Exterior Chimney Chase Code And Best Practices, Above And Below The Roofline--Ventilation, Insulatio

[Renovation]

Hi Folks,

How's that for a searchable title?

I've been researching exterior chase construction, and thought I'd pass on what I've learned.

I'm planning to optimize the draw of my exterior chimney by enclosing it, and bringing the chase below the roofline into the building's heated envelope, and insulation the portion above the roof.

The toughest question was whether to vent the portion above the roofline. From what I've seen, the standard practice is to leave the upper chase both unsealed and uninsulated, with a vent at the top to release whatever moisture enters from the loose construction. For example, here's Simpson's DuraTech chase-top flashing, which has a 13" hole for a 10" OD 8" ID Class A chimney pipe:

(broken link removed to http://www.northlineexpress.com/item/5SP-9630/8-Simpson-DuraTech-Chase-Top-Flashing-9630)

I wanted to do better, insulating the upper chase to hold heat and improve draw, to keep that big 8" wide flue from stalling with a cat stove's sometimes slow and cool flue gases. As I said above, venting is SOP, but is it necessary? Is it legal and advisable to seal a tight, sealed, and insulated chase, or might there be problems?

My research says that sealing is beneficial and legal. I'll include the evidence below, and a link to it's source, in case it's useful. Please feel free to make any comments or corrections.

It also has some interesting discussion of metal versus masonry flues, and chimney fires, which I included for general interest. I'll have to break it into parts, because of the character limit. :/


Best,

George

From The Journal of Light Construction:

http://forums.jlconline.com/forums/showthread.php?t=47395

Old 04-10-2009, 11:51 AM
tucker tucker is offline
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Default Re: Insulated Chimney Chase?
Sorry about the lack of info. It is a wood burning/optional gas log unit, a lennox superior fireplace to be exact, with double walled chimney . The clearances from combustibles are 1" to unit and 2" to chimney sections(I usually triple that atleast). The chimney section has no offsets, it is straight and about 13' tall with 8' of that above the roof plane. I am not asking about specific installation instructions like clearances, but more about the pro's and cons of insullating the above roof chase.... Chimeny draw vs condensation problems.

I have used this model a couple times and have always followed the manufacturers directions for chase construction, but instructrions have said insulation is recommended but optional in chase walls(installation guidelines also say do not use blown or fill type insulation). I have always insulated with unfaced fiberglass then used 1/4 hardiebacker for interior(although manufacturer recommends osb, plywood, or drywall).

I am just wondering about condesation issues since the chase cap is uninsulated and would be a cold surface, but I am also wondering about condesation in chase wall system....but now that I think about it wouldn't most of the warm air generated by the chimney section be fairly dry?

As for AHJ, I am in a no inspection area...& a six pack will get you a building permit ; )



#4
Old 04-11-2009, 09:06 PM
Hearthman Hearthman is offline
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Default Re: Insulated Chimney Chase?
You are describing a factory built fireplace listed to UL 127 for woodburning. These fireplaces are usually tested to also use optional vented gas logs listed to ANSIZ21.60 and in some cases, ventfree logs listed to ANSI Z21.11.2b . There is no such thing as "direct vent" with wood burning. Direct vent means self contained combustion/ makeup air and exhaust.

The listing does not require any type of fire resistive construction in the chase. However, if you do insulate the chase, you must protect the insulation from falling in contact with the fireplace and chimney. The building code dictates this construction. Basically, you want to bring the fireplace inside the thermal envelope of the home. This means foam, caulk, air sealing, insulation, a low flame spread sheathing and reducing air infiltration. Having said that, you cannot caulk listed firestops unless specifically allowed by the mfr.s listed instructions.
You must provide proper fireblocking around listed firestops per your local codes. In the absence of any locally adopted codes, you should still build to the IRC, which is now the default building code referenced in the US. Therefore, fireblocking would then comply with IRC 602.8 .
Once you get above the attic deck, there is no requirement by the codes to weatherize the chase walls since you are outside the thermal envelope. Doing so would provide some improvement in draft and minimize flue gas condensation. However, it would have little if any effect on condensation inside the chase. I enter these chases all the time and the only moisture problems I find are from leaks and faulty installation.

There is no need to triple the stated clearances off the chimney. In fact, you cannot at the firestops since they should be braced and supported laterally. The listing allows a max. chase temp. rise of 90*F above ambient so don't lose sleep over it. Properly installed, these fireplaces are much safer than masonry fireplaces and unlike masonry, come with a warranty and are listed. BTW, a masonry fireplace was tested to UL 127 back in 1984 and the test structure ignited!

