Questions and answers about how to build or inspect a wood-framed chimney chase.

This article series describes wood-enclosed metal chimneys, how a chimney chase should be constructed, and how those structures can be inspected for leaks, damage, or unsafe conditions.

Q&A about Wood Framed Chimney Chases Surrounding Metal & Insulated Metal Flues

These questions and answers about wood-frame chimney chase construction and safety were posted originally

at CHIMNEY CHASE CONSTRUCTION - be sure to review that article.

I can't find the source of leaks into my chimney chase; can't I just ventilate it to dry it out?

A few years ago I had to remove the vinyl siding and repair the osb that is framing my wooden chimney chase due to water infiltration.

I replaced the large cap on top of the chimney and had flashing installed.

However on the underside bottom of the wooden chimney box, I still get wet and rotting OSB (shown below) that seals the bottom of the chase from the underside.

Is it ok to put screen supported by lattice-work or a frame on the underside instead of a solid board (as shown below) to allow any moisture build-up to drain out?

Moderator comment on photo above: Watch out: OPINION: the "venting system" shown at the bottom of this chimney chase is ineffective, the wrong approach to solving a chimney chase leak (lets the water in and then tries to let it out), and may be unsafe, violating fire codes and feeding air to a potential chimney chase fire.

Anonymous by private email 4 September 2025

Moderator reply: No, that's a bad idea. Here are some other suggestions.

Moderator comment on photo above: notice the algae stains on siding - this is a damp or shaded area.

Moderator comment on photo above: we can't see enough of the chimney chase top flashing to know where the leaks are but certainly risks include those discussed at at CHIMNEY CHASE CONSTRUCTION - too-flat on top, poor seal around the metal chimney itself, inadequate drip edge sending water down siding surfaces, bad flashing where the upper roof abuts the chimney side, and depending on chimney chase wall construction details, possible wind-blown rain leaks through the chimney chase walls .

I've taken apart chimney chases like yours to remove moldy materials, repair, rebuild, etc. as you've done.

Key during demolition was to look very carefully for the points of water entry.

I've found most-often leaks at the chimney top, for example at a nearly-flat top of the chase where the round chimney penetrates the metal top flashing, or at improper edge flashing.

But I've also found leaks along the walls of the chase or at the abutment of chase walls to building sidewalls, sometimes related to wind-blown rain.

Watch out: Venting your chimney chase to try to dry it out risks violating fire safety and fire codes.

Also, venting the bottom may let water out but it won't do a great job of venting and drying out the interior of the chimney case anyway because there's no significant air exit at the top - you won't get much airflow.

There could be a subtle increase in fire hazard (updrafts) but I don't think the code people have considered this since the chimney materials themselves are supposed to be handling the exahust safely.

In most cases the chimney chase has only a cosmetic purpose: enclosing the metal chimney for appearance. Important is a very careful inspection at the chimney top to be certain that there are no leaks.

Bottom line: OPINION: if there are no ongoing leaks then trying to dry out the interior of the chimney chase isn't necessary;  If there are ongoing leaks then the bottom vent won't solve the problem, won't adequately dry out the leak, and makes the mistake of treating the symptom rather than the cause. Find and fix the leak point. 

Anon followup:

I really appreciate you taking the time to reply.

Three to 4 years ago I removed the siding and replaced the OSB on the bottom 1/3 of the chimney structure. All of the trim boards from top to bottom were replaced.

The metal cap at the top was replaced.

All of the replaced OSB and trim boards were treated with anti-rot sealant before replacement.

Moderator reply: Find the leak

OPINION: In general it is always a distant second-best to let water leak into a structure and then try to make the structure water resistant and to help it dry.

Certainly while chimney chase leaks occur, they do not occur on every or even most of those structures as long as all of the components are built properly.

We need to find the leak and fix it.

 

Warning about a prefab fireplace / chimney that's leaking

I have a prefab fireplace with gas logs. I recently discovered it is leaking. I am not sure how long this has been happening.

I replace the chase cover and it is still leaking the same as before. The chimney is on the side of the house. Any suggestions for how i could find out where the leak is coming from without replacing the entire chimney? Thanks for any help On 2018-11-09 by Rob

Answer by (mod) -

Rob

It's unclear just what sort of leak you are describing: flue gases, inadequate combustion air, what?

I don't assume the chimney needs to be replaced; but you should have it inspected by a certified chimney sweep who can tell you what's wrong and what repairs (or possibly re-lining if it's an older masonry flue) is needed.

