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Duct through roof, wall or basement?

| Posted in Energy, Heating & Insulation on September 21, 2004 03:27am

I’m in the process of sealing air leakage into the attic of a 10 year old house.  One problem is that the original builder has ducted the exhausts from two bathroom’s fans into the attic (i.e., two bathrooms, each with their own fan, each fan ducted separately into the common attic space).  Also, the ceiling mounted fans are not sealed to prevent air leakage into the attic.  I’d like to rectify both problems, i.e., a) discharge of moist air into the attic and b) air leakage into the attic.

I suspect the easiest remedy would be to install a separate roof vent for each bathroom fan, and discharge through the roof.  However, we do get the occasional heavy snowfall and I’d be concerned that the vents would be blocked.  Also, this adds roof penetrations which increase the probability of roof leaks.  And, although I would seal the fan enclosures to prevent air leakage, sealant deteriorates with time – I’d like to reduce the number of ceiling penetrations if I can.  Additionally, I’d prefer to keep potential ignition sources out of the attic space.

Another option would be to relocate the fans to an exterior wall and discharge through the wall.  After closing off the unneeded ceiling penetrations, this would eliminate both sources of air leakage into the attic.  One of the bathrooms has no exterior wall, and would require the addition of a chase through an adjacent room.  Also, I came across advice that when discharging through a wall the discharge should be at least 3 feet below the eaves.  This is to prevent the moist exhaust from being drawn up into the attic through the eaves ventilation holes.  As the eaves are approximately at ceiling height of the bathrooms in question, this  would require running the duct down 3 ft through a cold exterior wall – which doesn’t seem a good idea.  Or, adding a duct chase on the inside of the wall – which doesn’t seem very appealing.  (note – we have a hip roof, so eave ventilation on all four sides.)

A third option suggested to me was to a) relocate the bathroom exhaust fan to an interior wall; b) run the duct down inside the wall, or in a chase, into the basement; c) run the duct through the joist space to an exterior wall and d) discharge through the wall.  This would definitely be the most work to retrofit.  And, if run through interior walls would be the most inaccessible.  (However, this doesn’t seem to prevent putting plumbing supply, drain or vent runs inside walls:  I’d suspect a non-issue.)  Benefits include eliminating these sources of air leakage into the attic, and no chance of the exhaust being drawn in through the eaves.   Both this and the second option keep the ducts in warm space, minimizing condensation.  The source indicated that this was a common prairie design, motivated by a desire to minimize penetration of the plane between the heated space and the attic. 

So, my questions to the forum are:

1.   How common is it to run bathroom fan exhaust duct down through interior walls and out a basement wall?  Any examples of problems, or of successes?

2.  Given this is a retrofit, would exhausting through the roof be the best option?

3.  Are there roof vents available which are designed to cope with heavy snow accumulations?  (Is it simplest to just extend the stack up beyond the expected accumulation?  Or, is this a non-issue – does the warm exhaust air always melt a path through the snow?)

 

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Replies

  1. User avater
    hammer1 | Sep 21, 2004 05:52am | #1

    I've been using remote vent fans for a while. The unit mounts wherever, usually in the attic and a duct only is run to the room/rooms. Put it anywhere. One fan will do three rooms. You can exhaust the fan through the roof, standard plumbing pipe. Roof boots aren't a problem unless you have a flat roof. You can just put a chimney (stove pipe) cap on the stack to keep out rain and snow. Keep it up at least 30". The remote fans are very quiet and out perform any combo light/fan I know of. I don't have fog in my bathrooms anymore as long as I remember to turn it on.

    Beat it to fit / Paint it to match

  2. Sasquatch | Sep 21, 2004 05:55am | #2

    Why not redirect into a vent in the soffitt?

    Les Barrett Quality Construction
    1. John_S | Sep 21, 2004 05:03pm | #3

      Although better than discharging directly into the attic, much of the fan's discharge would be drawn back into the attic space through the eaves ventilation.

      1. Sasquatch | Sep 21, 2004 05:13pm | #4

        The reason for my suggestion was kind of an idea I have been toying with for a couple of years.  I go up into the attic about once a year or another, most recently to install the wiring for a ceiling fan.

        My house has two bathrooms upstairs which discharge bathroom fan exhaust into the attic.  The bathroom exhaust which gets used more causes a slight discoloration on the underside of the roof sheathing where the duct is fastened.  I assume this is due to a regular, periodic release of moisture into the attic at this location.  The other bwthroom fan exhaust causes no noticeable effect on the sheathing.  I had toyed with the idea of running a duct to the soffitt and suggested it from that perspective.

        I still think that the amount of moisture leaving the bathroom would be significantly dispersed before possibly being reintroduced into the attic although what you said makes sense.Les Barrett Quality Construction

        1. Hubedube | Sep 21, 2004 05:18pm | #6

          Your final assumption is correct. This exhausted moist air will be well dispersed by the time it is thrown down and away from any re-entry opening. ....   Soffit vents do work.

          Been there, done that.

          1. Sasquatch | Sep 21, 2004 11:43pm | #7

            I'll do that the next time I go up there.  Each time it gets easier, since I am slowly building a catwalk with scraps each time I go up there.Les Barrett Quality Construction

      2. Hubedube | Sep 21, 2004 05:14pm | #5

        No it won't.

        The fan force will throw the discharged exhausted air "downward and away" from ever coming even close to coming back into the soffit. No problem.

  3. User avater
    rjw | Sep 22, 2004 01:27am | #8

    >> Also, this adds roof penetrations which increase the probability of roof leaks

    I walk 3 roofs a day, 5 days a week (less in winter) and probably see 15 -20 roof penetrations a day.

    It's pretty rare to see a leak at one of those penetrations, and when there is one, it's usuallly the result of shoddy workmanship

    So the increase in probability is pretty darn low, and you have a lot of control over it.



    Sojourners: Christians for Justice and Peace

  4. swan1 | Sep 23, 2004 07:28am | #9

     John good idea to get vents out of attic ! if baths are used alot with hot showers ,most definetly remove it .  My brother has a friend that has a major mold problem caused by vents in attic ! not allowed by Mass. code . Don't know where your from

    1. John_S | Sep 23, 2004 03:32pm | #10

      We're in rural New Brunswick.  Venting into the attic space is contrary to the National Building Code of Canada.

      However (as I learnt after purchasing the home), the NBC was only enforced by legislation within cities in NB.  So, for homes built in rural areas such as our own the NBC was just 'good practice'.  This changed a couple of years ago and now the NBC is mandated throughout the province.

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