Wherein I Seek Your Advice (on venting a bathroom)

December 19th, 2013 at 1:02 pm

Hopefully I’ve shared some useful thoughts and info with you all up here…and for a fair price.  Well, it’s give back time, at least for any of you who have insights into home repair.  Need I tell you, that ain’t moi.

We’re renovating a bathroom and the contractor (hey, I’m a job creator!) tells us the easiest way to add a vent would be to have it vent the moisture into the attic.  Sounds fine to me, and easy.  But the internet, at a rate of something like 10 to 1 says absolutely not.  You must vent your bathroom to the outside, otherwise you’ll eventually be stuck with all kinds of horrible mold problems.  And I must say: I found these arguments convincing.

The placement of our bathroom will make outside venting a lot harder and the contractor doesn’t seem to think it’s worth it.  What do you think?

UPDATE: The verdict is in.  With very few exceptions, venting to the outside is the way to go.  Thanks to all who responded.  Truly helpful information!

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29 comments in reply to "Wherein I Seek Your Advice (on venting a bathroom)"

  1. Everett Volk says:

    I assume you mean ventilation fan, not plumbing vents. However, if you mean plumbing vents have you contractor consult a plumber re Studor vents. If you mean ventilation fan, ask an HVAC guy for a consult. If you can vent into the attic, it seems you should be able to subsequently a) vent through the roof (worst case), b) vent through a soffit (possible not allowed), or c) tie into an existing vent. Good luck!


  2. Charlie Paulsen says:

    We used to have a house with a readily accessible attic. The baths vented though flex tubing in the attic to vents though the roof and hence the outside air.

    I unintentionally dislodged one of the tubes, and we had mold problems ever after, even though the tubing was screwy for only a month or so. Possibly, if your attic is extremely well ventilated that would not be a problem, but if I were in your shoes I’d vent it to the outside, even if it’s hard (i.e., expensive).

    Keep up the good work.


  3. Smith says:

    Take it outside. If it’s so easy to reach the attic, then go outside through the attic. You may already know most local building codes insist bathrooms have ventilation either through the presences of a window or a vent (usually accompanied by a fan). The trouble (for you and perhaps especially your contractor) is it’s a somewhat different skill set and an order of magnitude difference in work to break through an outside side wall vs. running a vent to the attic. You need to make sure the opening doesn’t cause leaks in your walls, leading to serious and expensive damage. But it’s not very different from vents for a clothes dryer, or kitchen stove fan (though they sometimes just filter instead of venting outside). Get a second opinion (cost estimate), find someone with experience doing exactly that, it’s going to be cheaper with someone who knows what they’re doing. You might also consider making your dwelling more energy efficient by adding an attic fan at the same time, but keep that separate. I’ve done house framing, but never this exact work, so I can’t give you a dollar figure.


    • Smith says:

      The simplest cheapest way if you already have a window is to rig something up with a small self contained fan and damper flap in the window itself, helps especially on hot humid days. I haven’t find one ready made, in home depot or by googling, but should be a multi-million dollar product. Anyone with info on that, please post a link.


  4. Fred Brack says:

    “I have always depended on the kindness of strangers.”


  5. Perplexed says:

    Fortunately (or maybe I should say unfortunately), home repair and economics are closely related fields; the answer is: it depends on lots of other variables. i.e. is this a “full bath” with shower, tub, hot tub, sauna & steam room, or is it “powder room” which won’t generate any near as much humidity? It also depends on how well your attic is ventilated (area of vents relative to the size of the attic, vent fan (& temperature & humidity controls, etc.).

    Moisture in the attic is a concern. “Dry rot” is an oxymoron, decay fungi in wood cannot grow without moisture, specifically, if the moisture content of the wood exceeds 19%, it will allow the fungi to grow (temperature is also a factor, decay fungi pretty much like the same temperatures we do, maybe just a little cooler), that’s why the industry standard for “dry” or “kiln dried” wood is set at 19% moisture content. The other temperature concern is that if your attic is well insulated (as I’m sure the attic of an energy conscious economist would be) the attic can get quite cold and the moisture can freeze an accumulate (and therefore not ventilate); when it thaws, the moisture content levels can increase unless you have adequate, humidity controlled ventilation.

    So it depends on “which model” you use! Unless the sequester has seriously damaged their capabilities, the U.S.D.A. Forest Products Laboratory in Madison, WI (you know, those big bad government types that can’t do anything useful) has loads of research, publications, and people that can help you drill down if you choose to pursue it further. Venting it directly outside is almost always safest (although you can do that by going through the attic and putting in a roof vent too.)

    Really, a bathroom remodeling so close to the holidays? Some real risk-seeking behavior going on there! Good luck with it..


