Venting a Bathroom When There Is No Attic Above It
Post-and-beam houses and flat-roofed additions leave nowhere to run a duct. The two routes that work, the one that ruins a roof, and how fans get sized.
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- Mountain View Bath Co
In a conventional house with a pitched roof and a ventilated attic, exhausting a bathroom is barely a design decision. The fan goes in the ceiling, a flexible duct runs across the attic to a roof or gable vent, and nobody thinks about it again.
A good number of houses around here do not have an attic. Post-and-beam construction with a low-slope roof, which describes both Mountain View Eichler tracts and a fair amount of other mid-century building, puts the finished ceiling immediately below the roof structure. Flat-roofed additions on otherwise ordinary houses have the same problem in one room. So does a top-floor condominium with a concrete deck above.
There are two good answers and one bad one.
Route one: straight up through the roof
A fan housing sized to fit the available depth, a short vertical duct, and a proper roof jack flashed into the roofing system.
The critical phrase is “flashed into”. On a low-slope roof, a penetration is a deliberate detail integrated with the membrane or the built-up system, sequenced with the roofing rather than cut through it afterwards by whoever is holding the hole saw. Done properly it lasts as long as the roof. Done as an afterthought it is a leak with a schedule.
Practical points: keep the duct run short and as straight as possible, because every elbow costs airflow. Insulate the duct where it passes through unconditioned space so warm moist air does not condense inside it and run back down into the fan. And use a roof cap with a proper backdraft damper.
Route two: horizontally out through a wall
Where the bathroom sits on an exterior wall, the duct can run sideways and terminate in a wall cap. This is often simpler and cheaper than a roof penetration, and it entirely avoids putting a hole in a roof.
The constraints are that the run has to reach an exterior wall without an absurd number of bends, the termination has to be positioned away from windows and air intakes, and the wall cap needs a damper that closes properly, because a cheap one lets cold air and insects straight back in.
The route that ruins things
Venting into the roof cavity, the joist space, or a soffit. This happens more than it should, usually as a shortcut, and in a house with no ventilated attic it is destructive. Every shower pumps warm moist air into a sealed structural cavity where it condenses on the underside of the roof deck. The damage is invisible for years and then it is not.
If you are opening up a bathroom in an older house and find the existing fan discharging into a cavity, that is worth investigating before the new work covers it up again.
How big a fan
The standard specification is 50 cubic feet per minute for a fan running intermittently on demand, or 20 CFM for one running continuously, ducted to outside air. This follows ASHRAE 62.2 as adopted into the California residential code.
The City of Mountain View is explicit that a bathroom with a tub or shower requires mechanical exhaust and that an operable window does not substitute for it. That surprises people in a climate this mild, but humidity control is about what happens inside the room during a shower, not about the weather outside.
A few notes on choosing one:
- Sone rating matters more than people expect. A fan rated at 0.3 to 1.0 sones is quiet enough to leave running. A noisy fan gets switched off, and a fan that is switched off ventilates nothing.
- Rated airflow assumes a duct. Manufacturer CFM figures are measured on a short straight run. A real installation with a couple of elbows delivers meaningfully less, which is a reason to size up rather than exactly.
- A timer or humidity sensor is the single best upgrade. Most moisture leaves the room in the twenty minutes after the shower stops, which is precisely when a manually switched fan is already off. A humidity-sensing switch or a run-on timer costs very little and does most of the actual work.
Where it belongs in the project
At rough-in, with the walls and ceiling open, and after the duct route has been decided. Retrofitting a vent path through a finished post-and-beam ceiling means opening the ceiling again, and where the route ends up going through the roof it means involving a roofer at a point when nobody planned to.
It is a small line item that is cheap to get right and awkward to get wrong, which is a fair description of most of the best decisions on a bathroom.
More on the houses this affects most in the Eichler bathroom guide, and more on lighting the same difficult ceilings in the lighting and ventilation guide.