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Community and Q&A

Condensation at the bottom of the main waste stack in basement

ERIC WHETZEL | Posted in General Questions on

During our recent cold snap here in Chicago, I noticed condensation dripping from my main waste stack in the basement. It’s a single story home with a full basement, and with Passive House levels of air tightness and insulation. The waste stack goes through the Master Bath on the first floor, surrounded in the wall with Roxul batt for sound deadening. There’s 2′ of cellulose in the attic. Never any evidence of condensation/moisture on the bathroom ceiling or floor.

I’m pretty sure it’s condensation since it entirely goes away once temperatures move above 35 F.

At 20-30F there are small amounts of condensation sitting on top of the lip of the first elbow in the PVC directly below the 1st floor sub flooring. Not enough to drip down onto the basement floor, but it’s definitely there.

Only when temperatures were at 0F did I notice any condensation falling off the PVC onto the basement floor. And it was only when temperatures fell to -24F, without the wind chill, that more than a foot of the PVC pipe began to show condensation and drop water onto the basement floor.

We also had enough snowfall on the roof that it was slowly falling into the vent stack on the roof (confirmed by looking up on the roof), but the stack was never completely blocked (never any sewer gas smell or trouble flushing toilets).

The waste stack has a straight shot from the roof to the elbow just below the 1st floor sub flooring in the basement, so I’m pretty sure snow was falling in from the roof and landing at the elbow before sitting there and slowly melting.

When I googled “condensation on waste stack vent” there were any number of hits, typically involving people trying to hunt down the source of mysterious moisture in a basement, and always during the winter.

I’m just wondering if this is normal? Anyone else seen or heard of this?

Apart from when temperatures were at or below zero, there’s not much water involved. Even at -24F we’re talking about wetting the floor, not collecting it in buckets or even significant pooling.

Worth doing anything about it? As long as my subfloor stays dry, do I care if the basement floor gets a little bit of water during extreme cold snaps?

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  1. GBA Editor
    Martin Holladay | | #1

    The pipe is filled with cold air. So the pipe is cold. If the condensation bugs you, insulate the pipe. Problem solved.

  2. ERIC WHETZEL | | #2

    Thanks Martin!

    I assumed as much, but just wanted to make sure.

  3. Expert Member
    BILL WICHERS | | #3

    Like Martin said, insulating the pipe should solve this. I’m guessing that’s a 3” pipe so your choices are commerical style split fiberglass insulation (expensive), or the wrap-on pipe insulation. I’d go with the poly foam spiral wrap stuff, the fiberglass wrap is a pain to install and also dusty.


  4. Expert Member
    Peter Yost | | #4

    Hi Eric -

    Your post prompted me to wonder what the thermal conductivity is of plastic pipe compared to say, metal.

    Great resource:

    "Mild steel" (iron?) more than 300 times more conductive than PVC; fortunate that it's plastic and not metal?


    1. Expert Member
      BILL WICHERS | | #5

      Despite the big difference in thermal conductivity, the energy loss charts in that document show very little difference between copper and plastics. I suspect that’s due to the very thin walls of pipe (fractions of an inch). I’m sure the plastic pipe sweats at a slightly lower temperature than a comparable metal pipe would, but not by much.

      The plastic pipe is going to be easier to insulate than a cast iron pipe would be since the PVC fittings are much closer to the diameter of the pipe than cast iron fittings are.


  5. Jon_R | | #6

    While insulation is the easy solution, I wonder if there is a way to stop the convective loop that is bringing cold air into your house (while preserving the function of the vent).

    1. joshdurston | | #7

      How is there a loop, the vent is a single pipe, and each entry into the house should be protected by a trapped drain? Unless the pipe is big enough to have convection within a single standing pipe.

      1. Jon_R | | #9

        Could be the latter. Warm air rises, cold air enters to replace it. Or maybe cold air comes in from the sewer/drain (where a trap could be used)?