  6. Richard Careaga says:

    Check with the building inspector about (1) whether the Uniform Building
    Code requires exterior ventilation in new construction, (2) what
    exceptions apply and (3) whether a permit is required for remodeling of
    an existing bath. Downside of asking is that you might not like the
    answer; upside is that if you don’t like the answer, you’ll know that
    future buyers/lenders might call you out on a non-standard vent.

    From a practical perspective, the problem with in-attic ventilation is
    getting the moist air mixed into as large a volume of air as soon as
    possible so that you avoid condensation. The reliable ways of doing this
    are, in my guess, likely to be more expensive that penetrating the roof
    for a vent stack.

    Make sure you know whether you contractor is a licensed general
    contractor with new residential and residential remodeling experience
    and not just a handyman.

    My deepest sympathies, having lived through a bathroom remodel.

    Good luck


  7. Mark S. says:

    Tom from ‘This Old House’ says never vent into an attic:

    http://www.thisoldhouse.com/toh/how-to/intro/0,,689843,00.html

    I’d listen to Tom.


  8. Dan Tyler says:

    Definitely run the vent through the attic to where it can vent to the outside. Even if your attic is well ventilated, dumping a bunch of warm, moist air into the attic is not going to be good for the framing and sheating. Moist bathroom air is likely to hit the dew point when it touches the underside of your roof sheathing on cold days and nights. Water condensing and/or freezing on untreated wood is an invitation to problems.


  9. Bob Moreo says:

    Residential codes require it to vent out. If he can get it to the attic why can’t he keep going to the roof?


  10. Doug says:

    Your contractor is looking for an easy way out. He/she will be long gone (with your cash) when mold builds in your attic. You need to vent the fan outside. Based on your contractor’s response, I wouldn’t trust him/her to do a proper job with a new roof vent.


    • Perplexed says:

      Can we at least give him the benefit of the doubt? It is possible that a smart economists like Jared has hired a contractor who has come highly recommended, assessed the entire situation and found that 1. the house is adequately ventilated, 2. the moisture contribution would be minimal (e.g. powder room; bathroom with tub only and no shower that will likely not be used a great a deal) and is trying to save Jared some of those hard earned dollars by not spending money where the conditions don’t warrant it. The possibility does exist that he could just be a very knowledgeable professional trying to act in the best interest of client and giving him advice on the options. We can’t really assume they’re bad guys without knowing the specifics. Just saying.


  11. mwbugg says:

    I bought a house in St. Louis where the previous owners had installed an unvented bathroom in the basement. We had to put in a vent to get up to code when we sold. This one didn’t have a tub or shower and was rarely used so it didn’t generate much moisture. If you take showers in yours, definitely vent to the outside.


  12. john stephanus says:

    You most definitely do not want to simply vent it into your attic.

    The mold issues are real and material.


  13. george kaplan says:

    Vent to outside. Make sure birds, squirrels etc cannot enter.


  14. Jill SH says:

    I live in an old house (1820) in NH, with an un-insulated attic. (Insulation in attic floor keeps house warm.) Vent with fan from bathroom goes above ceiling/across attic floor and outlets through soffit. Never any problems.
    As a state rep, definitely recommend you check building codes.


  15. Erik says:

    Check the code first. In this case, don’t trust the contractor – get a second opinion. And if it’s not up to local code, find a different contractor to do the work. If this one’s suggesting cutting corners in this case, he probably cuts corners elsewhere.

    If it’s actually permissible by code, I’d still say: if it’s a full bath, vent to the outside, no question, full stop. If it’s a half-bath, you may be able to get away with venting to the attic… but make sure the attic is well vented in turn. And you really want to vent to the outside if practical in any case.


  16. Mike Claxton says:

    OMG, DON’T VENT IN THE ATTIC! I spent 5 years dealing w/mold before a remodel and the contractor immediately said, vent to the outside; the mold was from venting on the inside.


  17. Kevin Rica says:

    How would I know? I’m a macro/energy economist. However, since I’m doing at least one bathroom this spring, this is a very useful topic.


  18. Code Compliant Guy says:

    Vent the bathroom in the shortest most direct route to the outside. Never vent in the attic.
    Make sure the “vent cap” i.e. the termination sheet metal on the outside of the house works
    well and is set for easy maintenance if necessary.

    Make sure your venting unit has enough horsepower to push to moisture laden air out the vent and
    through the termination cap efficiently. You can get combination Light Fixture/Vents that are low noise and energy efficient that will do the job. Mine makes a pleasant low hum – no racket. No condensation dripping on
    the intake grill. I would ask around to see if there is a new product that uses LED lights with an
    energy efficient vent motor.