Watch out: Meanwhile to be safe don't use the gas log fireplace and leave fuel to it shut OFF.

And be sure you have working smoke and CO detectors properly located and tested.

Rodents damaged insulation in my chimney chase: what do I do about it:

I recently discovered that there were mice or another rodent have been in the chimney chase and eaten through, knocked off and otherwise damaged the fiberglass insulation that was there. It's a two story house.

What am I to do? What are my options ? On 2018-09-25 by David

Suggestions by (mod) - Remove the insulation, check fire blocking

Most likely you want to remove rodent infested insulation as it's unsanitary and a potential health hazard.

Before reinsulating which might require partial disassembly to provide access, you want to be sure that the design of the chimney Chase and its insulation and any fire blocking required therein will meet local fire and safety codes.

On 2017-03-23 by Todd

Started to replace old cedar wood planks on side of chimney chase and found 25+ year old Sheetrock is this common ? Sheetrock in bad shape due to age ! And can I go back with exterior grade plywood or is Sheetrock required ?

Reply by (mod) -

No, Todd I don't think that's a common construction method. However I think that the drywall may have been put where you found it as a fire barrier. You should consider that and you should discuss with your local building inspector what she considers safe for the type of Chimney that you are enclosing.

Can I recover heat from my chimney from the chimney chase?

Good day,
I am building a 2 x 4 enclosure covered with 5/8" fire rated sheetrock around a selkirk supervent double wall chimney, running through a living space on the second floor just below the attic.

All joints (3) are fastened with bolted joint rings and spacing codes meet regulations.

The only reason I can find for the closure is to prevent someone or something from hitting the chimney causing it to part consequently causing a fire. With the closure in place I don't see how this is possible.

Question: can you install grills in the sheetrock to recover the heat comming off the chimney, which will also prevent the interor of the enclosure from geting to warm/hot? On 2016-11-13 by joseph dorsett sr

Warnings by (mod) - no

Joe,

Before cutting any openings in a chimney chase I would review the fire blocking requirements likely to be specified by your local building inspector.

Watch out: if your chimney chase venting or other chase details violate fire blocking you may be creating a serious safety hazard as well as a fire code violation.

On a practical matter of heat recovery, my OPINION is that there should not be enough heat loss out of an insulated metal flue that you'd bother to try to recapture it anyway.

And

Watch out: if your heat recovery scheme over-cools the chimney itself you may interfere with chimney draft, risking an unsafe heating system or fireplace or other heating appliance that could produce fatal carbon monoxide CO gas.

 

On 2016-11-09 by Jon

My wood-framed fireplace has a 25-years old double wall steel liner that is corroded. Can I use the flex-stainless-still products to replace the double-wall-steel liner.

On 2016-11-09 by (mod) - Check the UL Listing of the product you are thinking of using

Jon:

Where a mutli-wall metal chimney has corroded, you might find, as I have, that an exact replacement (such as a simple two-walled metal flue with just air as insulation) is no longer considered safe nor code-approved for a wood fireplace chimney. Neither may be a single-wall flexible stainless chimney re-liner.

Check the UL Listing of the product you are thinking of using, as well as checking with care the manufacturer's product description of its intended use.

For example you wouldn't use a single-wall flexible stainless product designed to *re-line* an existing masonry chimney flue when what's needed is an actual fireproof, insulated, multi-wall metal chimney.

In my OPINION UL or CSA listed flexible stainless chimney liners are suitable for re-lining an existing masonry chimney flue, and are most-useful where the chimney includes bends or offsets that would make inserting a solid smooth-walled chimney relining kit difficult.

But I also think that despite warranty claims, a flexible liner probably has a shorter life-expectancy than a heavier-gauge smooth-wall metal flue liner.

Can I extend or replace a damaged masonry chimney top by building a wooden chase and installing a metal flue?

I have a brick chimney that had some damage on the top. I'm thinking about replacing the top part of the chimney with a wood chase, so it's half wood half brick. The primary reason to use wood is to save some money. The existing flue is 8 by 8 clay. Is it possible to build a wood chase with the same 8 by 8 clay that connects to the bottom half? If not, is it possible to connect the metal flue used in the wood chase to the 8 by 8 clay flue in the bottom half? On 2016-10-03 by Richard

by (mod)

Richard:

I am way too scared of a house fire to say, based on e-text, that building wood (hence combustible) "chimney" to extend / repair a brick chimney is safe and OK. You are likely to set the house afire.

