Chimney Academy · How chimneys work

Insulated Chimney Liners

A stainless liner dropped into an old chimney is only half of the job. The material wrapped around it, or poured in beside it, decides how hot the brick gets, how warm the smoke stays and whether the installation matches its listing.

6 minute readUpdated October 2026

Chimney Liners

On this page

A liner inside a sleeve of insulation
Looking down the flue
Three things the insulation is doing
When it is required and when it is a choice
The clearance problem insulation can solve
Blanket wrap or poured: how they differ on the job
What to look for on a proposal or report
How we approach a relining job
Questions homeowners ask
Sources

The short answer

An insulated liner is a chimney liner, usually stainless steel, with a layer of insulation around it: a blanket wrapped on before it goes down the flue, or a lightweight mix poured into the space around it. CSIA says liners serving wood and other solid-fuel appliances must be insulated, while gas and oil liners do not need it but can benefit in an exterior chimney. Insulation keeps heat away from the masonry and nearby framing, keeps flue gases warmer so they draft better and condense less, and is often what lets a liner meet its zero-clearance listing.

A liner inside a sleeve of insulation

Most insulated liners start as a round stainless steel pipe, rigid or flexible, run down an existing masonry flue. The old flue around it, often clay tile, stays in place. Insulation goes into the gap between the new pipe and the old flue walls in one of two ways:

Blanket wrap. A sheet of high-temperature insulation is wrapped around the liner on the ground, covered with a protective mesh or foil, and lowered with it. DuraVent's DuraLiner instructions call for a minimum of 1/2 inch of its foil-faced blanket.
Poured insulation. The bare liner is set in place, then a dampened lightweight mix is poured into the space around it from the top. DuraVent's TherMix, for example, is a vermiculite-based masonry mix that cures into a lightweight, semi-hard mass.

Some liners come with insulation already attached. Whichever method is used, the insulation is not a separate add-on. It is part of how the liner was tested and listed, which is why the details matter. For the difference between the flue and the liner, see flue vs. liner.

Looking down the flue

A simplified plan view of a relined interior chimney. Real chimneys vary: some have no clay tile, some have several flues, and the insulation type and thickness come from the liner's instructions.

Three things the insulation is doing

Keeping heat off the masonry and the framing

A thin metal liner carries heat straight through to whatever surrounds it. Insulation slows that heat on its way to the brick and, beyond the brick, to the wood framing of the house. This is the job that makes insulation mandatory for solid fuel, where a chimney fire can drive flue temperatures well beyond everyday levels.

Keeping the flue gases warm

Warm gases rise; cool ones stall. According to DuraVent's gas relining guide, insulating a liner preserves the heat in the exhaust from the base of the chimney to the top, which helps draft get started at the beginning of each heating cycle and reduces condensation. The TherMix bulletin makes the same point for wood, describing warmer flue gases as a way to reduce condensation and creosote and to cut down on back puffing.

Meeting the listing

Chimney liners are tested under UL 1777. DuraLiner's instructions state that it is listed to UL 1777 and approved for zero clearance between the liner and the masonry and between the masonry and combustibles, with the insulation it specifies. Leave the insulation out and that clearance approval no longer describes your chimney.

When it is required and when it is a choice

Appliance on the liner	Insulation	What drives the answer
Wood stove, wood insert, coal or other solid fuel	Required	CSIA states that liners for solid-fuel appliances need to be insulated; liner instructions set type and thickness
Gas or oil appliance in an interior chimney	Not required by CSIA; check the liner's instructions	Lower flue temperatures and lower heat transfer
Gas or oil appliance in an exterior chimney	Not required by CSIA, but beneficial	Cold masonry cools the exhaust; DuraVent notes that exterior chimneys facing north run much colder than interior ones
Any liner where framing is closer than the code air space	Usually needed to use a zero-clearance listing	The liner and insulation have to be listed for contact with combustibles

Manufacturer instructions can be stricter than the general rule. DuraLiner's instructions, for instance, require the flexible liner to be insulated above the first flue tile no matter what fuel it serves, and strongly recommend insulating all flexible liner for better draft and fire safety. The liner options guide covers how the appliance shapes the rest of the liner choice.