    2. mackstann | | #8

      The code is pretty strict about not even remotely blocking waste stack vents. And I'm not sure it is convection. I would think that the steady state condition in cold weather would be for the warmer sewer gas to be wafting up the stack, and sewer gas shouldn't be cold enough to cause this much sweating. So maybe this is outdoor air coming down, due to cold and windy conditions. And this is in the windy city...

  6. siohouz | | #10

    Noticed that problem today on abs pipes, it was 15 f here in Canada and very windy, going to try to seal ceiling-pipe junction in the attic with some spray foam or caulking. I'll keep you posted on the results!

    1. Expert Member
      BILL WICHERS | | #11

      You’ll probably need to insulate the pipe for a distance from the roof, probably several feet. If you only insulate the immediate area where the pipe penetrates the roof you’re unlikely to do much to improve the sweaty pipe issue.


  7. john_prospect | | #12

    Correct me if I am wrong, but vent stacks are generally all about letting air IN to the drain system, not out. That is why you have one-way valves available for in-building vents. Insulating the pipe in the attic will just maintain the cold temperature of the air on its way down the pipe to the basement. Insulating the pipe in the basement will keep the humid interior air from reaching the cold plastic, but might just move the problem downstream?

    For those with more infrastructure knowledge than me, if the vent stack is connected clear to the sewer main, could there be additional suction coming from somewhere else in the neighborhood, drawing air down Eric's vent?

  8. onslow | | #13

    Mr. Prospect,

    Yes, the vent is meant to allow air into the system to balance things out. Yes, insulating the pipe will only move the problem downstream. Barely possible that the city network is adding some suction to the the system, but unlikely since multiple houses would all share access to the same network and thereby distribute the excess "suction" if present. I think it just plain physics that heavy cold air is settling into the pipe as warmer air rises out of it.

    As for traps in the lines from house to street mention earlier, I don't think that is a thing. All the houses I have lived in that were connected to city sewers were straight shots. Rodding a sewer would have to occur after a trap and I have not seen that in the last three roddings. (crappy luck with houses) My current septic tank is a straight line connection back to the house, though the tank itself is sort of a giant trap between house and field pipes.

    Most likely the insulation has created the problem by bring the coldest section of the stack into a region where the humidity levels are high enough to condense out on the cold pipe. During very cold weather the air in the pipe will chill deeply, particularly when the system is being used lightly, just not much warm stuff going down the drains to warm the air. With heavy hot water usage, it is possible to have a reverse condition where the pipe will frost heavily at the section outside the roof. Worse still is the possibility of an ice plug forming in horizontal sections that have drooped into a reverse pitch relative to the main stack.

    Basically, warming the pvc pipe is a better cure than insulating it. If it resides in a chaseway, then preventing moist air getting to it will be challenging. If it is in a outside wall, which though crazy, is done frequently, making sure that wall bay is well sealed would be necessary to avoid long term moisture damage. Stacks should be as straight up and down as possible. If you have vent octopus in the attic, all the horizontal arms must be pitched sufficiently to drain back to the warm parts of the pipes. The attic air must vented well enough to avoid condensation on the pipes to avoid dampening the insulation.

    While raising a family in Chicago area, a very cold winter could cause a frost plug in the vent pipe above the roof, and sometimes a giant frost ball around the pipe just under the roof line. The frost in the attic was thanks to poor sealing around the stack chase all the way from the basement up to the connected attics. Humid air was feeding frost on all cold surfaces. Fortunately the cold snaps were not too long or frequent and the insulation dried out by summer.

    The question about thermal conductivity of the iron vs pvc is a good clue to why the sweating problem is occurring. The warmth of the house is not being well conducted far enough up the stack assembly to counter winter air temperatures.

    My original stack in Chicago was cast iron up to the top of the second floor level and then iron pipe up through the roof with a lead boot over that. This had been in place for over 50 years before we bought. For our first 10 years no problems were had with frost. When we put on the addition and moved the vent location sideways 6 ft the crew used pvc. The pvc section was now in a cold attic with little transfer of heat from the iron part. Lots of moist air from laundry and shower venting now exited via a sometimes very, very cold pvc pipe. Condensate would freeze out into frost at the mouth of the pipe and restrict air flow, sometimes completely. I could tell by the way the toilet flushed if it was time to go up on the roof and knock off the frost plug. I just lived with the frost film on the underside of the roof.

