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Preventing Bathroom Mold - long term solutions

bathroom exhaust fan mold 1034x422

Bathroom exhaust fans perform a critical, though often neglected, function.  Their first responsibility it to exhaust warm, damp air created during shower/bath usage. Second, they are called on to exhaust bathroom odors.  Both of these depend upon a simple process of drawing air from inside the bathroom and expelling it to the outside. Simple enough.

Where do bathroom fans go wrong?

  • Non-existent.  Occasionally I’ll find homes with no exhaust fan whatsoever.  Often the homeowner’s solution is to open a window and hope for a decent breeze.  The problem is much more common in pre-1950’s homes.  Most building code requirements have called for bathroom fan installations for decades.
  • Under-powered.  Many bathroom exhaust fans are under powered to begin with.  The remainder often lose power and velocity over time due to worn out motors or clogged assemblies.  Even in million $+ homes the bathroom fans are nearly always cheap, hardware store variety.
  • Dumb Installation.  Designing a proper bathroom exhaust fan isn’t rocket science. However, there are design flaws that can dramatically reduce their effectiveness.  The most common problem is the absence of ducting.  While this doesn’t diminish the fan’s ability to exhaust air from the bathroom, it certainly can cause major mold and wood decay in the adjacent framing. Other problems include long and convoluted ducting paths, which can greatly reduce the fan’s ability to move enough air.

Results of bathroom fan failure:

Failing bathroom fans can lead to a number of comfort issues, however the most concerning problem is the likelihood of mold growth.  This occurs when the bath fan fails to properly exhaust the excessive moisture created by shower usage.  Lacking an appropriate escape path, the moisture soon condenses on the ceiling and walls.  This creates the speckling illustrated in the photo below.  Thankfully, because the mold growth is due to condensation, the problem is confined to the warm side of the ceiling or walls.  Even in fairly severe cases, the backside of the sheetrock will remain completely mold free.  This occurs because the wall or ceiling becomes the first condensing surface, effectively trapping the moisture before it can escape to through the wall.  Now technically, a small amount of moisture migrates through the sheet rock, but this moisture will easily travel through the insulation on the other side.

Bath Fan Failure Leads to Mold Growth

Determining the correct size / CFM of a bathroom exhaust fan.

According to the Home Ventilating Institute, a correctly sized bathroom fan should achieve a minimum of 8 air changes per hour.  This means that within 60 minutes, the entire volume of air within the bathroom will be replaced 8 times.  Of course, this doesn’t apply to bathrooms with saunas hot tubs or other exotic, moisture producing devices.  The vast majority of bathrooms will need a fan capable of producing 60-120 CFM.

Below is an example of a typical size bathroom and the associated calculations:

Fan Controls: Constant flow vs. timer vs. switch

The best fan in the world won’t do much good if it isn’t on when it needs to be.  In the past, the answer was straightforward: either a simple switch or an old fashioned dial timer.  Neither work very well, as people tend to turn off the switch after showering or the timers just failed.  Both result in inadequate air flow and mold problems.

Today, we have far better options.  Roughly these fall into the categories of continuous flow or digital timer.  As the name implies, a continuous flow bath fan runs all the time at a low CFM (typically 30-40), but ramps up to full speed when the bathroom is in use.  Panasonic models do this the best and can be set to run an additional 30-60 minutes after the occupant leaves the bathroom.  This allows the fan to exhaust the residual moisture built up during shower usage.

Digital timers are simply an updated version of the dial timer.  Because they are digital rather than mechanical, they tend to last far longer.  They work well in situations where the occupants can be trusted to operate the timer correctly.  With children or tenants, the constant flow models are superior.

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