Moisture on ceiling sheetrock around LP Furnace vent pipe (4" PVC)

[frule]

I have an LP furnace as auxiliary heat for my heat pump. The furnace and air handler are indoors in a closet. The vent and fresh air pipes from the furnace go up through a sheetrock ceiling, then into the attic and on outside up through a metal roof.

Where the vent pipe (4" PVC pipe) goes through the sheetrock ceiling there is moisture and a slight drip when the furnace has been running. So, I assume that it is condensation and that the pipe in not insulated in the attic.
I don't think that the pipe is leaking (but it could be).I know the roof isn't leaking where the pipe exits (heavy rain brings no leakage).

Problem is that I have no access to the pipe from the attic. Should I just cut the sheetrock away from the offending pipe and try to insulate it from below, then repair the sheetrock? I can't think of another option. Best way to insulate the pipe? Other advice?

Thanks!

[jasfrank]

Is the pvc pipe a single length or has it got a connector in it? Sounds to me like it is leaking, not condensate dripping.

[TDKPE]

jasfrank said:

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Is the pvc pipe a single length or has it got a connector in it? Sounds to me like it is leaking, not condensate dripping.

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Mine leaks at a joint when it's running. But since it's in the basement and just drips on the floor, I haven't gotten around to fixing it.

[frule]

jasfrank said:

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Is the pvc pipe a single length or has it got a connector in it? Sounds to me like it is leaking, not condensate dripping.

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There is a joint (a 90* to turn it up to the ceiling, but that is below the sheetrock, so not the problem. It only drips when humidity is high and the furnace has been running. The last couple of days have been cold (so furnace running) but very low humidity and no drip & sheetrock has dried. Plenty of moisture has gone up through the vent-pipe in this low-humidity time.....and no drips.

I assume any other joint would be above the roofline, but I cannot see if there could be one in the attic itself.

[blackhat]

Is it the vent or the intake pipe that is leaking? Vent- it has a leaky joint, intake, its sweating. Vents passing through a space subject to freezing temps must be insulated, intake should have been. You need to get access to those pipes.

[Snipe Hunter]

Rules of thumb:

The flue should be sealed, no more than 10ft long with no more than two elbows and sloped 1/4" per ft towards the heating unit to prevent condensation buildup in the pipe.

It's going to collect condensation no matter what, follow those rules to control it.