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.
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.
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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