Attic insulation around furnace flues

[steve ostenso]

I am curious when and what types of insulation do you allow around the base and use for wrapping the flues?

[H.G. Watson, Sr.]

I am curious when and what types of insulation do you allow around the base and use for wrapping the flues?
Note title "Attic insulation around furnace flues"

Generally, and as I am reading your question, NONE. However, HIs don't "allow" or "disallow" anything.

Please be more specific.

Lets start with where are the furnace(s) located?

What is the fuel-type for the furnace(s).

What type and rating for the furnace(s).

What type of venting or chimney connections and venting or chimney system (manufactured or built)?

Generally, at origination connection ("base", "of the flue") there are required air space (cooling, heat sink) and clearance from combustible for connection to the exhaust (combustion) at fuel-burning equipment (even condensing, special venting) for specified distances based on type, rating, and category, and these specifications are specific to the manufacturer's instructions and listing of the equipment (furnace). Furthermore, generally at origination there wouldn't be any fully "zero clearance" at the actual connection to equipment, even multi-wall venting or chimney systems (even those multi-wall - self insulated or air cooled) so-called "zero clearance" chimney systems at point of connection to equipment) and/or special venting systems at the point of connection to equipment.

If you're asking about a b-vent in the attic from floors below - penetrating the attic floor and/or the roof, it may not be insulated, and requires guarding from same, assuring required air-space clearances, and metal (generally listed by manufacturer) fire stops & support..

[steve ostenso]

Most of what I am seeing is your typical rambler or split level home, gas furnace in the basement, metal flue running through the attic to the roof. Living in Minnesota it seems like many houses built in the 80's and 90's have blown fiberglass right against the flues (no barrier) where they enter the attic and many flues are also wrapped with fiberglass batt insulation. I've been told that doing this helps keep attic temps down to reduce the chance of ice dams forming in the winter. I understand this but it seems to me that if you have adequate venting it shouldn't be a problem.

[Scott Patterson]

The problem with insulation on metal flue pipes is that it can cause a hot spot and actually channel the heat a good distance to the wood framing and possibly cause the wood to ignite.

[H.G. Watson, Sr.]

Multiple problems.

1. required clearances & required AIR SPACE around the multi-wall in this case 2-wall air cooled ventiing system.

2. Insulation around same negatively affects DRAFT.

3. #2 above condensation, incomplete evacution of the products of combustion - corrosion, esp. of the inner wall/flue liner - also collections of condensation and corrosion at the exterior. - then there are the increased hazards of condensate throughout and the equipment down below attached to. Deterioration - LEAKS such as water vapor (H20), C02 (too much can smother) and deadly CO (carbon monoxide), etc..

4. #2 & #3 above = leakage of products of combustion (and INCOMPLETE combustion) SPILL into home.

FIre stopping, and preservtion of fire-resistant construction as well.

Remember insulation works both ways. Spun fiberglass insulation is like a sponge as far as its ability to collect and hold significant volume of water.

In either cse it FILLS the REQUIRED OPEN air space around the b-vent in the un-conditioned attic space.

The b-vent system requires air space clearance as well as clearance from combustibles.

What the OP proposes is not only not allowed - it is dangerous & hazardous. The entire system needs evaulation for remediation or remove & replace.

[Eric Barker]

Has anyone ever personally seen or heard of a problem that resulted from insulation against a vent?

[Bruce Grant]

Hello Eric, I have to admit I call it when I find it although I have never seen a home burning down where they or I could pinpoint the cause to this. But, then again the powers that test and recommend are not likely making it up. I also have to admit I have never seen an iceberg sink a ship but recommending bodies suggest ships avoid close proximity to them as well.:D

[Scott Patterson]

Has anyone ever personally seen or heard of a problem that resulted from insulation against a vent?

Hey Eric, I think Cory Friedman in your area has some photos of attic framing that is charred by heat being funneled to it from a flue pipe.

I know Dale Feb has some but his are all from I think fireplace flue pipes.

I personally have seen a charred ceiling joist that was about a foot away from a gas flue pipe. The gas furnace was in a garage and the flue pipe went up through the above attic. It had about a 5' rise from the furnace to the charred area in the attic. In this case the B-vent was also dented on the side so I know the airspace around the double wall pipe was compromised. It was covered with blown in insulation.

[Eric Barker]

Scott - I'll run it past Cory to see what his take is - he and I are usually on the same page. When it comes to fireplace vents I cut no slack.

[Tom Rees]

That condition Scott is talking about is called Pyrolosis. I put this definition in my reorts to explain it to people who tell me that insulation doesn't catch on fire.
Definition of Pyrolosis and How it Causes an Increase in the Risk of Building Fire
Pyrolosis is defined as the chemical or thermal decomposition of a material when it is exposed to heat.

