Where does your bath exhaust vent terminate?

It used to be a very common practice to terminate bath exhaust vents at the screened bird-blocking of the roof overhang. Everyone has likely seen bird-blocking, but perhaps you did not know what you were looking at.

Bird-blocks are essential components of the roof/attic ventilation system, and shows between the ends of the roof trusses or rafters at the roof overhang. These vents are where air enters the attic and travels to the roof vents or ridge vents. Also, as their name implies they fill the space between the rafters/trusses to keep the birds out of the attic as well as provide a path for ventilation.  I think the blocking also gets its name from stick-built type roof structures where the rafters had a “bird’s mouth cut in the rafter to sit flat on the wall top plate.  So this blocking was between the bird’s mouths of the rafters.  So they are either a “description” or a “function”—maybe both.

It probably does not hurt the ventilation of the attic too much to “borrow” one of these openings for the termination of an exhaust fan, but it can have some serious unintended consequences.

Let’s assume for example that the vent is indeed going to blow air out the hole of the bird-blocking. If the pipe is butted tightly up against the blocking it will likely work pretty well. However, this warm moist air is being blown into an area that is typically under negative pressure. In other words a lot of this warm moist air is being blown into a space where it is likely going to be sucked right back into the attic.

Another problem we have to consider is that all of this exhausted air has a lot of dust and lint in it. In a relatively short period of time the screen in the bird-blocking is going to plug with lint and then the exhaust fan will stop moving air from the bathroom. This will lead to moisture issues in the bathroom as well as the whole home. The odors we usually appreciate being vented out of the bathroom as quickly as possible will also “linger.”

It is for these reasons that an exhaust cap with a back-draft damper is necessary, and that it be located such that the air being exhausted is not going to be sucked right back into a space we don’t want it. We want to have control over the maintenance of the exhaust system as well as where the air ends up.

As you can see in the following picture the vent screen is completely blocked with lint.

Plugged vent screen

I slid the pipe over 3” just so you could see what happens when the exhaust is installed in this manner. There is a small area at the center of the lint where the screen is visible but this is only so because the pipe scraped off the lint when I slid the pipe over.

Often these vents are buried in insulation and you only get a hint this method has been used by the amount of fungal growth on the soffit at the exterior or on the roof sheathing in the attic.

vent at soffit

When the fan in the bathroom will not hold a piece of tissue paper up against it, this is sometimes the reason why–and it is certainly an indication the fan is not venting properly regardless.

By Charles Buell, Real Estate Inspections in Seattle

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The Macro-view and the Micro-view in home inspections

When inspecting a home, the home inspector must not rush to judgment and must pay attention.  Having a “system” of checks and balances is required in order to not miss the details that would allow the correct conclusions later on.  “Concluding” is the easy part, having the “correct conclusions” can sometimes be illusive.

Photography is a huge part of the checks and balances that I utilize to help me keep track of myself–and perhaps help me answer questions later when I am home working on the report.  Another important aspect is to avoid distractions and maintain a protocol–to do each inspection in the same order as much as possible.  Sometimes deviations are unavoidable, and sometimes taking a few steps back and getting back on track is good idea.  Most clients understand when you tell them, “I have gotten a bit off my protocol and now I need to get back on it.”

At a recent inspection, while inspecting the roof, I noticed that there were no exhaust fan ventilation caps anywhere.(Sometimes the roof vents are used for exhaust fan vents though.)

Roof with no "proper" exhaust fan caps visible

Roof with no “proper” exhaust fan caps visible

To myself, I make mental notes that perhaps the vents are terminating in the attic, that there are no ventilation fans, or that they terminate at other locations.  Nothing to dwell on at that point—just something to store away in my brain until I have more information.

When walking around the home I noticed only two vents terminating at the exterior—a dryer vent and a bathroom exhaust fan vent–in the vicinity of the bathroom window.

Exhaust fan termination cap on the exterior of the home

Exhaust fan termination cap on the exterior of the home

Well this answered the question of where the bathroom exhaust fan vented to–but still didn’t answer the kitchen range hood exhaust vent question.  Since I had not been inside yet, I could not yet conclude that there was none or that it did not terminate in the attic.  Because of the risk of damage to the home from improperly terminated exhaust vents, finding where exhaust vents terminate is very important to me as a home inspector.

