How will bath exhaust fans or ERV ports affect steam showers? Will a steam-filled space leak air if the fan or port isn’t in use? The answers might surprise you. In this video, Green Builder Matt Hoots of Sawhorse, Inc. and Corbett Lunsford from Building Performance Workshop test what effects, if any, a bath fan or ERV port might have on steam shower operations. A manufacturer had advised the homeowner against installing a fan inside the shower. The building team was pretty confident that the fan would not have an impact on steam shower operations if and when the fan wasn’t running. And the fan would of course be helpful to dry out a very moist environment after the equipment was used.
Inside a steam shower, the goal of course is to generate steam and therefore pressure is created. So should a fan be outside? Actually. Matt and Chris explain that a fan outside the shower could actually cause pressurized steam to leave the shower as it draws air from this pressurized space, and that amount of steam might then overwhelm the fan.
Matt and Chris use an exhaust fan flow meter to show that air pushes up against the fan, but they use a more sensitive flow finder to show that it doesn’t leave. There is some air leakage, but it’s so infinitesimal that it’s not material - it literally shows INF on the screen. The fan’s backdraft damper is heavy enough that it doesn’t even allow 1 cubic foot per minute, or CFM, out.
There seems to be a debate even as to whether the fan could be useful when the steam shower is on, as 28 CFM is not very powerful, and won’t cause a draft. Chris points out that at this level a working fan plus a cracked door would allow air in, and would not cause air to rush out, and a fan inside could see steam from wandering out. A fan on the outside however might pull steam into a space that isn’t intended to be wet. Matt thinks an undersized fan used after the fact should work after using the steam shower and isn’t needed to be in use while the equipment is on. Matt does feel the fan should be turned on right after the steam shower turns off though - it’s key to dry out the space since no one wants mold and microbes. Both pros felt that 28 CFM was the right low flow fan choice for the task.
Thanks Matt and Chris for sharing this example and for testing everything out!
What are your exigencies with steam showers and exhaust fans and moisture? What are your questions or comments for Matt and Chris?
Corbett and Matt discuss ideal bath fan location for steam showers.
Testing air leakage in the steam shower at the bath fan opening
Corbett using the more sensitive air flow meter to test air leakage in the steam shower
Matt Hoots reminds us how important it is to reduce mildew and mold risk.
What is your experience with steam showers and bath fans/ERVs?
Date: February 2, 2026
Author: Rate It Green Team
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