However there are alternative ways to extend a chimney top including a wood chase (properly constructed) around an insulated, multi-wall, fire-rated metal chimney flue.

 

I had a chimney fire in the top 6 ft of my stainless chimney - can I repair it with cement or a sleeve?

apparently I had a chimney fire in the top 6 ft of my stainless chimney. The sweep found a small hole in a joint. The manufacturer was Marco and out of business. Can this be repaired with some type of cement or a stainless sleeve? I don't want to remove the stone veneer and replace the entire chimney.On 2016-07-21 by craig oliver

Warning by (mod) - probably not - it's unsafe!

Craig:

Watch out: The bad news is that probably anyone you talk with about this, if they have any sense at all (I have only a little sense), is not going to promise you that the chimney is safe (from fire or fatal CO poisoning) if you do any sort of home-made or patch job at all.

Best would be to remove and replace any damaged or questionable chimney sections of the chimney.

It might be possible to do this from the top, lifting, replacing, reinserting - avoiding having to wreck the stone veneer on your chimney chase. Be careful: you don't want a house fire.

What Caused Vertical cracks in brick veneer on wood chimney chase

I recently had a brick chimney replaced with a wood chase covered with brick veneer.

One month after it was installed vertical cracks started to appear running through more than 10 rows of brick at the corners. We have not had any weather below 40 degrees yet. What could cause the cracks so soon after construction? On 2015-12-31 y Joe W -

by (mod) - It is possible that the defect is only in the application of the brick veneer,

Joe:

It is possible that the defect is only in the application of the brick veneer, but because ANY movement in a chimney system can open up gaps and present fire or fatal carbon monoxide hazards, I'd ask for an expert inspection promptly;

From your question I can't know if the veneer cracks are due to improper veneer application or due to movement in the framed structure; either could cause cracking.

Can we change an old wood chimney chase to a brick enclosure?

my home was built 20 years ago and at that time the chimney was enclosed with wood. What problems might we incur if we try to now change it to be enclosed by bricks? On 2015-09-03 by lily

Reply by (mod) -

Chances are the old chimney needs to be removed entirely and a new masonry chimney built from foundation up through the roof line. You would not simply enclose the existing structure with brick.

We don't insulate the chimney chase below the roof line?

So it is not standard or code to insulate the chimney chase below the roof line? Mine had some rotted siding and when I was tearing the old wood off there was fiberglass insulation from the ground floor to the roofline fire stop. (July 17, 2012) Anonymous

Reply:

Anon,

right, since the chimney chase carries an insulated metal flue rated for "zero" clearance, possibly actually requiring an inch or more of airspace, and because the chase is outside the building, it's not normally insulated;

But a rot problem would better be blamed on construction or flashing errors that allowed leaks into the chase. I agree that once wet, insulation in such a space slows drying and increases damage, but it's the leak that caused the trouble.

 

Do we have to enclose the chimney flue in the attc

Does the chimney flue need to be completely enclosed in the attic? (Oct 24, 2012) grant

Reply:

Grant, no, assuming you mean by "enclosing" using a fire rated insuated metal chimney designed for the type of fuel or hear your chimnney is venting, but fire clearances must still be respected.

 

Do I have to enclose a metal insulated chimney with drywall?

We are planning on installing a wood stove on the ground floor of a two story home and running the stove pipe straight up.

I want to get as much heat as I can from the chimney pipe on the second floor.

The chimney pipe will be in a corner of the room and I would like the other two sides to have large wall vents to allow heat into the room.

Does the chimney pipe have to be completely enclosed by sheetrock? (Dec 26, 2012) Paul

Reply:

Paul,

It all depends on the type of vent pipe you are using and their clearance to combustibles rating. For Instance.. HT 103 chimney pipe for wood hardly gives off any ambient heat at all(Dec 28, 2012) Steve

Paul said:
The chimney pipe requires two inches clearance. Although there may not be much heat, it still seems like a waste to fully enclose it in sheetrock.

 

 

Chimney sweep professional says 1980's prefab fireplace chimneys are coming apart

I've been a sweep for 34 yrs. Finding in my area prefab fireplace double wall chimneys coming apart in condos build in the early 80's.

At that time there was no codes or inspections. Am now blamed for latest find. The owner thinks I damaged her chimney with my 8" poly brush. Do you know of any information to support my case?

