When to stop running bathroom fans?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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).