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Venting a bathroom vent through the attic and out the sloped roof

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  • Venting a bathroom vent through the attic and out the sloped roof

    I'm looking for the newer thinking on this task because I've been thinking it anew and it's scheduled for this Friday.

    Background facts: '80's single fam 2 1/2 story, whole house air, Panasonic 200 cfm vents (2 baths) on 2nd floor. Attic is fairly vented with new tab shingles, new ridge vent and scores of those 2" round soffit vents. And btw, the house sits on an unground spring and the roof sheathing is almost always wet on the underside in the winter. Two dehumidifiers run in the basement with springtime overflows. Bathrooms are mouldy and peely still, after I did the bath work 10 months ago. This new venting was planned to replace the 4" vinyl multi-bent hose to soffit.

    This month I gave the roofers who put on the new roof, the Nutone black roof vent caps http://tinyurl.com/8hgt6k which they cut in. My job is connect the bath vents to the caps.

    It seems the feds want an r-6 on vent pipes in attics. (Link) Aside from other moisture reducing tasks in the attic, my thought is to go 4" steel vent pipe, 3 screws, mastic, then wrapped with r-6 f.g. foil insulation. Why not a thermal bridge, say, 4" no-hub rubber coupling to break the heat/cold conduction? Why not pvc, maybe better?

    Also, vent lit suggests a two foot long sweep coming out of the bath vent, to build up speed I guess. Does tight 90 cut that much? Distance from bath ceiling to roof, vertically, is about 4 feet.

    Thoughts?

    tia,

    Victorian John

    P.S. How do you guys copy a link to a text word that turns blue and we just click the word to bring up the link?
    Last edited by Victorian John; 12-31-2008, 06:21 PM. Reason: P.S.

  • #2
    Re: Venting a bathroom vent through the attic and out the sloped roof

    you type

    [ url = the web adress for the link ]

    the word you want people to click on

    [ / u r l ]

    but without the spaces.

    like this
    Francois


    Truth is just one man's explanation for what he thinks he understands. (Walter Mosley)

    Comment


    • #3
      Re: Venting a bathroom vent through the attic and out the sloped roof

      Originally posted by Victorian John View Post
      Background facts: '80's single fam 2 1/2 story, whole house air, Panasonic 200 cfm vents (2 baths) on 2nd floor. Attic is fairly vented with new tab shingles, new ridge vent and scores of those 2" round soffit vents. And btw, the house sits on an unground spring and the roof sheathing is almost always wet on the underside in the winter.
      You've got some serious moisture problems, and it's being excacerbated by poor roof venting.

      A decent ridge vent has 18 square inches of net free vent area per foot, while a 2" button vent has 1.5 in², so balanced venting would require at least 6 button vents per foot of soffit (in other words, continuous) on each side of the roof to match the ridge. Insufficient intake vent allows the exhaust (ridge) vent to create a negative pressure in the attic, which will draw moist air through every crack and opening in the ceiling (wiring & plumbing penetrations, chimney opening, ceiling lights, attic hatch).

      You need to seal the upstairs ceiling thoroughly, or you're going to rot your roof sheathing.

      This new venting was planned to replace the 4" vinyl multi-bent hose to soffit...My job is connect the bath vents to the caps.

      It seems the feds want an r-6 on vent pipes in attics. (Link) Aside from other moisture reducing tasks in the attic, my thought is to go 4" steel vent pipe, 3 screws, mastic, then wrapped with r-6 f.g. foil insulation. Why not a thermal bridge, say, 4" no-hub rubber coupling to break the heat/cold conduction? Why not pvc, maybe better?

      Also, vent lit suggests a two foot long sweep coming out of the bath vent, to build up speed I guess. Does tight 90 cut that much? Distance from bath ceiling to roof, vertically, is about 4 feet.
      I would recommend either stainless steel or PVC duct. The PVC can have glued joints (best) and the ss should have something like foil/butyl tape at the connections. The smoother the pipe, the less pressure loss, and the plastic is less likely to condense than the metal.

      A tight 90° elbow is the duct equivalent of 5' of straight, while a minimum 10" radius wide sweep is equivalent to 1½'. The wide sweep should have a straight leader between it and the fan that's twice the length of the duct size (8" for 4" duct) to reduce turbulence.
      Robert Riversong
      Master HouseWright

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      • #4
        Re: Venting a bathroom vent through the attic and out the sloped roof

        One of the vent styles I use is Pvc pipe on the top of the ceiling joist tilten down toward a gable end with the fan end a bit higher.
        this allows any moisture to flow out and also allows the attic insulation to cover the pipe helping with any condensation issues. This of course does not work with a hip roof system.
        Mark Parlee
        BESI(building envelope science institute) Envelope Inspector
        EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
        EDI Seminar Instructor
        Level one thermographer (Snell)
        www.thebuildingconsultant.com
        www.parleebuilders.com
        You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

        Comment


        • #5
          Re: Venting a bathroom vent through the attic and out the sloped roof

          if the run is only 4', or even if it ends up at 7' with a bend or two, I would say just about anything will be fine in terms of air flow.
          Where the condensed water goes is something to think about. It's not cold enough here that i've ever seen an issue on a bath vent, but YMMV.

