Air Sealing Around a Chimney Safely

Where a chimney lets air through

A chimney is a large opening that runs from the basement or first floor to above the roof. Air can leak around it and through it in several places:

• The chase gap at the attic floor. Framing stops short of the chimney to keep wood away from the masonry or metal. ENERGY STAR calls the opening around a flue or chimney a potential major source of warm air moving into the attic.
• Gaps at each floor and in walls. The same clearance space may continue through floors and closets, forming a hidden shaft from the basement to the attic.
• Through the flue itself. A fireplace damper that is open, warped or poorly fitting lets house air go straight up the flue when there is no fire.
• Around the fireplace. Gaps where a surround, mantel or hearth meets the wall, and old cleanout doors that do not close.

The first two are framing and attic problems. The last two belong to the fireplace and chimney, and they are handled differently.

Why the gap exists in the first place

Chimneys get hot, and wood framing, flooring, paper-faced insulation and stored boxes can all ignite with long exposure to heat. The clearance around a chimney keeps those materials at a distance. ENERGY STAR notes that building codes usually require 1 inch of clearance from metal flues and 2 inches from masonry chimneys. Our guide to clearances to combustibles explains why these distances shrink over time when nobody is watching.

Look at the gap before you cover it. Darkened or charred wood on the framing beside the chimney, brittle insulation, or soot streaks on the masonry inside the attic are reasons to stop and have the chimney inspected first. Sealing over heat damage hides the evidence without fixing whatever caused it, whether that is a cracked flue liner, a missing mortar joint or framing built too close.

Air sealing has to keep that space intact. The goal is to cover the top of the gap with something that does not burn and seal its edges with something that tolerates heat, while leaving the gap itself empty of combustible material.

The sealed gap in one picture

Materials federal guidance names

Federal energy guidance is consistent about what goes around a chimney:

• Sheet metal or aluminum flashing. ENERGY STAR calls for lightweight aluminum flashing over the gap. The Building America Solution Center's do-it-yourself guide says sheet metal is needed for gaps larger than 1/4 inch, cut in two pieces that overlap by about an inch and fastened to the framing.
• Fire-rated or high-temperature sealant. ENERGY STAR specifies special high-temperature, heat-resistant caulk. The Building America guide calls for fire-rated caulk at the edges and seams of the metal and around the chimney perimeter.
• A metal dam for the insulation. ENERGY STAR says to build a metal dam before pushing insulation back into place. The Building America guide sets the dam at least 4 inches above the finished insulation level.
• Unfaced mineral wool. The Building America guide wraps a masonry chimney with unfaced, fire-resistant mineral wool batt before attic insulation is reinstalled.

Ordinary fiberglass batts with paper or foil facing, cardboard, scrap lumber and rags do not appear on these lists. If you use any foam product near the chimney, it should be one whose label allows that use, and it still does not replace the metal over the gap.

How the work goes, in order

The Building America guide lays out the job in a sequence. In summary:

1. Find every flue and chimneyCheck the roof, the furnace and water heater vents and the attic so you know what passes through the attic floor.
2. Pull back the insulationExpose the gap and the framing around it. Note anything stored or packed into the gap and remove it.
3. Cover the gap with metalFit two overlapping pieces around the chimney and fasten them to the framing, either at the bottom of the framing or on blocking added at the top.
4. Seal the edgesRun fire-rated or high-temperature sealant along the metal seams and where the metal meets the chimney.
5. Build the dam and restore insulationInstall the metal dam, wrap a masonry chimney with unfaced mineral wool where the guide calls for it, then put the attic insulation back outside the dam.

ENERGY STAR's attic safety tips apply throughout: work early on hot days, wear a particulate respirator, step only on joists or truss chords, and watch for roofing nails coming through the deck. Furnace and water heater flues can be hot while the appliances run.

Factory-built and metal chimneys

Factory-built fireplaces, and stoves vented through metal chimney pipe, are listed systems. The manufacturer's installation instructions specify the clearances, the attic shield or firestop where the pipe passes through a ceiling, and what may touch the pipe. Those instructions override general air-sealing advice. Do not wrap, foam or tape a metal chimney pipe unless the manufacturer allows it.

If you cannot find the manual, the label on the unit or the chimney pipe usually gives a model number. Our guides to factory-built fireplaces and single-wall, double-wall and Class A pipe explain the parts.

Closing the leak through the fireplace

A fireplace that sits unused most of the year can lose a steady stream of warm air up the flue. The fix is at the damper, not with tape or a sheet of plastic over the firebox opening. A throat damper that no longer closes tightly can sometimes be repaired, and a top-sealing damper mounted at the chimney's crown shuts the flue against a gasket. The differences are covered in throat dampers vs. top-sealing dampers, and our damper repair and replacement page explains the work.

Never seal a flue that an appliance still uses. A furnace, boiler or water heater vented into a chimney needs that flue open all the time. If a fireplace is permanently out of use, see caring for a fireplace you never use before closing anything.

After the house gets tighter

Air sealing changes how air moves through a house, and fuel-burning appliances depend on that air. ENERGY STAR warns that in some situations sealing a home or its ductwork can cause combustion appliances such as water heaters to backdraft, and that a tighter home can trap pollutants including carbon monoxide. The Department of Energy's guide to air sealing says that before sealing, it is essential to determine how much fresh air is needed, including for combustion appliances and fireplaces.

• Have combustion safety checked by a home energy professional or heating contractor as part of a sealing project.
• Keep carbon monoxide alarms working on every level. See placement and testing.
• Watch the fireplace the first few times you use it afterward. Smoke rolling into the room is a sign to stop and read why smoke comes into the room.

Questions homeowners ask

Sources

1. ENERGY STAR: Furnace flues require special sealing techniques
2. Building America Solution Center (U.S. DOE / PNNL): Air seal flue shafts and chimneys in the attic
3. ENERGY STAR: Tips for working in the attic
4. ENERGY STAR: Identify the problems you want to fix
5. U.S. Department of Energy: Guide to Air Sealing (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.