Watch out: Actual fire also "decomposes" combustible material, but in a fire the material is burned or decomposed both visibly and rapidly - dramatically. In contrast, pyrolosis can decompose material also lowering its combustion or ignition point to begin an actual fire, with no visible change in the external appearance of the material, particularly in the case of wood framing.

Wood exposed to heat, such as wood framing too close to a metal chimney in a building, is chemically transformed in an important way: its ignition point or combustion point is lowered - the wood can actually catch fire at a lower temperature. This means that by pyrolosis, wood and some other combustibles found in buildings are chemically changed by exposure to even relatively low but warm temperatures over time.

The chemical change of pyrolosis lowers the temperature at which a substance will catch fire. What may be surprising is that wood exposed to temperatures as low as 200 degF. can over time be changed such that its ignition point or combustion point is significantly lowered.

[Ashley Eldridge]

That condition Scott is talking about is called Pyrolosis. I put this definition in my reorts to explain it to people who tell me that insulation doesn't catch on fire.
Definition of Pyrolosis and How it Causes an Increase in the Risk of Building Fire
Pyrolosis is defined as the chemical or thermal decomposition of a material when it is exposed to heat.

Watch out: Actual fire also "decomposes" combustible material, but in a fire the material is burned or decomposed both visibly and rapidly - dramatically. In contrast, pyrolosis can decompose material also lowering its combustion or ignition point to begin an actual fire, with no visible change in the external appearance of the material, particularly in the case of wood framing.

Wood exposed to heat, such as wood framing too close to a metal chimney in a building, is chemically transformed in an important way: its ignition point or combustion point is lowered - the wood can actually catch fire at a lower temperature. This means that by pyrolosis, wood and some other combustibles found in buildings are chemically changed by exposure to even relatively low but warm temperatures over time.

The chemical change of pyrolosis lowers the temperature at which a substance will catch fire. What may be surprising is that wood exposed to temperatures as low as 200 degF. can over time be changed such that its ignition point or combustion point is significantly lowered.

Agreed, it is the long term exposure to heat as a result of lack of clearances that is at issue here. Most of the B Vent is going to be tested and listed with a 1 inch clearance to combustibles. You are right to make a note of that.

Ashley Eldridge
CSIA Director of Education
Chimney Safety Institute of America (CSIA) | Plainfield, IN (http://www.csia.org)

[H.G. Watson, Sr.]

Probably already was bad connection or breach of inner wall or other problem with bypass - insulation at attic floor and around vent was and is inappropriate treatment of symptom and dangerous.

CO poisioning - increased wet time initially at that insulated junction in the coldest and too warm in warmest months; overheating outer later following inner breach. Interior alum breach. The outer must be 1" air space so as to transfer heat to the air and cooler air cushion so inner can heat up and draft. Outer under insulation - pitting where condensation pools as cools gravity and sponge like characteristics. Interior breach sec. becomes essentially compromised section similar to SW larger size - no longer simple gravity draft currents - increased wet time - & no draft corrosion/deterioration quickly speeds up and outer wall section overheating.

Proper stops, blocking & supports below, insulting chase walls in occupied floors below as exterior walls, fail to eliminate other bypasses to attic, extending chase to roof and necessary sufficient attic ventillation for cold attic - oftentimes insulation retrofits fail to baffle eave intakes, blown insulation travels. Not ever wrapping metal vent system or metal manufactured chimney system with insulation. wet zinc sacrifices early.
Initial vent exposed in cold attic below roof deck not appropriately sized (unenclosed) to begin with, often problems. Removal of SW below, b-vent all the way from draft hood(s)/collars, increasing overall vent primng efficiency (heating up vent faster -thus efficient draft quicker, less spillage in basement), and continue chase which separates the vent system from occupied/living spaces below through attic, insulating chase wall are better possible solutions.

Air space clearances to combustibles also for worst case scenarios of fire below or in vent, preventing ignition (for a time) of adjacent structure.


Verifying minimum venting and max venting as often replacements in equipment over the years has not been appropriately resized, and sizing tables for interior vs. exposed venting not addressed as changes to home made.

[Eric Barker]

I don't think that anyone disagrees with what is specified. But no one has said anything about ever personally seeing a resulting problem due to a Type B furnace / water heater vent being in contact with insulation where it penetrates the attic floor.

[H.G. Watson, Sr.]

I have personally seen completely deteriorated bvent at such a location as OP describes and reviewed sections from same with interior flue wall (alum) completely breached.

