Condensation on bathroom vents - related to exhaust fans?

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Old 07-10-20, 08:51 AM
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Condensation on bathroom vents - related to exhaust fans?

We're having condensation on the HVAC supply vents in our two upstairs bathrooms.

For context, it's been hot and very humid - highs in low 90s and dew points in the 70-75 range. The bathrooms are in what I believe is called a "shed" dormer, so there is very little attic space above them (presumably just a little more than the width of the rafter boards). This part of the roof also gets a lot of direct sunlight.

The vents are both a little under two feet from bathroom exhaust fans (see pictures - areas circled in red is where the condensation is forming). What I think is happening is warm air is infiltrating through the exhaust fans (either from the outside or the attic cavity), hitting the metal supply vents which are much colder than the room temperature when the system is running for long times and causing the condensation.

I have some concerns that that the fully recessed fan (the one behind the light fixture) may not not sealed properly to it's exhaust duct or the duct might have leaks into the attic. In that bathroom we get a smell (like hot attic) coming out of the fan fixture on hot sunny days. I have sealed all the gaps between the fan box and the drywall with foam and HVAC tape, but that didn't seem to help much. The fan box itself looks pretty well sealed, but there are seams along the corners. Unfortunately due to the design of the fan, I can't check the internal fan damper or the connection to the exhaust duct without cutting into the drywall. We do not notice a smell from the other fan. Both fan boxes are metal and pretty hot to the touch on hot sunny days.

I think both fans vent up through the roof since there are exhaust hoods visible on the roof in what looks to be the same area as the fans. I haven't been on the roof, but I'm pretty sure the fan without a smell has an external damper since I can hear it open and close at times. I don't know on the other one. We have not noticed other condensation other than on the vents or mold issues in the bathrooms.

I'm not sure if this is to be expected with this kind of weather and placement of the vents/fans. Would it be best to just replace the supply vents covers with plastic ones and run the fans periodically during hot days? Is there anything I can do to for the fan with the smell without destroying drywall? Is it something that likely needs immediate attention or it can wait (assuming we can tolerate the smell) until we're doing other work that might require drywall repair?

Thanks in advance for any advice!


 
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Old 07-10-20, 09:53 AM
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You would need to get in the attic and wrap the ductwork with pipe wrap and tape all the edges. Possibly make a box out of foam and tape the corners, and slip that over the top of the fixture... Sealing the edges with foam or tape to air seal it.

Also take the grilles off and use foil tape to seal the interior perimeters of the penetration.
 
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Old 07-10-20, 01:23 PM
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Thanks. Accessing the attic will require tearing down ceiling drywall since there isn't any real attic space at that point - just the width of the beams as it's a nearly flat roof. I've already sealed around the edges of the fixture under the trim.

My question was more about whether this could be causing a bigger problem other than increased heating/cooling costs and the supply grill rusting over time? If not, I will likely wait until we are ready to replace the fan or do other work in the bathroom before ripping up the ceiling. Otherwise, I might go ahead and look to just replace the fan now.

The non-recessed fan is more easily accessible,so I will see if I can pull that out and check what the ducting looks like and if it's possible to put insulation around the box. Since there is no smell, I am wondering if that could just be air coming in through a poorly sealing damper being heated up by the metal fan box.
 
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Old 07-10-20, 01:31 PM
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Its only a problem if it makes a puddle on the floor or if the drywall gets wet and starts to mold/mildew.

Its condensing due to warm air contacting a cold surface... not cold air contacting a warm surface. Seems like the same thing but its not.
 
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Old 07-10-20, 04:17 PM
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Thanks. The condensation is only on the vent covers, not the drywall. The "hot wood" smell isn't great, but it clears out quickly when you turn on the fan. I know if a fan isn't vented properly you can have the opposite problem in the winter (warm air from the bathroom getting into your attic), but we don't really ever shower in that bathroom. We'll look to address this when we're doing some other work in the bathroom then. Appreciate it!




 
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