My bathroom originally vented into the attic instead of outside. I didn't have too much steam with this. A 3 inch vent was put on and exhausted out through the soffit. It wasn't effective. A new fan was installed and a 4 inch vent connects to the 3 inch vent. The hose also goes over one of the rafters instead of straight to the soffit. I still get steam even if the door is open a bit. A tissue does stick to the fan and I can feel airflow outside. Is the problem the 4 inch connecting to a smaller tube or that it is going over a rafter and not straight? Or both? Could there be another reason why the steam isn't venting? I don't want to drill another hole in the soffit of I don't have to.

  • Reducing tube size(4in to 3in) is usually not a good idea, increasing is(3in to 4in). Does vent go to outside of soffit or just ends on top?
    – crip659
    Sep 25, 2021 at 22:41

2 Answers 2


Venting to the soffit is not a problem, but you should use the appropriate vent terminal. If you have aluminum or vinyl vented soffit, then I suppose some steam could get up into the attic, but it's probably not a problem.

The issue here is the CFM (cubic feet per minute) of air your system is able to move from your bathroom to outside. Several issues of concern will determine the efficacy of your system to achieve your goal: to reduce the amount of steam condensing on your bathroom surfaces.

First, a 3 inch vent is really not sufficient, and the longer the run, the less sufficient it becomes.

Secondly, if you're using uninsulated duct, the chances are the steam is not even making it outdoors. It's most likely completely condensed inside the cold walls of the duct and running back down into your fan.

Lastly, you need to consider the size of your bathroom and choose the appropriate CFM for your situation. I recommend a fan of at least 70 CFM. The more CFM, the faster steam is going to be removed from your bathroom.

The bottom line is you're always going to see some steam. It's a matter of getting it out quick enough to avoid condensation on your bathroom surfaces, which can lead to water damage and mold. Of course, using the correct wallboard materials and paints goes a long way to mitigate this as well.

I always recommend putting your bathroom exhaust fan on a countdown timer and setting it for 30 minutes when leaving the bathroom after your shower/bath. This assures that the fan will continue venting the residual steam when you're done, while not wasting power by leaving your fan on all day.

Also, leaving your bathroom door open allows more fresh air to come in, helping your exhaust fan push more air out.

So to recap:

  • get a high CFM fan
  • use 4 inch insulated flexible duct
  • keep the vent run as short as possible
  • use a countdown timer to continue venting in your absence

If the vent tubing that goes from the fan's discharge to the soffit does not run level and has a dip in it, the dip can produce a partial trap in that vent pipe and over time fill with water from the water ladened steam that flows to the outside when the fan is running. I had this problem in my home and in the winter the hose would be blocked by the water collecting in the dip in the tubing. One more thing, some of the bathroom fans are only rated at 50-60cfm which is OK for a tiny bathroom or powder room with very little exhaust tubing added. When you add a long piece of exhaust tubing/pipe it will decrease the fan's discharge air volume and make the fan useless.

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