Myriad examples in multitude post CO poisioning/death/and fire investigations, reporting & stats, late 70s - early 90s, esp. as aging bvent, WH replacements with larger cat I storage type WHs and pilot-less and higher eff. FA furnace replacements made, no inspection or review of venting systems and unfinished, unconditioned basements, later insulated & finished, and improperly executed attic insulation "upgrades".

[Scott Bennett]

A wealth of info on venting different fuels:

Venting Pipe Installation Guides and Manuals at VentingPipe.com (http://www.ventingpipe.com/installing-vent-pipe-and-chimney-pipe/c3749)

[Jerry Peck]

I don't think that anyone disagrees with what is specified. But no one has said anything about ever personally seeing a resulting problem due to a Type B furnace / water heater vent being in contact with insulation where it penetrates the attic floor.

Well, if you are looking for someone who has personally seen it catch fire ... that is asking a lot as one would need to be there at the exact moment it catches fire, but ... I, and I am sure others have too, have seen the wood charred, but, not, I could not determine "when"it would fire, only that it would catch fire "sooner" rather than "later".

[Harry Janssen]

Every one has answers or ideas,but what does it state on the manufacturers rating label? that is what we need to know,in order to answer the question,our personal opinions do not really matter
We have several companies that rate items,ULC,Warner,Hersey,CSA,if we follow what they claim is correct,we have done due diligence.

[Jerry Peck]

Every one has answers or ideas,but what does it state on the manufacturers rating label? that is what we need to know,in order to answer the question,our personal opinions do not really matter
We have several companies that rate items,ULC,Warner,Hersey,CSA,if we follow what they claim is correct,we have done due diligence.

Type B gas vent requires 1 inch clearance to combustible material and insulation.

I have not seen a Type B gas vent which required any clearance other than 1 inch.

[Andrew Constantine]

This is one of those code required/low probability items. A 1 inch clearance from the flue and any "combustible" material is the typical code requirement. That said, fiberglass insulation is not combustable, however, several good points have been made here regardomg conduction.

The thing that most people don't consider is that the temperature of the vent pipe drops the further away from the unit it is. Put your hand on the flue of an active unit 4-5 feet above the unit. It might be 130 degrees. Not exactly a hot spot for combustion.

But, as an inspector, (IMO) it is our job to point out defects and any insulation touching the flue pipe should be called out, especially since so many areas have this in their building codes.

Andrew Constantine
Charlotte Home Inspections (http://InspectProHomeInspections.com)
Charlotte Home Inspections (http://Charlotte-Home-Inspections.com)
:rolleyes:

[Scott Patterson]

The clearance requirement is actually embossed on just about every "B" vent that I have ever seen. That clearance requirement increases when you get to fireplace flue pipes, it is also printed/listed on those pipes the majority of the time.

[Jerry Peck]

This is one of those code required/low probability items. A 1 inch clearance from the flue and any "combustible" material is the typical code requirement. That said, fiberglass insulation is not combustable, however, several good points have been made here regardomg conduction.

Andrew,

This is is no.more of a low probability than properly nailing roof decking is, or placing and tightening nuts on anchor bolts is, or ... the list is endless.

I would not want someone inspecting my house who looked at things with a 'is this a low probability item' attitude and mindset.

Pretty much defeats the purpose of having the inspection.

[Mike Kleisch]

The closest I can get is a house that caught fire, years ago, where the fire started in the attic. Where the fire started was pinpointed to the area the gas vent pipe was going through the attic. They were unable to determine if it was a connection issue with the vent pipe, no shielding to prevent contact with insulation, or if the vent was tight to framing members, in other words they could not say for certain the gas vent caused the fire (“whew”, says the builder, house was about a year old). Electrical wires were ruled out, along with fixtures (bath fan, lights), as well as arson.

It's really interesting how they reconstruct this stuff to determine how it started, if you saw the pictures there was nothing left of the area, but in talking with the fire inspector he could show the path of destruction. What I don’t remember is if it was a water heater vent or not, but that comes to mind, it was a ranch home, and if I remember right, with all the mechanicals in the basement, below the area where the fire originated.

[Jerry Peck]

[FONT=Arial]The closest I can get is a house that caught fire, years ago, where the fire started in the attic. Where the fire started was pinpointed to the area the gas vent pipe was going through the attic.

Yes, but how old was the house? ;)


This is one of those code required/low probability items. A 1 inch clearance from the flue and any "combustible" material is the typical code requirement.

