As far as necessary to remove moist air and odors from the house. The International Residential Code (IRC) requires all bathrooms to have a ventilation system.
Warm moist bathroom air must be ventilated to the exterior of the building. The proper combination of pipe length, pipe properties, and fan volume should meet code and remove moisture-laden and odorous air–and replace it with dryer air brought into the bathroom from the rest of the house.
Bathroom ventilation is important for home air movement and replacement. It needs to be installed properly to operate effectively.
Hopefully, the following information will help you get it done right.
Bathroom Ventilation Pipe Length
Most standard bathroom fans can move air 50 feet. Fifty feet sounds like a lot and in the majority of houses, should be more than sufficient.
Note: It took 23 feet to vent our bath fan out of the north gable end–in a small bilevel house. A shorter route would be through the roof, but I prefer the pipe to be covered with a layer of insulation when it is 25 degrees F below zero.
Stronger fans and straighter pipes can get you up to 100 feet between the fan and the outlet. Some bigger more creative house designs require longer pipe runs with more elbows and bends to exhaust the air.
Bathroom Ventilation Pipe Properties
Most bathroom fans require vent pipes that are 4” in diameter. Roof hoods and wall hoods are sized to match. Hoods should be weatherproof and rodent and bird resistant.
Types of Piping to Use
Bath ventilation pipe affects air flow. Not only size, but product composition and design. Air moving against something creates friction. The more friction created, the slower air moves, and the longer your bath fan needs to run.
Bath fan piping is usually one of three types:
- Smooth Galvanized Steel. Smooth galvanized pipe is the most popular choice for bathroom fan ventilation. It is rigid and provides the best air flow. More bends and corners slow down the air movement. Use sheet metal screws on all joints, along with duct tape to prevent air leakage.
Courtesy: Builders Best/Amazon
- Spiral Steel Wire. Multiple types of construction–all foil or combinations of materials. These are probably the most popular pipes for DIY projects because of ease of installation. They easily go around corners without needing elbows. One downside to these pipes is a slight loss of air pressure because of the ribbed construction. The construction also holds more dirt than smooth wall pipe.
- ABS Plumbing Pipe. ABS pipe works well, but has a static problem. The movement of air past any PVC plastic product creates a static charge that attracts and holds dust. (PVC windows can be wiped clean. Not so much the insides of piping.) ABS pipe can–and should–be glued together.
Design of Pipe Runs
The route your ventilation pipes take to the exterior of the house is quite often determined by house design. Electricians, plumbers, and heating contractors along with framers can complicate matters by installing their products in the way of logical pipe runs.
In fairness, designers and architects are not supposed to mark each nail location. They usually locate fans and light placement, then let the contractors do the work. But occasionally some small tweaks would be helpful.
Bathroom locations in the house can also make ventilation routes difficult. Some builders are using wall fans and running vent pipes down into the joist space, then venting out of a side wall.
The best ventilation run is the one with the fewest corners. Every time air flow is forced to change direction, it will slow down a bit. (Think of your air as flowing water. Hitting a 90 degree corner makes it move slower.)
Bathroom Fan Capacity
Bathroom fans use many different measurements to help a customer make a purchasing decision. One of the most important numbers is Cubic Feet Per Minute. CFM measures the amount of air the fan will move through a 4” diameter pipe.
Fairly common CFM ratings are 50, 80, and 110. The higher the CFM rating, the quicker air is removed from the bathroom. And the less time your fan will run. Panasonic offers a fan with all 3 options available at the flick of a switch. Meaning that regardless of pipe length, you can have the power to remove air efficiently.
This item Panasonic FV-0511VF1 WhisperFit DC Fan 50-80-110 CFM Retrofit Bathroom Exhaust Fan – Quiet Energy Star-Certified Energy-Saving Bathroom Ceiling Fan
Courtesy: amazon.com
A small bathroom, like one of ours, is 5’ x 10’ x 8’ high giving it a total air volume of 400 cubic feet. A 50 CFM fan will theoretically exchange all of the air in the bathroom in 8 minutes. Theoretical because as fresh air is sucked into the room it mixes with the remaining air; meaning it will take a little longer to vent all of the moisture from your bathroom.
Exit Locations for Bathroom Ventilation Pipes
Regardless of where your pipe exits, it will need a hood–to keep out the elements like rain and wildlife like squirrels. Roof vents should come with flashing kits to prevent water leaks. Wall hoods will require sealing.
Roof Venting
Venting your ceiling fan through the attic and roof usually provides the shortest route and pipe length. Quite often you can end up with a pipe length of less than 6 feet. When deciding on a roof vent system keep in mind snow depth. Two feet of snow covering a six inch cap is not a good thing.
Years ago it was acceptable practice to vent bathroom fans into the attic. Moist attics can produce mold, mildew, odors, rot, and in extreme cases attic rain. Although building codes changed, they did not mandate replacement of existing systems.
But if you have one of these, extend the pipe outside the building envelope to help preserve your attic framing and insulation.
Bathroom Fan Roof Hood. Courtesy: Amazon
Note: Apartment buildings–specifically midrise and high rise are almost invariably vented to the roof in a stack or HVAC system. Please see our article “Where Does Vent Air Go In an Apartment” for more information.
Wall Venting
Venting bath fans through walls is becoming more popular. Especially when the bathroom has an outside wall where a wall mounted fan does not even require a length of pipe–just the fixture and hood.
Vent pipes for ceiling mounted fans can be run down a 6” plumbing wall, between floor joists, and out the wall. Hoods for wall venting fans are less expensive, easier to install, and keep clean.
Soffit Venting
Venting your bathroom through the soffit usually provides shorter pipe runs, but it also has drawbacks.
- Restricted Access. Not much room in the soffit overhang to work.
- Location. The IRC requires 3 feet of clearance from air intakes like windows and furnaces.
- Blowback. Exhausting moist bathroom air at the soffit line is an invitation for the wind to blow it right back into the attic.
Room-to-Room Ventilation
Venting your bathroom into another room in the house was–at one time–acceptable. Not anymore. And who wants to move smelly moist bathroom air into his/her bedroom? If you are changing your bathroom ventilation, make sure you exhaust the air outside.
