, so the space runs out before you reach the wall, and it runs out of the depth your target R-value needs some distance before that.
How far is exact:
w = (d + c − h) / tan θ
d is the depth you want, c is the ventilation clearance that has to stay open above the insulation, h is the heel height and θ is the pitch. Here it is for 16 inches of insulation and a 1 inch clearance:
Width of the band where full depth cannot be reached, at 16 in target and 1 in clearance
Pitch
From horizontal
Band, no raised heel
Band, 12 in raised heel
What the heel buys
2:12
9°
8.5 ft
2.5 ft
71% narrower
3:12
14°
5.7 ft
1.7 ft
71% narrower
4:12
18°
4.2 ft
1.2 ft
71% narrower
5:12
23°
3.4 ft
1.0 ft
71% narrower
6:12
27°
2.8 ft
0.8 ft
71% narrower
8:12
34°
2.1 ft
0.6 ft
71% narrower
10:12
40°
1.7 ft
0.5 ft
71% narrower
12:12
45°
1.4 ft
0.4 ft
71% narrower
At a 4:12 pitch with no raised heel that band is 4.2 feet wide. On every side, all the way round. On a 40 by 25 ft house that is over half the ceiling area at less than full depth, and it is the half that sits directly above the join between ceiling and wall.
Now put a 12 inch raised heel under the same roof and the band drops to 1.2 feet. That is the whole argument for a raised-heel truss, and it is almost never made with a number attached.
The shallower the pitch, the worse it is, which is the opposite of what people assume. A 12:12 roof reaches full depth in a third of the distance a 4:12 does.
And read the last column down. The heel narrows the band by the same percentage at every pitch, which is not a coincidence: the pitch only scales the whole distance, so a heel that removes a given number of inches of required rise removes the same proportion of the band whatever the roof is doing. A raised heel is worth the same fraction on a shallow roof as on a steep one, and far more inches on the shallow one.
What to do about the band, in order of cost
A raised heel, at the design stage. It is the only thing that removes the band rather than mitigating it, and it costs almost nothing when the trusses are being ordered and is impossible afterwards. The calculator will show you exactly how many inches of heel make the band disappear at your pitch.
Air sealing, at any stage. If the insulation at the perimeter is going to be shallow whatever you do, the return on sealing the top plate, the wiring penetrations and the partition intersections is higher there than anywhere else in the attic. Shallow insulation over a sealed ceiling beats deep insulation over a leaky one.
A different material at the edge. Rigid board or closed cell foam at the perimeter reaches a given R-value in less depth than loose fill, which is the one place on a ceiling where paying for R per inch makes sense.
What not to do is fill the clearance. The airspace above the insulation is a ventilation requirement and blocking it at the eaves is how a vented attic stops being vented. Baffles go in before the insulation does.
Measuring the attic rather than the house
The area for this calculation is the ceiling plane below the attic, and reaching for the house's square footage overstates it on almost every house.
Two storey sections have a room above them, not an attic. The attic covers the top floor only.
Vaulted and cathedral ceilings are a different assembly. They have their own clearance problem, they are not reached from the hatch, and they are not part of this.
Attached garages and open porches are usually outside the thermal envelope, even where the attic space above them runs on unbroken from the one you are standing in. The insulation follows the heated space, not the roof.
The practical method is to take the upper floor plan, subtract the vaulted parts and the unheated parts, and use what is left. Or pace the attic itself, which has the advantage of also telling you what the pitch and the heel actually are.
What that does to the average, and why it is not catastrophic
The band is not at zero R-value. It tapers, so it is at somewhere between nothing and the full depth, and the average across it is roughly half.
The calculator works that out by walking across the band in small steps and taking the mean of what actually fits, clamped at zero where the roof is below the clearance line and at the target where it is above. Then it weights that against the full-depth field to give a whole-ceiling average.
The honest conclusion is that the average is worse than the label and better than the panic. On a typical house the whole-ceiling figure lands somewhere between two thirds and four fifths of the target, depending almost entirely on pitch and heel.
What it is not is a reason to blow deeper at the edge. The clearance above the insulation is a ventilation requirement, not a suggestion, and filling it converts a vented attic into an unvented one with no design behind it. The roof ventilation calculator on this site quotes the section and covers the baffle that holds the gap open.
The right response to a wide band is a raised heel at the design stage, or accepting the geometry and spending the money on air sealing instead.
The hatch is a hole, and the code says how big
Almost every attic has a hole in the insulation by requirement:
Buildings with combustible ceiling or roof construction shall have an attic access opening to attic areas that have a vertical height of 30 inches (762 mm) or greater over an area of not less than 30 square feet (2.8 m2). The vertical height shall be measured from the top of the ceiling framing members to the underside of the roof framing members. The rough-framed opening shall be not less than 22 inches by 30 inches (559 mm by 762 mm) and shall be located in a hallway or other location with ready access.
Not less than 22 by 30 inches, which is 4.6 square feet, wherever the attic has 30 inches of height over 30 square feet or more. And it has to be somewhere with ready access, which means somewhere you heat.
Where located in a wall, the opening shall be not less than 22 inches wide by 30 inches high (559 mm wide by 762 mm high). Where the access is located in a ceiling, minimum unobstructed headroom in the attic space shall be 30 inches (762 mm) at some point above the access measured vertically from the bottom of ceiling framing members.