    If you have right angles in your vent pipe, you need more power to push the moisture out.
    The longer the run the more problems you may have.

    You want to seal the vent pipe joints by wrapping them with appropriate tape – good idea, solder the joints
    if you want a Cadillac job.

    There are local/national codes on all the above points.

    You can have it all, moderate cost, energy efficient, pleasant lighting, and health living.

    Code Compliant Guy


    • Jared Bernstein says:

      Excellent advice, code guy! Any thoughts on monetary policy??


      • Code Compliant Guy says:

        Yes, I do have a lot of thoughts about monetary policy – particularly “targeted stimulus”.

        I have been taking a 5 year course on Macro/Micro from my favorite: Econblogs. Paul Krugman, Barry Ritholtz, Calculated Risk, Yves Smith, and a guy named Jared Bernstein who
        is good across the board on monetary policy, fiscal policy, politics & music.

        Here in the Bay Area, we have several successful “targeted stimulus” projects. We just opened the Fourth Bore of the Caldecott Tunnel – Highway 24 between Alameda County and Contra Costa County. Just Google “Fourth Bore of the Caldecott Tunnel” to check it out.

        Local taxpayers paid $120 Million of the cost through “Measure J”. We got another
        $194 Million from ARRA. The Tunnel cuts down travel time, saves on energy, and less pollution from the years of stop-and-go traffic. The project provided good construction jobs and opened a major land development just on the other side of the 4th Tunnel on the Contra Costa side with energy efficient homes and sports facilities donated to the Local Government. It was a win-win-win-win strategy.

        Oh by the way, the other three Caldecott Tunnels? They were WPA projects that provided
        jobs after the Great Depression. Those tunnels are still working just fine.


  19. Tom in MN says:

    Outside, that is for sure. Around here if you don’t have the ceilings, etc, completely sealed with a vapor barrier, on the first day after a cold (0 or less) spell, you get a rain storm in the attic from all the frost on the underside of the roof melting. We thought we had ice dam problems, turned out to be open framing letting warm hot air up from below.

    Same guys that added our vapor barriers told me Panasonic makes nice ceiling fans, which I’ve been happy with. You size the fan to change the bathroom air 8 times an hour, see http://www.efi.org/factoids/bathroom_sizing.html and make sure it’s sealed up to not leak at all into the attic.


    • Perplexed says:

      -“Around here if you don’t have the ceilings, etc, completely sealed with a vapor barrier, on the first day after a cold (0 or less) spell, you get a rain storm in the attic from all the frost on the underside of the roof melting. We thought we had ice dam problems, turned out to be open framing letting warm hot air up from below.”

      You may want to check with the Forest Products Lab. in Madison Tom. They’ve done a number of studies on the effectiveness of vapor barriers, (many in Minnesota I believe). The conditions you describe above are often indicative of a lack of adequate attic & soffit ventilation which can result in high moisture content levels in the framing members or trusses in the attic, even without the “rain storm” in the attic you describe. There does not need to visible water or condensation for elevated moisture content levels to support decay fungi. Your local code agency can tell you what the roof and soffit vent requirements are for your area. Unless you have an indoor pool or hot tub and are generating a lot of “above normal” moisture, the vent requirements should result in the attic air having a humidity level not tremendously higher than the outside air. The fact that when the temperature dropped below 0 and lots of moisture condensed and “rained” down in your attic suggests that the warm moist air had no way to get out, even with a big difference in temperature. If you have adequate roof top or ridge vents, your soffit vents may either not be there or be blocked (this happens often with blown-insulation if the contractor doesn’t take proper care to keep the soffit vents from getting blocked,


  20. D. C. Sessions says:

    OK, first off: I’m in the desert Southwest, where attic moisture buildup isn’t a problem. Usually. In fact, we cool the house for about four months of the year with evaporative cooling, and the indoor air (including from the bathrooms) gets vented … through the attic. Which, after all, cools it as well and reduces the heat transfer into the house.

    But we do that when the dew point is below 45F and the outdoor temperature is above 85F, and we do that with an airflow of 6000 cfm, something like 50 times the rate of a bathroom fan.. You’re looking at slowly venting the saturated wet air from your bath to the attic when it’s freezing outside (one of us lived in your area for several years) and that means condensation. Which will, mold aside, degrade your insulation and quite possibly get enough moisture to accumulate on top of your ceiling to either dissolve the gypsum or loosen the plaster.

    Either of which will cost you vastly more than running a low-volume vent to the outside.


  21. JFC says:

    Guess I’m a little late with this but….
    Vent to your attic near an existing attic ventilator fan if you have one. I did it in my house several years ago and it works fine. If you don’t have one, vent to the outside.


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