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About five years ago I had one of our bathrooms re-painted. It turned out that the painter didn't use mildew resistant paint, and after a few years the paint on the ceiling started cracking. I had an electrician in who told me that this paint had likely dried out, not become wet, due to the ceiling fan running for too long and sucking moisture out of the air. Whether this is true, I'm not sure.

So I re-finished the ceiling, but a couple years later we now have a bit of cracking on the wall, above the shower head. To me this looks like moisture, not it being dried out.

What I'm curious to know is - how long should we leaving the bathroom fan running for to avoid cracking of the room's paint? Or are we likely to see cracking on our bathroom paint regardless?

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  • 17
    I think your electrician should stick to electrical work. Or at least talk about paint cracking due to the fan running in the direction of a garden that needs a load of organic fertilizer spread over it... ;^)
    – Ecnerwal
    Aug 3, 2023 at 17:52
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    Fans move air. They do not "suck moisture out of the air". That would be a dehumidifier. Aug 4, 2023 at 3:51
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    Just to be sure - does your "fan" include a heating element? I would assume not. But a heating element sure can cause cracking.
    – Nobody
    Aug 4, 2023 at 12:01
  • @Nobody Good call. I used to have an old fan with just such an element. I never used it but it's a real thing.
    – JimmyJames
    Aug 4, 2023 at 20:33
  • I'm not sure it matters but I think we are assuming latex paint here. It might be worth clarifying.
    – JimmyJames
    Aug 4, 2023 at 20:35

3 Answers 3

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The problem you would have from not having mildew resistant paint is poor resistance to mildew. You don't have mildew so lack of mildew resistance is not the problem. QED

The idea of a fan sucking moisture out of the air is ridiculous. Is this the moisture that using the shower adds to the atmosphere? Is it implied that the paint would have cracked if you never used the bathroom? The fan doesn't "suck moisture out of the air." It ejects the moist air and replaces it with the same air you have in the rest of your house.

The paint is likely cracking because there is a problem with the surface to which it was applied. It may not have been suitably prepped like made free of dust or not had the right primer applied prior to painting. The drywall itself may also be cracking and the cracks show up through the paint. If your refinishing the ceiling was just adding more paint, the paint underneath may still be letting you down. It may also be the wrong type of paint that just can't stand moisture. But it's definitely not cracking because you're running the fan for too long.

How long a fan should run depend on things like how powerful the fan is and how big the bathroom is but typically 30 minutes should do the trick. Running it for longer shouldn't hurt and running it prior to the increase in humidity is also helpful: if you have cold temperatures, it helps to warm up the fan duct to the outside to reduce condensation (at least in theory).

Mildew resistance is nice but it's not absolutely critical. Good quality paint should stand up to a reasonable amount of humidity and a properly installed/spec'ed/operated fan should keep the moisture to a reasonable level. It seems like manufacturers are stepping away from mildew resistance in paints but that might just mean that people don't bother paying for it.

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  • Indeed, in New construction that's done to prevent air leaks at random, its entirely normal and (even code required, in some LAHJs) to run exhaust fans all the time.
    – Ecnerwal
    Aug 4, 2023 at 1:18
  • I have a 19" box fan outside our showroom, it is plugged into a 2 hour timer. When finished with a shower we turn it on especially in the winter as it adds moisture to the house. We have been doing this for over 50 years and yet to have a moisture problem in a shower room.
    – Gil
    Aug 4, 2023 at 2:02
  • @Gil - Although it would add complication there would be a benefit to having a way to choose whether to exhaust the bathroom fan to the outside or back into the house. The problem with a fan is that the same volume of air expelled will draw an equal amount of outside air into the house. During the winter this means drawing cold air into the house while meanwhile expelling valuable heat and moisture outside. Even in summer you could at times be replacing warm humid air with hot equally humid air, in theory at times it might be better to expel back into the house, although that requires caution. Aug 4, 2023 at 13:25
  • "fan outside our showroom ... When finished with a shower" you... may want to proofread before posting, @Gil! I don't think that came out the way you meant it! :ROFL:!!!
    – FreeMan
    Aug 4, 2023 at 16:15
  • I suppose running the fan during or right after applying the paint could cause the paint to dry faster but I can't imagine that causing it to crack.
    – JimmyJames
    Aug 4, 2023 at 20:29
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You can get a fan with a run on timer and trickle feature.

In the UK we normally wire to the bathroom light, the run on timer will keep the fan going for a period of time after light turned off in order to clear condensation. Trickle will run the fan all the time at a very low setting.

Furthermore, some fans have a short visit delay, the fan won't turn on for a short delay to avoid wasting heat and extend the life of the fan.

If users aren't turning the light on to shower, then tell them to do so or fit a PIR to detect occupancy and wire that up.

As for your painter, I've never heard such rubbish. I would give it a decent sand, use some zinsser peelstop and a good quality eggshell to give a tough moisture resitant finish. You can get paint with mould inhibitor, or you can buy + add seperately. If the bathroom is ventilated adequately, you won't need inhibitor.

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aquaticapetheory addressed your actual question. The cracking isn't likely due to moisture (or lack thereof).

I thought I'd chart out the protocol I implement for my family. I have installed digital timers on all bath fans in my home. Note that there are variables which affect your protocol:

  • Regional climate
  • Seasonal climate
  • Home construction attributes
  • Fan effectiveness (you're cleaning the blades annually, right?)
  • Introduced moisture volume (occupant count, shower length, etc.)
  • The presence of an energy recovery ventilator (ERV)

In general, here's what I direct my family to do when using the shower:

  • Spring, Summer, Fall (temps above ~0° F and variable outdoor humidity): 2-4 hours
  • Winter (outdoor temps below ~0° F): 30 minutes to 1 hour

These generalities were derived using the sniff test--how the air in the bathroom smells after a period of time. The key is to remove the bulk of the steam as quickly as possible, and then address lingering dampness due to wet shower walls and towels. Sometimes I'll run the fan for a shorter period some time after the initial cycle is complete because I detect elevated humidity from evaporation.

The bottom line is that no one can tell you what your home needs. Vent as long as necessary, but no more, in order to conserve energy (both electrical and HVAC).

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  • My bath fans are automated - I have Rh sensors in the bathrooms and one in the living room. The fan runs until the bath Rh is within ~5% of the living room and that's generally close enough. On occasion, I'll back to the bathroom after the fan has kicked off and is still feels damp - I hit the switch and turn the fan on for an additional 15 min which usually does the trick.
    – FreeMan
    Aug 4, 2023 at 16:17
  • Automation can be ok, but considering the known poor quality of indoor air I tend to run fixed cycles for that as well.
    – isherwood
    Aug 4, 2023 at 16:21
  • Do you really mean 0F here? Where I live, the air gets pretty dry below 25F.
    – JimmyJames
    Aug 4, 2023 at 20:39
  • I did, and I'm not sure I get your point. If you don't dry out your bathroom during a shower you get thick ice on your windows. :)
    – isherwood
    Aug 4, 2023 at 21:02
  • @isherwood: 0°F is 32° below freezing. If your threshold is 0 then you're getting frozen condensation already. Double pane windows will affect this. The only other reasons to change length of fan run time are seasonal interior humidity differences and drawing in outside air that's at a different temperature than the inside air and having to heat or cool it (unless you have an ERV - which you mention in passing). I think the single criterion to use is interior humidity and using smart sensors as FreeMan describes is the best way. Confession: I've never in my life used bathroom fans. Aug 4, 2023 at 23:18

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