Bathroom vent fan exhaust to attic?

Factual Questions
1

I have a small, centrally located bathroom on the top floor of my house. It has a skylight “stack” that I would like to install an exhaust fan in. The attic space above is uninsulated, and ~2’ from the ceiling plaster to the flat roof sheathing, so it is not really feasible to vent a fan with a horizontal run to the exterior. The attic has adequate venting.

  1. Would I be insane to route the exhaust from a small “fart fan” in this bathroom to the attic crawlspace? The fan will be switched independently of the light and used infrequently, only as necessary to dispel odors or vent the bathroom of water vapor from showers.

  2. Moisture in the attic is my number one concern, but I’m also worried about heat losses. Should I install a “baffle box” to minimize heat losses thru the fan while it’s not in operation?

  3. What am I not thinking of? What would you, as a general contractor or frequent viewer of TV remodeling shows do differently? Feedback appreciated.

Thanks.

2

[This guy](http://www.askthebuilder.com printer_546_Bathroom_Fan_Ventilation.shtml) says not to do it. The phrase “horrible attic mold and wood rot in the roof framing and roof sheathing” seems pretty self-explanatory.

3

You seem to understand that moisture in the attic is a bad thing. I wouldn’t worry too much about the heat loss, there is usually a small barometic damper on the fan exhaust that would keep air from leaving when the fan isn’t on.

I have a very similar situation. Ok, exactly the same. What I am considering doing is tearing out one side of the skylight box (it’s just framed in and covered with drywall) in order to gain access to the attic area and do it the right way (i.e., exhaust duct out of the building). I’m in no hurry, so don’t expect me to report back anytime soon. Might do it next year.

Have you thought about how you are going to get power to the fan? Seem like the inaccessible attic might be a problem there also.

4

Don’t do it. You would be needlessly introducing moisture into your attic space. IMHO, this is a really bad idea. Your attic space is constructed of untreated wood. You don’t want to be encouraging wood rot or mold.

(Also, I’m having a new house built, and have been reading a lot of house construction books lately. I’m pretty sure that I’ve read that you really don’t want to do this.)

I wouldn’t worry about the heat loss issue. Any baffles you install are just going to cut down on your airflow and reduce the efficiency of your fan.

I would try to figure out a way to vent your bathroom to the outside.

5

Finagle’s link seems to cover the issue pretty thoroughly.

Fixed link:
http://www.askthebuilder.com/546_Bathroom_Fan_Ventilation.shtml

6

Thanks for the link Finagle (and thanks for fixing it Robby). Although it sounds like the info pertains to a hardwired exhaust fan used regularly as opposed to an independently wired fan used only occasionally, you’ve confirmed my suspicion that this is a bad idea in general. Should the upstairs bathroom eventually get used very frequently (e.g., teenage daughters living upstairs), moisture could become a significant problem. I think I am going to look into venting a fan to the existing skylight and better insulating/sealing the inside skylight window.

Dag Otto, we can easily get power from a light fixture we are relocating. Good luck with your project, though. Hope it is easier than mine.

7

Perhaps you could run a duct thru the attic to the outside?

8

I should check the links first shouldn’t I. :smack:

9

Not to hijack, but I’ve been wondering lately: If the attic is unconditioned space, what keeps it dry from moisture? Is it simply the “breathability” of the attic space that prevents moisture from collecting? And, I WAG, there’s no major surface on which mositure could collect? Is this all correct? Or, is it yet another worry of being a homeowner? - Jinx

10

Moisture comes from 2 sources: (1) Rain and other weather outdoors (2) human beings (breath, etc.) and water fixtures (showers, etc.) indoors.

For (1), the outside sources, you have roofing and siding, and roofing felt and house wrap to serve as barriers.

For (2), the inside sources, you generally have a vapor barrier at the edge of all conditioned spaces (i.e., between your living space and the insulation). In some cases, this is a plastic membrane; in some places, this is just the kraft paper wrapper on the fiberglass insulation batts.

Either way, even though the attic is not insulated/heated/cooled to isolate it from outdoor heat and cold extremes, it’s moisture-proofed.

11

My attic is thoroughly vented to outside air.

12

The attic isn’t protected from atmospheric moisture as much as it is protected from additional moisture (rain) by the roof. Attics are normally vented to the outside, so the air (and just about everything else in the attic) should be in equilibrium with the outside air. With this situation, you are not going to see water condensing on truss members, etc. (there is an exception - surfaces colder than the air, such as exposed water pipes and HVAC ducts, may cause condensation, which is why cold ducts and pipes should be insulated AND protected with a vapor barrier).

Now take the situation where you have a cold attic and you are exhausting warm, humid air into the attic. As the warm air cools, if the dry bulb temperature reaches the saturation temperature, the water may condense. It might not always condense because it is mixing with the air in the attic which could be anywhere from bone dry to almost saturated, but there are times when it will.