          I think Robert is right (as usual) that you have much bigger issues than the bath venting. If the sheathing is wet, there's stuff to fix on that house.

          Doing good air sealing at the attic floor level (caulking topplates of all walls to the adjacent drywall, finding & sealing up accidental gaps, covering recessed lights, blocking up the tops of the baloon-framed walls & the plumbing wall that has no top plate, etc. etc., look up "air sealing" to find more info) is a good step.

          Having said that, keeping the 'underground stream' out of the basement in the first place would solve the problem closer to its source. There are several approaches that can work; all are expensive; this house will have major problems until one of them is employed, if I understand your description correctly.
          Doug

          Favorite tool this week: Leatherman Wave

          Blog:
          Three types of gas tank hot water heaters for your renovation
          Three types of furnace for your renovation
          Deconstruction: the thrifty, green start to your remodeling project

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          • #6
            Re: Venting a bathroom vent through the attic and out the sloped roof

            Thanks Frenchie, Riversong, Mark and Doug. The job went well. It helped that Lowe's carried not only the lightweight 4" pipe, but also the light fittings including the 22 1/2 and street 45's and 90's. (3 bucks for a long sweep vs. 11 bucks for schedule 40).

            The street ends fit the roof caps and Panasonic outputs perfectly.

            BTW, my best buddy is a tech rep for air-handling industrial equipment, and it didn't dawn on me to ask him until you guys jogged my memory. Anyways, ASME standards are 5X the diameter for the straight out of the blower and equipment is tested and speced at that length. Turbulance and lambda don't ya know ;>

            Thanks again. PVC, the only way to go. Not to choke on a knat, but vinyl-coated slinkies are soooo 70ish -;>

            John

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            • #7
              Re: Venting a bathroom vent through the attic and out the sloped roof

              Forgot to mention, anyway it's really just an afterthought for your job--I have stopped using through-roof vents wherever possible, having installed one that took on water 3x a year during our summer afternoon thunderstorm weather patterns. The water came right down the vent pipe and out the fan cover in the bath. Definitely something to consider depending on your weather, I decided gables & soffits make more sense.
              Doug

              Favorite tool this week: Leatherman Wave

              Blog:
              Three types of gas tank hot water heaters for your renovation
              Three types of furnace for your renovation
              Deconstruction: the thrifty, green start to your remodeling project

              Comment


              • #8
                Re: Venting a bathroom vent through the attic and out the sloped roof

                Originally posted by Riversong View Post
                I would recommend either stainless steel or PVC duct.
                When you use PVC for a bath vent, do you use sched 40 or the light duty PVC? Is it acceptable for drier vents as well? I find using rigid metal to be a PITA, the light duty PVC seems like it would be a lot easier and faster, with equally good results.

                -Tobias
                Last edited by toobiloo; 01-04-2009, 10:10 PM.
                "I smell a lot of 'if' coming of this plan." -Jayne Cobb

                Comment


                • #9
                  Re: Venting a bathroom vent through the attic and out the sloped roof

                  schedule 20 is plenty
                  Mark Parlee
                  BESI(building envelope science institute) Envelope Inspector
                  EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                  EDI Seminar Instructor
                  Level one thermographer (Snell)
                  www.thebuildingconsultant.com
                  www.parleebuilders.com
                  You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

                  Comment


                  • #10
                    Re: Venting a bathroom vent through the attic and out the sloped roof

                    Originally posted by toobiloo View Post
                    Is it acceptable for drier vents as well?
                    No, dryer vents must be aluminum or galvanized steel.
                    Robert Riversong
                    Master HouseWright

                    Comment


                    • #11
                      Re: Venting a bathroom vent through the attic and out the sloped roof

                      Originally posted by Mark Parlee View Post
                      schedule 20 is plenty
                      My answer above was for bath venting not dryer vents.
                      Thanks Robert
                      Mark Parlee
                      BESI(building envelope science institute) Envelope Inspector
                      EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                      EDI Seminar Instructor
                      Level one thermographer (Snell)
                      www.thebuildingconsultant.com
                      www.parleebuilders.com
                      You build to code, code is the minimum to pass this test. Congratulations your grade is a D-

                      Comment


                      • #12
                        Re: Venting a bathroom vent through the attic and out the sloped roof

                        Got it. Thanks to you both.
                        "I smell a lot of 'if' coming of this plan." -Jayne Cobb

                        Comment


                        • #13
                          Re: Venting a bathroom vent through the attic and out the sloped roof

                          Doug,
                          I think soffit vents are not allowed in some states such as CA. The idea being exhaled saturated air being picked up by the soffit vents and thrown back into the attic.
                          John

                          Comment


                          • #14
                            Re: Venting a bathroom vent through the attic and out the sloped roof

                            I was told the reason not to use PVC for dryer vents is that it will build up static electricity and clog up with lint. Anyone know if this is true?
                            Geoff

                            Comment


                            • #15
                              Re: Venting a bathroom vent through the attic and out the sloped roof

                              Butcher,

                              My buddy told me of a top 10 military-industrial company that tried to use pvc for a vacuum system on the assembly benches. The workers were getting zapped all the time. They had to rip it out.

                              For bath fans, saturated air is self grounding. I don't know about dryers though.
                              John

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