I believe lack of support straps inside chase is the real reason for pipe collapse and separation. I have given up cleaning metal flues over 2 stories now. thank you robert (Nov 4, 2014) Robert A Wilson ll

Reply:

I agree Robert. Sometimes the solution is a very large saddle (cricket) or even roof mods. At the very least, use absolutely bulletproof flashing and detailing to minimize the leak risk.

 

OK to run air conditioner refrigerant lines inside the chimney chase?

Having a 2nd floor split system HVAC unit replaced. Contractor wants to run new refrigerant lines inside of the chimney chase from the crawl space to the attic. Is this allowed? Won't it ruin any fireblocking? (Nov 13, 2014) paul

Reply: NO

Paul

I would NOT do what is suggested for several reasons if the chimney is or is ever likely to be in use.

If the chimney has been abandoned and if fire sealant is used at penetrations that might be ok.

(Nov 13, 2014) paul said:
I could not find any IRC codes that say don't run a refrigerant line in a chimney chase other than the fire blocking issue.

Reply:

Paul:

Re no code prohibition for refrigerant lines in chimneys: in my OPINION there are so many possible variations in SNAFUS in construction that building codes do not and cannot list every thing NOT to do.

Rather codes list more general guidelines including "follow the manufacturer's instructions". Probably a chimney chase - if we're speaking accurately - is less worrisome than running refigerant piping in an actual chimney where heat from combustion gases could ruin the piping and overheat refrigerant.

 

How do I find the source of an unwanted draft at my fireplace? I covered the outside air intake.

I have a minor draft at my fireplace. Blows back in the room enough to make it unusable.

It is an insert with a 22 degree offset. I have covered the chimney to see if it is down draft not the problem.

I have also covered the outside air intake not the problem. Help! (Dec 20, 2014) Keith

Reply:

Keith I don't think we could find a draft by e-texting.

A careful on-site inspection by a fireplace mason or installer or possibly an experienced home inspector may be of some use.

Or you could try some smoke testing. You could perhaps also do the test in reverse: fans blowing into the home to pressureize it and smoke released in and around the fireplace, spotting smoke appearing outdoors.

See our detailed advice at AIR LEAK DETECTION TOOLS  that includes examples of detecting air leaks

See SMOKE PENCIL / SMOKE GUN SOURCES for sources of smoke testers

Watch out: covering your fireplace air intake will increase the heating cost for your home and can lead to improper or even unsafe fireplace operation.

 

Question: is there a fire-retardant paint to use inside of a chimney chase?

I just replaced my chimney siding (T11) after 7 years since the new roof was installed. The usual cause due to rain rot around the edging. I noticed the discarded T11 where the side facing the flu appeared to be painted with some form of (possibly) fire resistant paint or it could just be discoloration from the Heat.

Is there a paint available? The new job is tight as skin. Let me know if I needed some type of interior coating.

You can send me a reply via my email tomhaeussner@yahoo.com (June 6, 2015) Anonymous

Reply:

Anon:

In a properly constructed chimney chase with the proper-rated and type of chimney installed there should never be enough heat emitted to char the interior surface of the T111 siding. I would ask for an inspection by a certified chimney sweep to be sure your chimney is the proper type and properly installed. The risk is a house fire.

If your chimney is venting a UL listed appliance you do not need and should not require any type of insulating or fireproofing coating on the interior of the chimney chase, though some writers such as my friends over at the JLC (Journal of Light Construction) like putting a fire-resistant covering inside of a chase (keep those clearance distances though).

To build a fire-retardant chimney chase one might use FIRE RETARDANT PLYWOOD and sure one could try spraying a fire-retardant coating, but those are not common practices.

Codes allow a maximum of 90 degF temperature rise above ambient inside the the chimney chase - well below the combustion or pyrolysis point for plywood.

A greater fire risk in chimney chases might occur if birds or squirrels can get into the chase where it's a lovely place for nesting.

Do check that the chimney chase has the code required fire blocking.

[Readers with masonry chimneys will want to take a look at Chapter 10 of the 2003 (or later) IRC: Chimneys and Fireplaces.]

 

Question: ok to run pipes through the chimney chase?

Can I run my sewer pipe thru the fire place chase 22 July 2015 Tom Brackett

Reply: no

Tom:

I doubt your local code officials will permit running a sewer line through a chimney chase. There are issues with fire blocking and clearances to combustibles.

...

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