    I now have built with all AAVs save the one vent line code required in our area. No stacks no sweating no problems. The sole vent line is through a side wall from a heated part of the envelope thanks to outsulation on walls and roof. The cold section of vent pipe is now only 12". The noise the AAVs is a bit odd at first but they become background noise just like the fridge noises and whatnot.

    Hope this provides some value to the group.

    1. Expert Member
      BILL WICHERS | | #14

      >”As for traps in the lines from house to street mention earlier, I don't think that is a thing.”

      It may not be in all areas, and may not have always been required, but traps in sewer mains existing homes are not uncommon. It will be a 3” or more likely 4” trap, usually under the basement floor (or at least somewhere low in the structure) with two cleanouts maybe 12-18” apart. The trap assembly itself is shaped like a big “U” with the cleanouts on the tops of the U and the inlet and outlet pipes coming off opposite sides of the U slightly below the cleanouts on each end. The main trap will be the last thing in the line before the line leaves the house to tie into the city sewer main.

      In my area, many of the older house used 4” cast iron drain lines out maybe 8-10 feet from the house and then transitions to 6” terra cotta line that eventually fills with tree roots.

      If anyone ever needs to replace old terra cotta sewer lines, I highly recommend using s directional drilling contractor to do the work. The directional drilling guys can dig down to the terra cotta line depth, then use the old like a pilot hole for the drill rod. During the pullback, they install a new HDPE (high density polyethylene) line that will be seamless with no joints (or thermally fused joints, but it still is like one solid pipe) inside the old clay pipe. The new HDPE pipe crunches the old clay line out of the way as it goes.

      The advantage is you have a hole on either end of the line you’re replacing but no big trench through your yard. This is a HUGE advantage, especially on long or extremely deep sewer lines.

      Directional drilling contractors usually work for utility companies. If you need to find such a contractor, call your local gas/water company, or electric/telephone/cable company and ask for a recommendation. The gas and water companies are usually easier to ask for this info, the other guys (especially the telephone and cable guys, and I know since that’s my industry :-) tend to have so many levels of people to go through that you’ll never find anyone who knows what you’re talking about. The engineering and “system” people as they’re called are the ones who’ll know. We call this stuff “outside plant” in the industry.


  9. onslow | | #15

    Zephyr7, I am from the Chicago area originally, so my experience with septic lines is based on four different houses there. All of them had clean out caps facing into the basement on the straight leg of the septic Y fitting. Two were waist high, two were partially embedded in the floor slab. Great fun when rodding roots.

    I can sorta see the value of a trap in helping to trap sewer gases, but as the vent pipe is generally going to be the homes ultimate collective vent for internal and the city waste line it seems unhelpful. Also seems like a good place for diapers and other unintended items to get stuck. In line clean outs with elbows pointing in both directions were relatively new and by the time I left Chicago behind, only good builders were doing them as outside clean out access points. What with more and more finished basements, the idea of dragging a big clunky roto-rooter into the carpeted rec room lacks appeal. Ironically, most of the homes with new clean outs aren't using the clay tiles so loved by willows and poplars.

    A much bigger problem in the Chicago area that has been addressed pretty aggressively is forcing disconnection of the foundation drains from city sewer lines. All through the 40's, 50's and even into the 80's foundation drains were hooked into the sewers. Storms could drop so much water into the systems they would back up. I know, I had to deal with 18" of poopy storm water in my circa 1958 ranch. The bummer was not having put in a stand pipe like everyone in Berwyn did. My basement floor was over 9' above the main septic line and I never dreamed that much height could be over run. Needless to say the floor drain received a rubber plug with a 1 1/2" center threaded hole for a 6' stand pipe that prevented future flooding.

    One trick for old clay tiles that was being pitched in the 90's was a resin soaked fabric sock that was pushed down the rodded clay tile path then inflated for a period of time to harden. The ends were cut off and voila (sp??) a clean path to the city line. Never did hear how it was holding up to the ever insistent willows.

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