Home Inspector Rochester NY – Properly Venting Bath Exhaust Fans


Improperly Vented Bath Exhaust Fan Damage

Condensation Related Structural Damage

Venting Bath Exhaust Fans

Stained Roof Sheathing

Welcome to my blog for homebuyers and homeowners. Here I will share stories about; how to select a home inspector, issues discovered during home inspections that affected the home’s safety and habitability, energy efficiency issues, sustainability issues, and occasionally on information that is totally unrelated to homes and home inspections. I hope that you enjoy this blog and return often.

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I feel this is a fairly important topic to discuss, as I’ve seen the results of improperly vented bathroom exhaust fans kill more than one home sale, and as a home inspector, the last thing I want to do is to kill a home sale. (the truth is home inspectors don’t’ kill deals…inept contractors & do-it-yourselfers kill home sales when home inspectors discover and report on the results of their shoddy workmanship)

Imagine how an inspector feels when they have completed the home inspection, except for the attic inspection, and discovered no issues of major consequence, but when they access the attic, they discover the that the underside of the roof sheathing is heavily stained with what appears to be mold, and much of the roof sheathing is delaminated and failing as the result of this.

What could be worse? In my, all too frequent experience, the only thing that could make this worse, is when the current homeowner has just had a brand new layer of shingles installed. This is because the only way to correct this is to complete a full tear off of all shingles and sheathing, and to start over.

So the current homeowner just spent $10-15K for new shingles to improve the curb appeal and the marketability of their home, only to have an offer withdrawn due to the buyer’s home inspector discovering both structural and potential health issues with their roof structure. What now?  Now the current owner must tear off and trash the new shingles, plus replace roof sheathing, plus another new layer of shingles.

This begs the question…..did anyone even bother to inspect the attic prior to the reroofing?

Tip: When you are shopping for a new roof covering – don’t hire the cheapest roofer – make sure that the roofer inspects your attic prior to simply adding new shingles, and that any attic ventilation deficiencies are corrected, and the results of these deficiencies are corrected as part of the reroofing contract.

Believe it or not, the above conditions could have been solely the result of an improperly vented bath exhaust fan.

That said….let’s get to the topic of the post:

All manufacturers of bath exhaust fans specify in their installation instructions that their fans must be vented directly to the exterior of homes.

Why is venting directly to the exterior so important, and what is the best practice to accomplish this?

The why is all about moisture control – getting the moisture from the interior of your bathroom safely to the exterior of your home. Moisture is a home’s worse enemy, and an improperly vented bath exhaust fan can affect the health of both the home’s occupants and of the home itself.

One key factor that we must take into consideration when designing an effective method to vent our bath exhaust fans in Rochester, New York, is that we have cold damp winters.

The best practice in a cold damp climate is to vent bath exhaust fans through insulated rigid duct material. (Both PVC and galvanized sheet metal are good duct material choices – never reduce the diameter of this – increasing the diameter is just fine)

Why insulated? This is all about moisture control. When the warm moist air from your bath exhaust fan enters your cold Rochester attic, condensation occurs. Insulating the duct will diminish the amount of condensation that occurs and make it more manageable.

Why rigid duct? The textured surface on the interior of flexible duct will collect condensation, and that textured surface will also increase static pressure which will reduce air flow and make your fan noisier. Flexible duct also has the tendency to droop and water will collect in the duct’s low spots. Standing water supports the growth of all kinds of nasty things that smell bad.

O.K. – insulated rigid duct – what else can you do to control moisture and effectively vent our bath exhaust fans?

Here are few more recommendations:

- Install a short rise in the duct near where it connects to the fan – not too close – establish a straight run of several feet prior to installing a sweeping bend to support the installation of a rise, as placing a sharp bend (as you would have with a 90 degree elbow) too close to the fan’s exhaust port will also increase fan noise. The purpose of installing this rise is to then allow you to pitch the horizontal run of the duct down towards the point that it passes through a gable end and exhausts directly to the exterior of your home.

- Let’s discuss exhausting this through a gable end rather than through the roof. Why is this preferred? You guessed it – it’s all about moisture control. Any moisture that might collect in the duct can now drain by gravity down towards the exterior exhaust point, and let’s face it – one less hole in your roof is always a good thing.

- Don’t even think about venting this towards a soffit vent or even to a manufactured under eave exhaust hood. Why not? If you have soffit vents, any warm moist bath exhaust vented near the soffit vents will likely be sucked back up into the attic through these attic ventilation intake vents and condensation will form on the underside of your roof sheathing. Why is this an issue? Condensation formation on the underside of roof sheathing supports mold growth, and some molds can be unhealthy to some individuals, and all molds can contribute to sheathing deterioration. (TRUE FACT: mold is generally more likely to kill your roof sheathing than to kill you….but that’s a whole other blog topic)

One more TRUE FACT – over 80% of the home’s I inspect have improperly vented bath exhaust fans, and many of these home’s have at least some condensation related staining (most likely some type of mold) on the underside of their roof sheathing. Left unattended and allowed to go unchecked, this can be a real deal killer when you go to sell your home.

If you have not had your attic inspected lately, maybe it is time to call in a qualified home inspector for an inspection. Recognize, a home inspector will provide a fair and objective inspection, as a home inspector is only interested in your safety and the health of your home – not primarily motivated towards selling you repair or remediation services. Please contact Advanced Home Awareness at (585) 245-1190 if you would like to schedule an attic inspection in order to assure your bath exhaust fan is vented properly and to gain a better understanding of the general condition, safety, effectiveness, and efficiency of your attic and its installed components.




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