Please help us avoid mold in the attic.

May 14, 2008 6:57 AM

We just bought a house, and the bathroom fan currently vents into the attic. I know this isn't good, and I have done a bunch of googling, but I still have questions. Like: who should do the work -- a roofer, a handyman, someone else? Is it better to vent through the roof or through the wall, as our handyman suggests? Any other things I should know?
posted by trillian to home & garden (10 comments total)

I don't know the particulars, but my husband, who is handy with tools, but not a handyman, roofer or other such professional, took care of venting our bathroom fan himself without major incident.
posted by SuperSquirrel at 7:06 AM on May 14

I would rather vent through the wall, because that's the shortest route, so less chance of moisture leaking through joints in the ductwork or whatever.

A handyman would be your best bet for this.
posted by stupidsexyFlanders at 7:09 AM on May 14

Personally, I'd worry any time I cut a hole in my roof. Perhaps I'm over-reacting, but I'd call the roofer.
posted by Echidna882003 at 7:16 AM on May 14

Firstly, it isn't necessarily as bad as you think. It really depends on (a) how much attic space there is, (b) how well ventilated your attic is, and (c) how much water vapour your bathroom produces. A small amount of water vapour being drawn into a large, well-ventilated attic space isn't likely to be a problem. It might be worth just checking with a moisture meter before you make any changes to the current system. The fan in my house vents into the roof space (there really was no other option) and we've had no problem with moisture there - the climate here is pretty damp anyway.

The two most common ways to vent a bathroom are either through an exterior wall, or via a fan in the ceiling (above the shower if you have one) and then downward through the eaves of the roof. It's really a matter of whichever best suits your house, or whatever the handyman prefers to do.
posted by le morte de bea arthur at 7:19 AM on May 14

You can vent through the wall or the roof. Here is what both options look like.

I like venting through a wall because you are less likely to have problems with leaks. (When roof-vented fans leak, they leak either from the outlet if it isn't properly installed or from the flashing around the roof vent. Plus, it's just one more thing to worry about if you ever re-roof.)

You can vent directly out of the wall pretty easily. But this may not be as easy for you since you already have a vent installed into your attic. It might be simpler to continue the vent to your soffit. Or up through the roof.

This is a job for a handyman who would be able to a) purchase the appropriate vent materials like a roof cap/more duct/proper insulation tape, b) cut the hole using either a reciprocating saw or a drill with a circular saw bit, c) attach the duct to the fan unit and thread the duct through the hole in the roof, d) install the roof vent to the duct with proper flashing around the vent, e) caulk everything securely and test for leaks.

The less bends you have in the duct, the better because moist air from the bathroom will hit that bend in the duct (which is normally colder than the air itself), will condense there and drip back down into your bathroom. You can insulate the heck out of the duct, which should help. But I don't like to bend the duct unless it is RIGHT at the point where the duct meets the fan.

You never, ever, ever want something venting into your attic. Ever. I don't care how well-ventilated it is. That is inviting tons of trouble and is against building codes for a reason.

If it helps, I knew nothing about ducts or fans or installing them five years ago. Then we had to install two of them. So we asked a lot of questions of friends who are contractors, read lots of stuff in how to books and online, and very carefully did it. Twice. Contractor friends inspected the work when we were done and pronounced it successful. On a scale of 1 to 5 in DIY difficulty, I would put this at a 3 if you don't have to climb onto a steep roof in order to accomplish it.
posted by jeanmari at 7:59 AM on May 14

Agree with jeanmari- you're asking for trouble just leaving it.

I would make sure the vent line is as short and as vertical as possible, and choose whichever method accomplishes that. You don't want condensation building up in the vent line and corroding it or leaking back into your bathroom fan.

It isn't that hard to properly vent through the roof, but it is a skill that many people doing it for a living sadly lack. There is an art to visualizing where the flashing belongs and where to seal things up.
posted by gjc at 9:19 AM on May 14

jeanmarl - building codes vary by state (says my dad the architect).

Our fan vents into the attic, as do all the others in our townhouse subdivision. The duct ends directly (1-2") below the ridge vent. As far as I know it's been like that since 1987, and even with our muggy, soggy east coast summers there's no problem yet.

I'm very, very handy (I do all our repairs, built a wall, and install electric wiring and fixtures) but I wouldn't touch the roof simply because of insurance and liability issues. If the roofers screw it up and it leaks, I can get them to come back and fix it for free. If it's an insurance problem, I can sic my insurance company on them. If I did it myself... well, tough cookies.
posted by GardenGal at 7:14 PM on May 14

The previous poster was quite correct that depending on the application, a direct to attic vent isn't necessarily a bad thing. Attic size and air-flow are important here.

I don't know where you live.

If the bathroom fan is on a humidistat or thermostat and is designed to eradicate all waste air from the home, not just moist air from the bathroom, and will kick on on its own sometimes without you using the switch---then it needs to be vented outside.

If you can glue PVC together and operate a saw, you can do it. Minimize turns. More than 1 turn is too much. (So is a vertical climb, it's nonsensical to assume that the fan can push moist dense air up 15 feet of 3 inch pvc). From the fan, come up, 90 degree elbow to the gable, and louver. Done and done.

If the fan only operates to eliminate bathroom odor and ventilate immediately post-shower moisture from the air, then I wouldn't worry about it. Really, I wouldn't worry about it. Especially if you have any gable venting what so ever.

If your house is newer than about 10 or 15, then the bathroom fan is probably one of your few envelope perforations and is designed to ventilate the house. If it's older, chances are that it is only intended to remove bathroom humidity.
posted by TomMelee at 7:37 PM on May 14

An architect is neither a building contractor or a home inspector, sorry. Regardless of how well your attic is vented, venting moist air into an attic space is not a good idea. Any experienced home inspector can tell you many horror stories about this practice and I've interviewed dozens.

When warm moist air vents into a well-ventilated cool attic space** the water vapor condenses back to liquid. This settles on cooler framing surfaces or insulation. Over time, this can lead to wood rot. Same thing happens if you vent to the outside but don't insulate your duct. Water condenses in the duct. That's physics. If you don't believe me, ask the folks over at Fine Homebuilding. Venting to the exterior is important.

**Because in cooler weather, your attic space should be cool if it is well-ventilated. This indicates that the warmer living spaces below are properly insulated and not leaking into the attic space. If a duct terminates below a ridge vent in the interior of the attic, in cooler weather, any warm moist exhaust from the bathroom will condense before it exits the ridge vent and fall back into your attic. Or under your shingles.
posted by jeanmari at 8:02 AM on May 15

Thanks for all the answers! If any more are forthcoming, here is some more info: house was built in the '40s; climate is hot/humid in the summer and very cold in the winter; the attic has a louvre vent (?) on either side and a roof vent that is currently blocked by insulation. The bathroom was remodeled very recently, before we bought the house.
posted by trillian at 10:39 AM on May 16