Once in the kitchen, I quickly discovered that the vent fan was the recirculating type that does not vent to the exterior.  This is a pretty simple recommendation:  “have it properly vented to the exterior.”

Next came the bathroom.  Normally, unless I can’t reach the fan, I will test the fan with toilet paper to see if it is drawing air from the room or not.  If I can’t reach it I will sometimes put the tissue paper on the floor at the bottom of the door and then turn the fan on–if the fan is moving air from the room it will pull the tissue away from the door–usually.

Bathroom exhaust fan not pulling any air

Bathroom exhaust fan not pulling any air

These methods are not very “scientific,” but they do give at least some indication of function–beyond merely turning it on.  As you can see from the picture, this fan, while it turned on just fine, would not suck the toilet paper up against it.  Since I saw a nice vent cap at the exterior of the home, and the flap was slightly open, it was very odd that it would not pull ANY air from the room.

Some sort of blockage in the pipe?  Disconnected in the attic and buried in insulation?  Back draft damper in the unit itself blocked shut?  So many questions—and no time to jump to any conclusions just yet.

When I got to the attic I found the bathroom exhaust fan and followed its vent pipe.

Vent pipe terminating at roof?

Vent pipe terminating at roof?

It suddenly became very clear that the cap, next to the bathroom window, at the exterior was an abandoned cap from some previous vent fan installation—perhaps a wall mount unit.

I scratched my head for a moment when I saw the pipe terminating at the roof sheathing.  This installation was consistent with a vent cap on the roof—but remember, I didn’t see any vent caps on the roof.

I pulled the vent pipe down enough to see “black.”

Can you see the felt paper?

Can you see the felt paper?

This is a little like seeing “red”–for an inspector–but different.

I took the pipe the rest of the way down and in this next picture one can see the felt paper over the hole which is the underlayment for the roof shingles.

The improper roof vent in the first picture is above this felt paper

The improper roof vent in the first picture is above this felt paper


Now, and only now, we finally have the final answer as to the “how and the where” the fan is vented to–and why it would not pull any air.  All done in the “context” of the inspection, without looking at the house:  individual component by individual component.

Conclusions often come at the end–sometimes on the ride home.

By Charles Buell, Real Estate Inspections in Seattle

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Rats love greasy kitchen vent fan ducts.

The title of this post grosses even me out—-and I love to blog about rats.   I sort of lost all respect for rats when I had to deal with them in my barn in Cato, New York.  I was trapping them, and one morning I found one part way out of a hole in the floor with its head in the trap.  When I picked up the trap I discovered that the rest of its body was gone—-apparently eaten by its friends and relatives.

Every component of the home is composed of materials that are either right for the installation or not.  For some components there is some “leeway” depending on what the component is, there are others that if you don’t use the required materials bad things can happen.

For example the use of water supply lines to hook up the gas dryer is NOT a good idea.  When venting the dryer to the exterior the use of plastic type vent pipe is not an acceptable substitute for smooth-wall metal pipe.  However exhausting the Powder Room fan to the exterior can be done with either plastic or metal and not present much of a problem.

Kitchen Range exhaust hoods are supposed to vent to the exterior with smooth wall metal pipe.  The ducts from these hoods can get very greasy.  Years of cooking bacon and hamburgers can build up in these range hoods and ducts, and can represent a fire hazard.  For these reasons, plastic type vent pipe or even corrugated metal pipe, should not be used for venting kitchen ranges.

You want to be able to have these hoods and duct work cleaned, and if there are lots of “nooks n’ crannies” they cannot be easily cleaned—-save the nooks n’ crannies for English muffins.

A while back, I came across this installation of “foil-type” vent pipe installed on the Kitchen range hood.  I must confess I couldn’t tell whether this had “melted” or was due to “grease-eating-rodents—-but either way, I was fortunate that it wasn’t buried in a wall where I wouldn’t have been able to tell what kind of pipe it was—-let alone what condition it was in.  There is no “leeway” when it comes to kitchen range hood venting—-it must be smooth-wall metal pipe that cannot be damaged like this duct was.

Flexible metal duct should never be used for Kitchen exhaust fan ducting

Flexible metal duct should never be used for Kitchen exhaust fan ducting


Charles Buell, Seattle Home Inspector

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