4.6 square feet is nothing as an area. It is a great deal as an uninsulated, unsealed opening between a heated hallway and a ventilated attic, because it leaks air as well as conducting heat, and warm air leaving through it carries moisture into the roof.
The fix is dull and cheap: rigid insulation on the back of the hatch to something near the attic's own R-value, weatherstripping on the frame, and a latch that actually pulls it closed. A drop-down ladder is worse than a panel, because the frame is a permanent thermal bridge and the seal is harder to make.
The calculator subtracts the hatch from the insulated area, because it is not insulated area, and reports it separately rather than quietly folding it into a percentage.
The ceiling plane is not the floor plan
The area to insulate is the ceiling below the attic, and on most houses that is smaller than the footprint people reach for.
Anything with a room above it is not attic ceiling. On a two storey house the attic covers the upper floor only, which is frequently much less than the ground floor area.
Vaulted and cathedral sections are not attic either. They are a different assembly with a different clearance problem and they are not reached from the hatch, so they are not part of this calculation.
Attached garages and open porches are usually outside the thermal envelope and should not be in the area at all, even though the attic space above them may be continuous with the one you are standing in.
The reliable way to get the number is to measure the attic rather than the house: pace the ceiling plane between the framing you can see, or take it off the upper floor plan and subtract the vaulted parts.
The source, and what is arithmetic
Read 2026-09-10. Seattle Residential Code, Chapter 8, Section R807.1 Attic access, City of Seattle, adopting the 2018 International Residential Code. Sets when an attic access opening is required and how big it has to be: not less than 22 by 30 inches, wherever the attic has 30 inches of vertical height over an area of 30 square feet or more.
Seattle is used because codes.iccsafe.org returns HTTP 403 and cannot be fetched. Your jurisdiction may amend R807.
The wedge is not a code value and is not a citation. It is trigonometry: the roof plane and the ceiling plane are two straight lines and the distance at which they are far enough apart is a division. Anybody can check it with a tape and a level in their own attic.
The ventilation clearance is an input here and is quoted on another page. IRC R806.3 sets it, and the roof ventilation calculator owns that quotation, the baffle discussion and the net free area arithmetic that goes with it. One page owns each figure on this site.
Frequently asked questions
How much insulation does my attic need?
By area, the ceiling plane below the attic, less the hatch and less anything that is not heated space beneath. By depth, whatever your target R-value divided by your product's R per inch comes to, which is a separate question answered on the R-value calculator here. Then subtract the perimeter band, where the roof is too close to the ceiling for full depth.
Why can't I get full depth at the edges of the attic?
Because the roof plane and the ceiling plane converge there, and a ventilation clearance has to stay open above the insulation. The distance before full depth becomes possible is the target depth plus the clearance, less any raised heel, divided by the tangent of the roof pitch. At 4:12 aiming for 16 inches with 1 inch of clearance that is 51 inches from the wall.
What is a raised heel truss?
A truss built so the roof plane starts some height above the ceiling at the wall rather than meeting it there. It is the only thing that removes the shallow perimeter band rather than mitigating it. On the worked example here a 12 inch heel takes the whole-ceiling average from 70.6% of the full depth figure to 97.4%.
Does the shallow edge really matter?
It is a smaller effect than it looks and a bigger one than nothing. On the worked example the whole-ceiling average lands around 70% of the full depth figure. It matters more than the arithmetic suggests because the shallow band sits directly above where the ceiling meets the wall, which is also where most of the air leakage and thermal bridging is.
How big does an attic hatch have to be?
Not less than 22 by 30 inches under IRC R807.1, wherever the attic has 30 inches of vertical height over an area of 30 square feet or more, and it has to be somewhere with ready access. That is 4.6 square feet of hole in the ceiling plane, so insulate and weatherstrip it or it leaks air as well as conducting heat.
Should I insulate the attic floor or the roofline?
Different assemblies with different rules. Insulating the ceiling keeps the attic outside the thermal envelope and needs ventilation above the insulation. Insulating the roofline brings the attic inside it, usually with spray foam, and has its own barrier requirements under IRC R316. This page is for the first case.
Can I just blow it deeper in the middle to make up for the edges?
You can, and it does raise the average, and it is a poor trade. R-value is not linear in usefulness: doubling the depth over the part that is already deep buys much less than fixing the part that is shallow, and the shallow part is where the leaks are. Air sealing the perimeter is the better spend.
Does the roof pitch change how much insulation I buy?
It changes how much of it can be at full depth, which changes the total. A shallow roof has a wide band where the depth tapers, so it takes less material and delivers less R-value; a steep roof reaches full depth quickly. The calculator reports both areas separately rather than giving one figure.
Check these numbers yourself
Seattle Residential Code, Chapter 8, Section R807.1 Attic accessCity of Seattle, adopting the 2018 International Residential Code, read 2026-09-10. Sets when an attic access opening is required and how big it has to be: not less than 22 by 30 inches, wherever the attic has 30 inches of vertical height over an area of 30 square feet or more, located in a hallway or other place with ready access. Seattle is used because codes.iccsafe.org returns HTTP 403.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-10. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The perimeter band is trigonometry, not a code value. The roof plane and the ceiling plane are two straight lines and the distance at which they are far enough apart is a division; check it with a tape and a level in your own attic. The ventilation clearance is an input here and is quoted from IRC R806.3 on the roof ventilation calculator, which owns that section. R807.1 is quoted from the Seattle Residential Code as read on 10 September 2026 and your jurisdiction may amend it. What R-value you need is a separate question answered on the R-value calculator.