Bathrooms need some way of exhausting air. Mostly it’s to keep the humidity down, but, depending on who lives in the house, the bathroom also can be a source of a lot of volatile organic compounds (VOCs). Not all homes have bathroom exhaust fans, but let’s focus here on those that do. Ideally, these fans connect to ducts that send the air all the way to the outdoors. Not to an attic. Not to the garage. Outdoors. Have you been in your attic to see if yours goes all the way out?
Snorkels in the attic
I’ve been in a whole lot of attics. I can tell you that a great many homes have bath fans that do NOT vent all the way to the outdoors. Let’s look at a few of the photos I’ve taken over the years.
The snorkel method just dumps bathroom exhaust into the attic. The lead photo shows one coming out of the fan and aiming toward the ridge. The one directly above looks like it could be the trunk of an underwater elephant.
The one just above used hardpipe, which is more expensive than flex. That must be why this installer used a piece that was just barely long enough to get above the level of the attic insulation (or at least where the insulation should have been).
Too close to the roof deck
When you snorkel that air into the attic, you’re putting a lot of moisture up there. This can be a significant problem in winter because the surfaces are cold. And as I’m sure you know, water vapor loves cold surfaces. And when you dump the moisture close to the roof deck, you’re more likely to have problems.
The snorkel above is close to the roof deck, but the part of the roof deck showing here doesn’t seem to have problems.
How about this one? The roof deck near that snorkel is looking kind of dark.Now, you can’t deny that the house in the photo above has a serious problem with moisture in the attic. Frost on roof decks in winter is pretty common in cold climates. One reason this happens is that humid air from the house leaks into the attic. But what really amplifies it in the photo above is that that house is in a really cold climate, northern Manitoba in Canada. Brrrrrr!
This problem isn’t directly due to a bath fan venting in the attic, although I believe that shiny duct in the bottom left is connected to a bathroom exhaust fan. That one rafter bay is worse than the others, though, most likely because of leakage from the bathroom near where the fan is. Maybe the duct on the bath fan has fallen off? I don’t recall the whole story here.
In any case, you don’t want moisture getting into your attic. Yes, attic ventilation can help, but that’s just treating a symptom.
How about near the soffit?
Here’s another idea some installers use. Put the end of the bath fan duct near the soffit. That way all the bathroom exhaust will go out the soffit vent, right?
Wrong! Attic ventilation works according to the principle of the stack effect. Air comes in through the soffit vents and exits through vents higher up on the roof, either ridge, gable, or roof deck vents. That air coming in at the soffit will just push the bathroom air back into the attic. No good.
Here’s an interesting one I saw in South Carolina. The installers didn’t know where they were going when they started with that duct below. When they hit an obstacle, they just stopped.
How much air do you think that bath fan moves?
Oops! No duct at all
And then there’s the case of the duct that wasn’t there. I’ve seen quite a few ductless bath fan in attics.
At least the one below has some (completely unnecessary) filtration through the fiberglass.
Ductless can work fine for heat pumps. Don’t try it with bath fans, though.
How bad could it be?
Moisture damage in an attic can be severe. Of course, mold needs water to grow, so you can create some serious indoor air quality problems. The moisture also can rot the framing and roof decking. I recently heard from someone who had to replace their whole roof prematurely because of this very problem. So, just a little missing ductwork on their fans resulted in a $16,000 repair and roof replacement.
Another problem sometimes occurs as well. Before we bought our house from my father-in-law, he had the attic encapsulated with spray foam insulation. Within a year or so, he had to get another company to come in and do some remediation because he had mold growing in the attic and on the hallway ceiling. Part of the problem was that the bath fans vented to the attic.
Where should duct go?
A duct on a bathroom exhaust fan needs to terminate so that the exhaust air goes all the way to the outdoors. That could be through a roof, a gable end, or a soffit. And yes, it can work to exhaust the air through a soffit, despite what I said above about air coming in at the soffits. The key is to make sure you have ducting and a termination that all the air to exit at a high enough velocity to escape any drafts that occur for attic ventilation. I wrote about that when I installed an exhaust fan in a bathroom remodel a while back.
So, have you checked out your bath fans yet?
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
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