Venting bath fans through soffit – never say never?
I am installing three bath fans in a major remodel. All three are in close proximity, and two of them are 6″ vents. The conventional advice against venting through the soffit seems based on (a) soffits used to vent the roof/attic, and (b) condensation issues. In my case, the soffits are not venting the roof or attic — I will have 18″ soffits that are not venting the roof at all. The roof itslef is an unvented insulated panel build-up (3.5″ of foam sandwiched between existing and new deck), with 1″ of spray foam on the interior. There is no attic, except for just a small portion above the bathrooms, where the fans are located. So, my thought is that I can use the 2×6 cavities between the spray foam and drywall as chases to vent down (3/12 slope) and out through the soffits, as these will not be competing with roof ventilation and will enjoy enough insulation above them to avoid condensation problems. What am I missing? It will look much cleaner than having all those ven caps in on top of the new roof.
Replies
Nothing
Seems like you have thought it through and examined the options.
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So, you think the exhaust will travel down and out the vented soffit without piping?
And if it were to do that, what's to keep the air outside from coming back up and to the fan?
Certainly not that flimsy flapper on most bath fans.
If you want to do this, why not run insulated flex from fan to your chase, then down to a real soffit vent cap-if it doesn't have a decent backflow preventer-insert one in the line b/4 it exits.
What type of fans are these? Each with their own motor or a remote blower?
I don't think there's any real problem with venting through the soffit, but you do want to use "real" ductwork, even if you have to have some transitions fabricated.
I think running the bath fan vent tube down to the soffit is the best possible solution in a cold climate as long as you use a proper made-for-the-purpose soffit vent cover instead of just dead-ending the tubing in the soffit space. I try to install fans like this whenever possible.
One major advantage of this kind of installation over running the vents upward is that there will be no gravity flow of warm air when the fan isn't running. When the tube is run upward, there is an inescapable tendency for warm air to flow up and out. Those little flapper things in the fans really are nearly useless at preventing this. When the tube is run down to its exit - no chimney effect.
Make sure the joints in the tube are installed to encourage any condensation to flow out. Keep the seams in the tube on top. Tape the joints with proper metal tape or sheathing tape.
Ron
The channel be at outside temperature.
Not a good idea as the channel will be at outside temperature and the moisture from the bathroom will condense on all surfaces when they are below dew point, which will be most of the year, this will lead to mold and wood rot.
A better idea is to use a flexible plastic pipe purpose made for bathroom fans.
Perry
Flexible INSULATED with a good spring loaded gasketed damper (not the cheap flapper in the fan box.