Can someone tell me where the warm moist air generated by a shower goes once it is exhausted to the outdoors via a bathroom fan? How does temperature effect the exiting air? How about the velocity of the air? How quickly does the moisture dissipate?
I bring this up because there seem to be a lot of opinions out there regarding where bathroom exhaust should exit a structure but no supporting data.
There is a concern that warm moist air could be pulled back into the structure via the soffit intake of a ridge vent system.
What is unclear is whether or not such a risk is mitigated by moving the exhaust away from the soffit intake, or does the decrease in velocity and the thermodynamics of the moist air make it more likely to find its way into the ridge vent system?
Thanks for any help or thought on this… Figuring out where to put my bath exhaust has been driving me crazy!
Reduce the concentration of water vapor in the bathroom to improve visibility and reduce water exposure to the sheetrock walls.
If a fraction of that water vapor were sucked in to the soffit intake, what does it matter? It takes a lot more than a little extra humidity to cause wood to rot.
I’ve seen a bathroom fan mounting that exhausted directly into the attic. Never caused any problems as far as I know.
As for the thermodynamics : sheesh. That’s going to require extensive computer modeling or empirical testing to actually answer your question as posed. There’s no simple rule of thumb for something like that.
The concern is not rot, but rather mold growth that would then find its way into the living environment.
Venting directly into the attic can be a big problem, especially in cold climates where condensation is more likely to form. I understand in places like AZ it doesn’t pose an issue.
I guess my question boils down to: is it better to blast the moist air out near the soffit or some distance below the soffit?
To really study this empirically, you need a smoke source and to actually visualize the flows directly.
What probably happens in the case of a house roof, the air probably flows over the top in a series of constrained streamlines, like so:
Air from the bottom-most streamlines is what goes into the vents.
This means that to avoid any exhaust air getting sucked back in, you merely need a tall enough vent that the air exhausts into the streamlines that will not enter the vent. Like thisone.
But hey, while you’re trying to design a proper solution, how’s this for an idea. Install an intake vent down at the same level of the toilet seat either in the wall to the side of it or behind it. Use an inline blower so that the actual motor and the wiring is above the level of the toilet (so there isn’t a risk of water splashing in). Build it right, and it’ll suck all the farts out without even the person on the throne having to smell them.
Most residential construction isn’t really designed for optimum utility. Many of the ideas in use are only used because they are popular or cheap.
Oh, and I seem to have answered your question about distance from the soffit. Looks like it doesn’t matter whether it’s far or near since the air travels in those constrained streamlines. It’s the height of the vent above the roof surface that will affect where the air ends up.
Great for extracting malodorous air near the toilet, bad for extracting warm/moist air coming from the shower stall. The latter hugs the ceiling, so if your intake is just three feet off of the floor, all that high-humidity air will linger in the upper part of the room. Expect foggy conditions during every shower, and long-term drywall damage.
Well, if you’re in a cold enough climate, you can go outside on a winter’s day and watch the vapor dissipate. I’ve seen it come out of my dryer vent and it doesn’t appear to do anything too interesting – just sort of billows.
I suppose you could always block the soffit vents in immediate proximity to the exhaust if you’re truly worried about mold formation.
