A gap with a clearance of how many inches is required between a manufactured chimney and combustibles?

Typically 2 inches of air space all around, for the whole run. Some listed systems are tested for 1 inch. The label on your chimney is the binding number — a rule of thumb is not.

Is the clearance measured from the flue or from the outer casing?

From the outer casing, at the tightest point. Wrap a tape around the casing and divide by π to get the true outside diameter, then add the clearance on both sides to size a framed opening.

Can I pack insulation into the 2-inch gap to stop drafts?

No. The air space is a working part of the system — convection through it keeps the casing cool. Filling it traps heat and is a genuine fire cause. Use a listed insulation shield to hold attic insulation back instead.

Do I need a firestop at every floor the chimney passes through?

Yes — a listed firestop or radiation shield at each floor and ceiling penetration. It blocks the chase from carrying a fire between stories and, just as importantly, holds the chimney centered so the clearance is actually maintained.

My chimney label says 1 inch. Is that legal?

If the chimney is listed and labeled for a 1-inch air space, that is its tested clearance and it governs — the listing always beats the generic figure, in both directions. Install it exactly as the instructions say and confirm with your local building department.

Chimney clearance to combustibles: the 2-inch air space

For a listed factory-built chimney the answer is two inches of air, all the way around, for the whole run. The useful part is knowing how that gap gets built and how it quietly disappears.

By Francesco Zinghinì

A gap with a clearance of 2 inches is the typical requirement between a manufactured (factory-built) chimney and combustibles — framing, sheathing, subfloor, trim, insulation, anything that can burn. Some listed chimney systems are tested and labeled for 1 inch, and a few specialty products for more; the number that governs is the one on the label of the chimney you actually own, not a rule of thumb. For the full lookup by surface — masonry, factory-built, connector pipe — use the clearance-to-combustibles reference. For context: a masonry chimney is typically 2 inches to interior framing and about 1 inch where it runs up an exterior wall, and a bare single-wall stovepipe connector wants a full 18 inches.

It is an air space, not a gap you are allowed to fill

This is the part people get wrong, and it is the reason the requirement is written as an air space. The two inches are not a tolerance or a construction allowance — they are a working part of the chimney system. A listed class-A chimney has insulation or an air jacket inside it, but the outer casing still runs warm, and it sheds that heat by convection into the surrounding gap. Air moves up the chase, carries the heat away, and the framing stays cool.

Pack that gap with fiberglass batt, blown cellulose or spray foam and you have done two bad things at once: you have put a combustible or heat-trapping material in direct contact with a hot surface, and you have stopped the convection that was keeping the casing temperature down. The casing then runs far hotter than it was tested at. This is the mechanism behind a slow-cook fire in an attic, and it happens years after the install, which is why nobody connects the two.

Worked example: framing the penetration

Say you are running a listed 6-inch class-A chimney up through an attic floor. Do not trust the catalog — measure the outer casing. Wrap a tape around it: 32¼ inches of circumference, so the outside diameter is 32.25 ÷ π = 10.27 inches. Add the air space on both sides:

clear opening ≥ 10.27 + 2 + 2 = 14.27 inches

Now look at what the framing gives you. Joists at 16 inches on center with 1½-inch stock leave 14.5 inches clear between them. You have 0.23 inches of margin — under a quarter of an inch, split between two sides. It fits, barely, and only if the chimney runs dead plumb through the middle of the bay. Nudge it an eighth of an inch off center while you are muscling a section into place and one side is out of clearance, invisibly, forever.

Step up one size and the arithmetic stops working. An 8-inch class-A chimney with roughly a 12½-inch casing needs 12.5 + 4 = 16.5 inches of clear opening, which no 16-inch-on-center bay will ever give you. You head off the joists and frame a proper opening. This is not optional carpentry — it is the clearance requirement expressed in framing, and it is exactly the sort of thing that gets skipped when a chimney is installed by someone in a hurry. Budget for it: it is part of what a wood-stove installation or a new chimney actually costs.

The three places the air space disappears

1. Attic insulation. The commonest failure by a wide margin, and almost always innocent: an energy retrofit blows cellulose across the attic floor and it drifts against the chimney casing. The fix is a listed insulation shield — a sheet-metal collar that stands above the insulation line and holds it back — installed at the ceiling penetration. If your attic was insulated after the stove went in, go look. It takes five minutes with a flashlight.

2. The firestop spacer left out. Every floor and ceiling penetration takes a listed firestop or radiation shield, which does two jobs: it blocks the chimney chase from acting as a flue for a house fire, and it mechanically holds the pipe centered in the opening. Leave it out and nothing guarantees the two inches; the chimney simply leans wherever the sections want to go.

3. The enclosure gets finished tight. A chase built around a chimney, or a closet the run passes through, gets sheathed, trimmed and sometimes shelved — and the finish carpenter has no idea there is a clearance rule in play. Watch for a shelf board scribed around the pipe, a trim collar screwed to the casing, or drywall returned tight to it.

The chimney and the connector are different numbers

Do not carry the 2-inch figure down to the stove. The connector — the visible stovepipe from the appliance up to where the chimney begins — is not a chimney and has nothing like the same clearance. Plain single-wall connector pipe typically needs 18 inches to a combustible wall or ceiling because it radiates heat directly; listed double-wall connector usually needs about 6 inches. Confusing the two is how a stove ends up with a beautifully clearanced chimney above the ceiling and a pipe 10 inches off a stud wall below it.

What counts as combustible

Framing, sheathing, subfloor, roof decking, furring — obviously.
Insulation of every kind, including mineral wool unless the listing specifically permits it.
Trim, shelving, a scribed closet board, a wood mantel, cabinet sides.
Drywall does not burn, but it is usually screwed to something that does — measure to the framing behind it.

Checking an existing install without opening a wall

You can learn most of what you need in twenty minutes. Find the label plate on the chimney sections (usually near a joint, or on the support box at the ceiling) and read the required clearance and the model — that is the binding number. Then go to the attic and look at the penetration: is there a metal shield standing proud of the insulation, is the pipe centered in its opening, is anything touching the casing? On the roof, check that the storm collar and flashing stand off the casing rather than clamping insulation against it. Anything you cannot see — a run through a finished chase or a boxed-in wall — is what a Level 2 inspection with a camera is for, priced with the inspection cost tool and explained in the inspection-levels guide.

One last framing note, because it saves rework: when you head off joists to make an opening, size it to the clearance and then leave it alone. Do not later insulate the ceiling around it, do not run a wire through it, do not use the chase as a convenient path for a duct. The opening is part of the chimney, not spare space. And if you are choosing between chimney types on a new build, the clearance and the framing it forces are a real cost input — see masonry versus factory-built chimneys before the framing is set, not after.

Clearances for factory-built chimneys come from each product listing under the standards adopted by your local code; the model venting standard is NFPA 211, published by the National Fire Protection Association, and the residential code that adopts it comes from the International Code Council. For hearth-product installation practice see the Hearth, Patio & Barbecue Association, and for inspections, the Chimney Safety Institute of America. These are typical published planning values, not a certified spec: confirm the clearance on your chimney’s own label, follow the manufacturer’s instructions and local code, and have a CSIA-certified sweep or licensed installer verify anything concealed. Fire, structural and code judgment is theirs, not this page’s.