Chimney Chase Air Sealing in Maine Homes

Most homeowners assume their chimney is sealed where it passes through the ceiling. From below, it looks solid. But climb into the attic and you will find one of the largest single air leaks in the entire house - a rectangular or circular opening measuring 6 to 12 inches around the chimney, completely open to the attic.

We have measured individual chimney chases leaking 50 to 100 CFM under blower door pressure. That single opening can account for 10 to 15 percent of total house air leakage. Because hot air rises, the chimney chase acts as a supercharged air pathway - warm air from every level funnels upward and pours into the attic.

The reason it stays open in so many homes is that sealing requires specific fire-safe materials and clearances. Getting it wrong creates a fire hazard, so many contractors and homeowners skip it entirely. Here is how we do it right.

What Is a Chimney Chase

A chimney chase is the framed opening in the house structure that allows a chimney, furnace flue, or water heater vent to pass vertically through the building. In a typical Maine home, you might find one or more of these:

Masonry chimney. A brick or stone chimney for a fireplace, woodstove, or oil-fired appliance, with a framed opening 2 to 4 inches larger than the chimney on each side.

Metal flue pipe. A single-wall or double-wall metal pipe from a furnace, boiler, or water heater, passing through a framed opening 12 to 20 inches across.

B-vent or Type L vent. Double-wall vent pipes for gas and oil appliances, requiring specific clearances from combustible materials.

In every case, the gap between chimney and framing is the air leak. Building codes require this clearance, but they also require fire-stopping - and in most older Maine homes, nobody did it.

Why Standard Air Sealing Materials Cannot Be Used

This is the critical safety point that separates chimney chase sealing from every other air sealing task. Standard air sealing materials - spray foam, caulk, weatherstrip - are combustible. They cannot be used within the code-required clearance of a chimney or flue pipe.

The required clearances are:

• Masonry chimney: 2 inches from combustible materials (framing, insulation, foam)
• Single-wall metal flue: 6 inches from combustible materials
• B-vent / Type L vent: 1 inch from combustible materials (varies by manufacturer - always check the listing)

These clearances exist because flue pipes get hot. A chimney serving a woodstove can reach surface temperatures of 600 to 1,000 degrees. Even a gas furnace flue reaches 300 to 400 degrees during operation. Spray foam ignites at around 600 degrees and melts at much lower temperatures. Using it to seal a chimney chase is a fire waiting to happen.

We have seen it done wrong many times - spray foam around a furnace flue, fiberglass batts against a woodstove chimney, even newspaper jammed into the gap. None of these are safe or code-compliant.

The Code-Compliant Sealing Method: Step by Step

Step 1: Identify the Chase and Flue Type

From the attic, we locate the chimney chase and identify what is passing through it. This determines the required clearance and the materials we will use. We also check whether the flue is active (connected to an operating appliance) or abandoned.

If the flue is abandoned (no longer connected to any appliance), the clearance requirements may be relaxed, but we still treat it as active unless the chimney or pipe has been removed entirely. An abandoned flue can still channel outside air and moisture.

Step 2: Clear the Area

Insulation is pulled back from the chase. Debris, old nesting material (mice and squirrels love chimney chases), and any inappropriate materials are removed. We need to see the full perimeter of the gap to plan the flashing.

Step 3: Cut Sheet Metal Flashing

The primary sealing material is sheet metal - typically 26-gauge galvanized steel or aluminum. We cut the metal to cover the entire chase opening, with a hole in the center sized to fit around the chimney or pipe with the appropriate clearance gap. For masonry chimneys, the flashing fits tight against the masonry (metal against masonry is allowed at zero clearance). For metal flue pipes, the hole maintains the required clearance.

Step 4: Install the Flashing

The sheet metal flashing is laid over the chase opening and secured to the framing with screws or construction adhesive. We ensure the metal sits flat and makes full contact with the framing on all sides.

For irregular shapes or very large chases, multiple pieces of metal may be overlapped and sealed together.