Tucker, I don't think you will see any significant condensation issues in a properly constructed chase built to code and the fireplace installed to the listed instructions.
HTH,
Hearthman

 

[Renovation]

Old 04-12-2009, 09:18 AM
tucker tucker is offline
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Default Re: Insulated Chimney Chase?
Thanks for the info, report on experience, and specific code references!

Although the fire-resistant interior is not-required/might be overkill and almost all of the chases I have seen constructed in my area are lined with OSB, I like the idea of a fire resistant material on the inside. Last year in my area there were 2 chase fire's. I think the fire department concluded that the fires started with some kind of animal nesting on the actual unit and then spread through the combustible chase (OSB). All the firefighters in my small town where preaching the dangers of OSB on the interior of chases.

I am sure there where probably more variables involved and I cannot prove that a fire resistant interior would have prevented the fire, but either way its not that big of a deal to put a fire resistant interior on the inside of the chase in my opinion, atleast.

#6
Old 04-12-2009, 11:17 AM
BeachBoy BeachBoy is offline
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Default Re: Insulated Chimney Chase?
What happens as the metal flues reach end of service life and start to corrode through? A lot of chase fires waiting to happen.

The animal nesting problem is a good example of how unexpected changes or 'modifications' can get made to an installation that can cause an (often hidden) hazard. Building with that in mind is always a good idea. (Although I've never figured out an easy way to completely line a chase...)
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#7
Old 04-13-2009, 02:14 AM
Hausdok Hausdok is offline
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Default Re: Insulated Chimney Chase?
Quote:
Originally Posted by BeachBoy View Post
What happens as the metal flues reach end of service life and start to corrode through? A lot of chase fires waiting to happen.
That's the whole point of having regular Level 1 and occasional Level 2 chimney inspections and it's the homeowner's responsponsibility to see that it gets done.

It doesn't matter how you build them, if a homeowner ignores regular periodic maintenance on them they'll be just as dangerous as one that's crappily built.

ONE TEAM - ONE FIGHT

Mike


#8
Old 04-16-2009, 08:36 PM
Hearthman Hearthman is offline
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Default Re: Insulated Chimney Chase?
Doc makes a good point for a lot of other reasons, too. However, it takes a level III inspection to open a chase to inspect for clearances, water damage, etc.

Properly installed and maintained factory built fireplaces and chimneys have been in use for over 50 yrs with no problems. Improperly installed, neglected or abused fireplaces can light off the house on the first fire as I have seen.

Beach, what happens to masonry flues when they erode away? The masons don't read the code and the AHJs don't know what they're looking at so they get built howeve the mason feels like. They get away with using ordinary Portland cement mortars with wide, sloppy joints that soon erode away from the moisture and acids. In heater flues, you have even more acids. In woodburning flues, if you reach ~800*F, the mortar calcines and breaks down. The flue tiles cannot take thermal shock much less the hoop stress of a sudden thermal event such as a chimney fire so they rip apart. I could go on but I just wanted to make the point that ALL things built and installed by man can go wrong. ;-)

BTW, most factory built fireplaces carry a 20-25 yr warranty and some UL103HT factory chimney carry a lifetime warranty. What's the warranty on masonry fireplaces and chimneys?

As for the critter nests, always inspect those combustion air kit inlets to ensure they are intact and attached to the flex duct. I know of at least one unfriendly fire that may have resulted from birds entering the eyebrow where the mesh broke down and Tweety ripped through the slinky duct to nest on top of the fireplace.


Proper vertical and horizontal fireblocking per IRC 602.8 will do more to limiting a chase fire to ~$35K instead of ~$350K
Hearthman

#9
Old 04-16-2009, 11:26 PM
miterme miterme is offline
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Default Re: Insulated Chimney Chase?
What about the portion of the chase that is in the home, how is that to be fireblocked? Most of these new homes have prefabbed fireplaces going in that are framed in during rough. When the fireplace is installed inside that framing, there is a direct opening from the fireplace to the attic. Once the walls are sheetocked in, the chase is exposed from fireplace to the roof deck and attic. Its not possible to sheetrock inside the chase of course. Most of my installers install the factory fireblock sheet metal shield around their vent when it penetrates the attic/ceiling joists. Yet the space around that fireblock shield is just open joist, which they remove the installation from. The purpose of the fireblock shield seems pointless, but its installed anyways. Any input?

BeachBoy
Default Re: Insulated Chimney Chase?
It's not supposed to be open joist! The top of the chase should be sealed with blocking to prevent (delay actually) fire from moving between floors.
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