The clearance problem insulation can solve

Model code expects an air space between a masonry chimney and the wood around it. Under the 2021 International Residential Code model text, an interior chimney, or one built into an exterior wall, has to be held at least 2 inches away from combustibles by open air; a chimney standing fully outside the exterior walls gets 1 inch. Only the code's noncombustible fireblocking may go into that gap.

An inspection of an older chimney sometimes finds floor joists, a header or roof framing sitting against the brick instead. The same code section has an exception for that situation: a masonry chimney fitted with a lining system listed and labeled under UL 1777 for use in chimneys in contact with combustibles, installed according to its instructions, may have combustible material in contact with its exterior. In practice, that usually means an insulated liner. The TherMix bulletin, for example, describes the product as UL 1777 listed with several brands of liner for zero-clearance applications where the chimney is in contact with framing, and notes that it is not a chimney liner by itself.

Whether framing contact is acceptable in a given house, and which listed system answers it, is a question for the inspection and the local construction official, not something to assume from a product sheet.

Blanket wrap or poured: how they differ on the job

Fit through the flue. A wrapped liner is fatter than a bare one, so the flue has to be big enough, and its offsets gentle enough, to pass it. Pouring lets a bare liner go down first and fills irregular spaces afterward, which the TherMix bulletin presents as one of its advantages over blankets in chimneys with offsets.
Thickness. Each product sets its own minimum. DuraLiner's blanket needs at least 1/2 inch. TherMix calls for at least 1 inch between a listed stainless liner and a 4-inch masonry wall for a zero-clearance solid-fuel installation.
Time before use. A blanket is ready when the liner is connected. TherMix cures over 28 days, about two-thirds to three-quarters of it in the first week, and its bulletin says flue gases must not exceed 1000°F during the first 48 hours of use.
Weight and future work. The bulletin gives an installed dry weight of about 20 pounds per cubic foot. It describes the material as easily removed, but a poured fill is still a larger job to take out than a blanket if the liner ever has to be replaced.
Gaps and leaks. Poured insulation fills cracks and voids between the liner and the old flue. A blanket leaves the old flue's defects as they are, so masonry repairs come first either way. DuraLiner's instructions say to clean tar glaze creosote from the flue and make needed repairs before installing.

What to look for on a proposal or report

Insulation is hidden once the job is done, so the paperwork is the only record of it. A useful description names:

The liner's maker, model, material, diameter and listing.
The insulation method, product and thickness, and whether it runs the full height or only part of it.
The appliance and fuel the liner is meant to serve.
Whether the installation relies on a zero-clearance listing, and why.

The state's F370 chimney verification form, used when fuel-fired equipment is replaced, asks for the liner's manufacturer, model, listing, material and size, and says all liner documentation must accompany the permit application when a liner is installed. Keeping the liner and insulation instructions with your house records saves guesswork for the next appliance change or sale.

How we approach a relining job

All three liner families, stainless, cast-in-place and clay tile, are part of our liner work, and insulation is planned with the liner rather than added at the end. The assessment looks at which appliance and fuel the liner will serve, how big the flue is and which way it runs, the condition of the masonry and, where it can be seen safely from the attic or below, how close the framing comes to the chimney. A camera goes up the flue whenever the flue permits it, and you see the findings and photos before anything is booked. See chimney liners and, if a stove or insert is coming, inspection before a new appliance.

Questions homeowners ask

Does a gas furnace liner need insulation?

CSIA says liners for gas and oil appliances do not require insulation, though those in exterior chimneys benefit from it. Your liner's own instructions may still call for it in some sections, as DuraLiner's do above the first flue tile.

Can insulation be added to a liner that is already installed?

Usually not without pulling the liner, because a blanket goes on before installation. Poured insulation is placed after the liner is set, so in some chimneys it may be an option, but only where the liner and fill are listed together and the space allows it.

Does insulation make the liner last longer?

The sources reviewed for this guide describe insulation as improving draft, cutting condensation and creosote, and meeting clearance listings. They do not give a lifespan figure, so we do not offer one.

Is the old clay tile left in the chimney?

Often, yes. The new liner and its insulation go inside the existing flue. Loose or broken pieces and any glazed creosote are dealt with first, as liner instructions require.

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Sources

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

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