Step 5: Seal Metal-to-Framing Joints with High-Temperature Caulk

Where the sheet metal flashing meets the framing, we seal the joint with high-temperature (fire-rated) caulk. This creates the air barrier at the edges of the flashing. Standard silicone or latex caulk is acceptable here because the joint is at the framing, away from the heat source.

Step 6: Seal Metal-to-Chimney Joint with Fire-Rated Caulk

Where the flashing meets the chimney or flue pipe, we use high-temperature caulk rated for the expected temperatures. For masonry chimneys, a standard fire-rated caulk works. For metal flue pipes that reach higher temperatures, we use high-temp silicone or furnace cement rated to 500+ degrees.

The goal is a continuous seal around the perimeter of the chimney or pipe, closing the air gap without placing combustible material within the clearance zone.

Step 7: Reinstall Insulation (With Clearances)

Once the flashing is sealed, insulation can be placed over and around the chase - but not within the required clearance of the flue pipe. For metal flues, we create a dam or barrier using sheet metal to maintain the clearance zone, then insulate around the outside of the barrier.

For masonry chimneys, cellulose insulation can be placed against the masonry (masonry is non-combustible), but must stay 2 inches from any combustible framing elements.

Carbon Monoxide Safety

While working near combustion flue pipes, we test for carbon monoxide (CO). A cracked flue liner, disconnected pipe, or backdrafting appliance can allow CO into the living space. Tightening the building envelope through air sealing can change house pressure dynamics, potentially making an existing backdrafting problem worse. We always test combustion appliances for proper draft and CO levels before and after air sealing work. This is one of the key reasons we recommend professional assessments over DIY air sealing.

Not sure if your chimney chase is sealed? A free energy assessment includes attic inspection and blower door testing that reveals exactly how much air your chimney chase is leaking. Call (207) 221-3221 to book yours.

What About Abandoned Chimneys

Many Maine homes have chimneys that are no longer in use. These are still massive air leak pathways. Options include capping and sealing from the top (chimney cap plus attic floor sealing), sealing at the attic floor only (the flashing can cover the entire opening when no pipe passes through), or full removal for chimneys that are deteriorating or causing structural problems. We refer to qualified masons for chimney removal work.

What Chimney Chase Sealing Costs

Materials only (for one chase): $30 to $75 for sheet metal, high-temperature caulk, and fasteners.

Professional chimney chase sealing: $200 to $500 per chase, depending on size and complexity. Most homes have one or two chases.

As part of whole-attic air sealing: Chimney chases are included in our standard air sealing scope at no additional charge. The total project cost for attic air sealing runs $1,500 to $3,500, which covers every penetration in the attic floor - chimney chases, plumbing stacks, wiring holes, recessed lights, and all the rest.

Efficiency Maine rebates apply to professional air sealing work. Income-qualified homeowners may receive enhanced incentives. Visit our rebates page for current program information.

Do Not Skip This One

The chimney chase is often the single biggest air leak in a Maine home, and it is the one that homeowners are most likely to skip because of fire safety concerns. That is understandable - nobody wants to create a fire hazard. But leaving it open means living with a major hole in your building envelope that costs you money every single day of the heating season.

The solution is straightforward: use the right materials, maintain the right clearances, and have it done by someone who understands both air sealing and fire safety codes.

Schedule a free energy assessment or call (207) 221-3221 to have our team evaluate your chimney chase and every other air leak in your home. We have been doing this work since 2006, and we get it done safely and to code.

Related Guides

• Sealing Plumbing and Wiring Penetrations - The other hidden air leak pathways that run through your attic floor.
• Attic Hatch Air Sealing Guide for Maine Homes - Seal the access point you use to get into your attic.
• Air Sealing Recessed Lights in Maine Homes - Fire-safe sealing for IC and non-IC rated fixtures.
• Air Sealing Knee Walls in Maine Cape Cods - Where chimney chases combine with knee wall problems in 1.5-story homes.
• Sill Plate Air Sealing for Maine Homes - Sealing the bottom of your building envelope to match the top.