If the house was, say, 20 years old or older, than that vent was a "low probability" risk as it lasted 20 years or longer. :rolleyes:

[Stuart Brooks]

I don't think that anyone disagrees with what is specified. But no one has said anything about ever personally seeing a resulting problem due to a Type B furnace / water heater vent being in contact with insulation where it penetrates the attic floor.

Yes - gas condensation because the insulation allowed the flue gases to cool before they can vent completely.

Yes - Blown cellulose discolored and powdery; Fiberglass - none

[H.G. Watson, Sr.]

Yes, but how old was the house? ;)



If the house was, say, 20 years old or older, than that vent was a "low probability" risk as it lasted 20 years or longer. :rolleyes:

He said the home was about a year old (at the end of the second-to-last sent. of the first paragraph):


(“whew”, says the builder, house was about a year old).




The longer the condition exists the greater risk for inadequate priming, longer wet time, and corrosion to the inner liner, eventual breach of same, breaching outer wall, etc.

The risks are not only fire breach, and the conducting of same through fire resistant construction, but infiltration of the building and/or cavities with the products of combustion and/or incomplete combustion - this includes noxious/toxic gases, acids, condensation, compromising health of persons and damage to building, and compromise of draft, inadequate venting during the year, even when there is no fire-event, .and the fuel-burning equipment is in use.

Your 20-year old argument isn't supported one way or the other. Older homes more likely to have b-venting already far beyond its original life span, and the appliances/mechnicals being vented more likely changed. Standing pilot WHs vs. ignition, sizing, efficiency, esp. as the offending insulation "upgrades" made - less drafty lesser insulated home, lesser efficient furnace; for example longer burn times, higher temps, shorter wet time, more frequent firings vs. longer periods idle, shorter burn times, lower stack temps, longer wet time, less frequent, shorter burn times.

Then figure in the WH which may also have been changed, possibly multiple times, and likely "upsized" with the usual storage type gas fired WHs over the years.

Remember also, the OP is from Minnesota, not Florida.

[H.G. Watson, Sr.]

Type B gas vent requires 1 inch clearance to combustible material and insulation.

I have not seen a Type B gas vent which required any clearance other than 1 inch.

Clearances are required air space around the b-vent as well. Larger sizes require larger clearances (12"+ IIRC). Where b-vent is passing through floor-ceiling assemblies (such as in the OP's example in his subsequent post, where the equipment is in the basement, and is being vented through floor ceiling assembly of basement/first floor, through occupied space on the first and/or second level, through the floor/ceiling assembly of unconditioned attic, and through roof deck - these areas require support, firestops, which provide heat-sink and generally listed componants provide more than a mere inch clearance to corners, etc. and assure an air-space buffer to not just combustibles.

Sheilds, even metal - are also specified by each manufacturer regarding proximity to multi-wall venting systems in their instructions.

[Eric Barker]

Is there nothing at all that you guys let slide? Do you actually write up every little infraction you find? I gotta think that you've got some jimdandy reports. Glad to see that you're in the home inspection business rather than law enforcement - you'd probably put me away for life for being such a menace on the road. ;)

[Larry Morrison]

Sometimes it can be helpful to put the cart in front of the horse. Most All insulation manufacturers recommend a 3" clearance to any heat producing device be maintained. This is because of entrapping heat and transferring it to a flammable material (wood trusses).

There is an exception for rock wool type insulation, it sometimes can be used around vents, but they then the rock wool manufacturer refers you to the manufacturer of the appliance or vent. So it seams it is very limited and specific when this is allowed or recommended.

[Jerry Peck]

Watson,

Do you NOT actually read posts, especially ones you quote and comment on?

If you do, do you NOT notice the ;) and the :rolleyes: ?

:rolleyes: :confused:

[Darren Miller]

Here's what I say in my reports: note the mention of the mechanical code (oh geez, we aren't supposed to mention the word code...)-

Combustible materials (insulation) are too close to 'B' vent. Recommend removal of these materials so at least a 1 inch clearance is maintained around vent. Failure to remove these combustible materials could result in a fire or damage the B vent. As per the mechanical code, a shield "shall be installed to provide clearance between the vent and the insulation material." I strongly recommend installing such a shield for safety.

[Jerry Peck]

Here's what I say in my reports: note the mention of the mechanical code (oh geez, we aren't supposed to mention the word code...)-

Combustible materials (insulation) are too close to 'B' vent. Recommend removal of these materials so at least a 1 inch clearance is maintained around vent. Failure to remove these combustible materials could result in a fire or damage the B vent. As per the mechanical code, a shield "shall be installed to provide clearance between the vent and the insulation material." I strongly recommend installing such a shield for safety.

Darren,

I would change the first part to (I added the underlined word):

Combustible materials (and insulation)