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Unread 01-06-2021, 08:59 AM   #1
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keeping water vapor out of walls

A thread in the main forum got me thinking. I did not want to derail the OP's question so I asked here.

I have 4 shower heads in one of my showers. Each one is 2gpm. The entire bathroom becomes a steam room when I am done. I have a 210CFM fan as well as an 80cfm fan. I leave on the 80 almost all day and the 210 runs for 2 hours after a shower. The duct runs are both 8in diameter and 6in diameter. The run length is about 5 feet without any sharp bends and insulated. I get concerned about the vapor going into and through the drywall, but what can you do?? The bathroom ceiling (including shower) is not tiled or waterproofed. Originally I was tempted to redgard the entire bathroom and paint it. I never did. Are there any best practices for this?
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Unread 01-06-2021, 09:19 AM   #2
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Hi Mike,

What size pipe feeds your shower setup? You won't get anymore water than that provides no matter the number of shower heads.
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Unread 01-06-2021, 09:26 AM   #3
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Sounds to me like with the fans evacuating the humid air you’re probably fine, but if you wanted to do a test, get a hygrometer and check with humidity level immediately after showering, and then again at 30 minute intervals. That would give you a sense of what you’re dealing with and how much moisture is actually in the air. Ambient humidity can vary a lot, depending on the construction of the house, where you live, and what the season is. (Our house ranges from around 30-40% in the winter, to as much 60-70% in the summer. It’s an old house though, so I would imagine that newer/tighter construction would reduce this variability.)
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Unread 01-06-2021, 09:43 AM   #4
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good idea.

I have multiple sensors in the house to measure humidity. Upstairs and downstairs are both within a percentage point. I keep it at mid 30's year round with dehumidification, humidification, and A/C. The insulation is 2in of spray foam and blown in bibs inside a 2x6in nominal cavity.

I do not have a sensor in the bathroom though, but just outside of it.

I used to have a 90cfm fan as the main one and ditched it a year ago for the 210cfm. I did not notice THAT much difference though in reducing steam. This shower does not use a shower door though. It is constructed so the water sprays away from the entrance. I have a feeling the fan, being on the ceiling just outside the shower, has a hard time capturing steam as it leaves the shower entrance.

As to the plumbing pipe, I have 3/4in CPVC feeding the bathroom to a thermostatic valve and then a diverter. The valves are rated at 17gpm. My tankless heater can do 8 gpm with a 50 degree delta. My ground water is always 50. I have my own well and keep water pressure between 60 and 80psi.

I do get fairly good flow rates as long as nobody else is using water.
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Unread 01-06-2021, 10:23 AM   #5
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Mike, the "steam" you're seeing in the bathroom is not the moisture vapor that is going to penetrate your drywall into the cooler spaces toward the exterior of your structure. The vapor that will penetrate those surfaces you cannot see at all.

A couple heavy coats of good latex paint on the inside surfaces of your drywall ceiling is usually a sufficient vapor retarder in that application. Removing that visible moisture quickly helps a good deal, too, of course, lowering both the temperature and humidity that are causing the vapor drive in the room.

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