Is Loose-Fill Fiberglass Combustible?

[Brian G]

Yes folks, it's silly question time again. I know the paper on the back of roll fiberglass is combustible, but what about the fiberglass itself? Does a gas vent pipe require clearance from fiberglass insulation, or can it be in contact?

Brian G.

Never Tried to Burn Any (yet)

[Bruce Thomas]

Brian,

I've seen it smolder but you have to keep an open flame on it. It won't sustain a flame. Is this a double vent pipe?

Bruce

[crusty]

Double or single wall, contact is a no-no. The insulation in contact with the vent will cause draft problems due to the fact that it will cool the gases and prevent them from discharging properly.

[Scottpat]

It will melt at around 2300f

Of course at this temp the house will be gone!

[Brian G]

Originally posted by Bruce Thomas

Is this a double vent pipe?

I honestly can't tell. It's a big metal vent pipe (about 10 or 12 inch diameter I'd say) on an older gas fireplace. I did find a label in the attic (fallen off) that said it needed 2 inches of clearance from any combustibles.

It's a weird deal all the way around. There are two small intake vents on the wall with the fireplace, then on the backside of the same wall are two output vents. Not what one sees every day.

Brian G.

[Brian G]

Originally posted by crusty

The insulation in contact with the vent will cause draft problems due to the fact that it will cool the gases and prevent them from discharging properly.

Okay, you lost me on that one. How would the insulation work to cool the gases in the pipe, oh Crustoid of the Left Coast?

Brian G.

Yuh Done Went & Confusiated Me [:-boggled

[crusty]

Sorry Brian, confusing verbiage, it's a relative thing within the length of the flue. This says it better. "Any insulation against the exterior flue creates an insulated section of the metal flue that does not heat in the same even rate as the metal sections not in contact with insulation. The B-vent appliance exhaust draft is inconsistent at that section with insulation against it and the draft of combustion gases swirls becoming trapped by the uneven temperatures. The products of combustion must change composition in order for the lighter gases to continue to rise. The heavier materials do not rise as readily. Water vapor is very heavy and easily comes out of suspension forming moisture that condenses on the cooler B-vent sections. Moisture condenses on the cooler vent and rolls back toward the B-vent appliance damaging the condensing fan, vent or worse the heat exchanger."

[Bruce Thomas]

Ok Crusty,

I understand and it makes sence but how then do you prevent room air from passing through the opening created around the pipe. You now have 2 flues. One inside the pipe and one from the basement or appliance floor into the attic so conditioned air can pass directly into unconditioned space.

Bruce

[?]

[Brian G]

Hmmmm...I admit this is not my best subject, but it seems more logical to me that the "insulated" parts would heat up faster than the uninsulated ones, not slower. However, I reserve the right to be entirely wrong and full of it.

Brian G.

Not That It's Ever Happened Before or Anything []

[crusty]

I commonly see a piece of sheet metal with a round hole or thimble isolating the vent from the drywall and any type surround to prevent the insulation from coming in contact, much like what the insulators used around unrated can lights or even wood blocking. I'll try to get a pic of one the next time I see it.

[crusty]

Originally posted by Brian G.

Hmmmm...I admit this is not my best subject, but it seems more logical to me that the "insulated" parts would heat up faster than the uninsulated ones, not slower. However, I reserve the right to be entirely wrong and full of it.

Brian G.

Not That It's Ever Happened Before or Anything []

I'm not smart enough to know and have a short attention span for theory. What I do know and have often witnessed is that the draft is affected by the uneven heating, be it faster or slower and condensation with the associated efflorescence has an adverse deteriorating effect on the flue and often the appliance Brian. Flames on the insulation is of little or no concern.

[crusty]

LOL, thanks for the observation Mike. That wasn't from by comment library. Way too complicated for me too. I borrowed it from a friend's recent post on another BB.

[Brian G]

Say Mike,

Do you really use "exaust bouyancy"? I'd feel the need to explain that one to a layman client. Maybe it's just me, but I tend to automatically associate bouyancy with something that floats in liquid, even though I realise it has wider meanings.

Brian G.

I'll Say a Word, and You Say the First Word That Comes into Your Mind...

[kurt]

".....adversely effects exhaust buoyancy?"

I don't know why, but that phrase makes me giggle. The only thing that adversely effects my exhaust buoyancy is when my ass....

Oh, never mind, you don't wanna know.......

[Scottpat]

"exhaust buoyancy" this reminds me of what we use to do to dead bloated cows from about 50-75 yards away and rifle! If you hit them just right, they would shoot up in the air in a ball of flames. [:-jump]

[kurt]

Originally posted by hausdok

Uh uh,

(Somebody please tell me how we got here from fiberglass!!?? [:-boggled)

Mike

It's that damn term "exhaust buoyancy". It ain't natural. It gets me all stirred up....

[Paul MacLean]

OK, the non-technical guy is going to jump into this.

It's my understanding that insulation in contact with the metal flue can cause hot spots in the flue and damage the flue pipe. In any case, at least 1-inch clearance is needed between the the flue and surrounding insulation.

[Bruce Thomas]

Scott,

I always thought you were a sick unit, now I can prove it.

Bruce

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