Should a Chimney Chase Be Vented?

What the Fireplace Instructions Say

A chase is a framed box built around a factory-built fireplace's metal chimney, and the fireplace maker's instructions set how it is built. In the Superior WRT wood fireplace instructions, every chase-top drawing carries the same note: do not seal ventilation openings. The flashing sits on 1 inch noncombustible spacers, and the storm collar is caulked to the pipe; the openings around it are left alone. The same maker's Pro Series manual says not to seal openings on the rooftop flashing.

Those instructions also govern what happens inside the chase. They say the opening in the collar around the chimney at the top of the fireplace must not be obstructed, and that blown insulation must never be used to fill the chimney enclosure. So the answer to the venting question starts with your own fireplace's manual, and the documents we read point the same way: air moves around the chimney by design.

Venting at the Top

The ventilation openings at the chase top are part of how an air-cooled chimney stays cool. Caulking them shut to stop a leak, or setting a new cover tight to the pipe with no gap, changes the system the fireplace was tested with. If water is coming in at the top, the cure is a sound cover, a sealed storm collar and the right termination, not a sealed vent.

From the ground, look for gaps or louvers just under the termination cap. If someone has run a bead of caulk or foam all around the top, note it for the crew; it is not something to cut away from a ladder.

On a two-flue chase, each termination has its own collar and openings, and both stay clear.

• Termination cap, with its own openings clear.
• Storm collar caulked to the outer pipe only.
• Chase-top flashing on noncombustible spacers, ventilation openings open.
• Chase siding below, finished like the house wall.

Insulating the Walls, Not the Chimney

The other half of the answer is insulation, and it trips people up. The Majestic MPL11 fireplace manual says that when a fireplace sits on an outside wall, the chase walls should be insulated, because cold outside air reaching the chase leaks into the house, and that even a well-insulated chase cannot stop all of it. The same manual says never to pack the chimney's required air spaces with insulation, and to keep a 2 inch air space between the chimney and the materials around it.

Floors and ceilings get firestops. Superior requires a firestop spacer wherever the chimney passes through a floor or ceiling, and a firestop thimble where it enters an attic, to keep insulation from falling against the pipe. Section R1005.8 of the Florida Building Code, Residential, requires a steel insulation shield where a factory-built chimney passes through an insulated ceiling or attic. DuraVent notes that its attic shield is not needed when the chimney is enclosed in a chase there, which shows how the chase itself acts as the barrier.

How a Sealed, Wet Chase Rots

Trouble starts when water gets in and cannot get out. A rusted cover or a cracked storm collar lets rain into the box; caulked vents and stuffed insulation hold it there against the plywood and studs. Wood that stays damp in a closed, dark box softens from the inside long before the siding shows it.

The fix is never more sealing. It is stopping the water at the top, opening what the maker said to keep open, and replacing wet insulation and soft framing. Our chase cover guide covers the top, where most of that water starts.

Chases Around Other Kinds of Chimney

Not every chase hides a fireplace chimney, and the rules change with what is inside. A wood stove chimney such as DuraVent's DuraPlus may run through a chase in the attic, under its own instructions. A stainless liner that pokes out of a masonry chimney is different again: the DuraLiner instructions say not to build chases or other enclosures around liner pipe above the masonry. Whatever the chimney, its own maker decides whether a chase is allowed and how it breathes.

What You Can Check

From inside the house, look at the wall and ceiling around the fireplace after rain for stains or a musty smell. From the yard, look for rust streaks below the cover and swollen or stained siding. If you can see the chase interior through an attic, look with a flashlight for loose insulation touching the pipe or a missing firestop, but do not move anything or climb into a tight attic space near the chimney.

Write down what you see and when: a stain that appears only after wind-driven rain points to a different leak than one that shows after every shower, and that note saves time on the visit.

Getting the Chase Checked

Our chimney chase repair crew opens what needs opening, checks the top, the firestops, the shields and the insulation against your fireplace's instructions, and puts the repair in a written estimate after a free inspection.

Chases are carpentry and chimney work at once, and our experienced crew handles both. The company is locally owned, run by a family, and Proudly American; Spring Hill homeowners get a Proudly American family crew that explains what it found before it touches the chase.