bathroom ventilation fan question

[dale rex]

not sure if this is the right place to ask this or not, but here goes..............

Im havent installed any bath fans for many years and dont really do that particular job that often(i do mostly carpentry and painting,etc), but have a bath fan to install for a client that I am working for currently. ..It will be a complete installation , including venting. There is already a wire and switch from a light fixture, so i dont have to worry about that part. My questions are related to proper ducting. I know in recent years there is more of a concern about mold , and I understand the importance of proper ducting and venting. I am concerned about condensation inside the duct, in the cold attic in the winter(I live in pennsylvania), so I am prepared to insulate the ducting. Is it better to duct up and out the roof, or horizontal and out the sidewall? If I vent vertical, and any condensation forms inside the duct will it run down and into the fan? I am not up on the current methods for venting a fan like this, and want to do it properly to avoid any problems down the road. ( the last fan I installed I just used plastic dryer vent hose, and Im sure there are much better methods today and need some guidance. Thanks, Dale

[cniverson]

I have installed a few fans for our company. In our city it is code to insulate the duct work in a non-heated space. I always duct through the roof with a one way valve. I have replaced 2 soffits where the vent had been ducted to a soffit vent. Both times I have run across this, the soffit had dry rot around the vent. I believe dry rot could have been avoided had the duct to vent in soffit been caulked, and clamped. Duct tape alone failed both times.

[Woodticks are square]

I would not vent a bathroom exhaust to a soffit where soffit vents are used. This is the intake for air circulation that works in conjunction with roof vents. You would then be introducing all the bath moisture right back into the attic. Use insulated duct, and right after you come off the fan, form the hose into a trap to collect any condensation that collects instead of having it run out the fan and into the bathroom. Run the hose either out a side wall or out the roof. Broan makes a cool roof termination kit that uses a metal starter to connect the hose to.

[dale rex]

I was considering using insulated flexible duct, but figured the smooth walled galv duct would have less resistance and promote better airflow. I would of course have to insulate the galv duct with some sort of HVAC duct wrap. The 4" insulated flex duct would definitely be the easier of the 2 methods, and its only about 5 ft from the fan up to the roof, so maybe there wont be that much resistance in that short of a run.?

[Mark Parlee]

How far is it to the gable or is it a hip roof

[dale rex]

3 ft to the gable wall. Its an easy short run to either the roof or gable wall. Just am not sure which would the best way to vent, horizontal or vertical?

[Mark Parlee]

The gable is your best option. Use a 4" piece of PVC slightly higher at the fan end than the gable. This slight down-tilt toward the exterior will drain any incidental moisture and you simply cover the pipe with attic insulation to insulate. Use a Broan metal vent hood on the gable.

http://www.broan.com/display/router.asp?docid=451

[David Meiland]

A good thing to do with a bath fan is install a timer switch and tell the owner to make sure the fan runs for 10 minutes after they are done showering and leave the room. I generally have the electrician install this type:

http://www.amazon.com/Leviton-612-62.../dp/B00004YUMP

If the fan is on a toggle switch then it will usually get turned off fairly quickly, possibly leaving humid air in the duct.

With your short run you have almost nothing to worry about. Only problem I ever saw was a 25 foot run of uninsulated galv in a cold attic that sloped up away from the fan. They got drips sometimes....

[Martin Holladay]

Dale,
Whatever you do, don't follow Woodticks' advice to "form the hose into a trap to collect any condensation that collects instead of having it run out the fan and into the bathroom."

Never put a trap in a bathroom exhaust fan duct! Simply use rigid ductwork (PVC or galvanized), properly supported, and be sure that the duct runs downhill toward the termination in the gable wall.

I have inspected houses with flexible duct that includes an unintentional sag in the middle -- what Woodticks would call a trap. Guess what? The trap fills with water and totally blocks the duct.

Martin Holladay, senior editor
www.greenbuildingadvisor.com

[NateMoss]

Dale, we do a ton of bath remodels and vent fans all the time. I suggest a vertical route if possible with a roof termination kit (broan; mentioned in a previous post). However, if horizontal I would try to give it some upwards pitch. Also strap the pipe often to prevent sags. I installed a fan tech exhaust fan in my master and had no idea at the moisture it would draw out until I got up in the attic and checked it out. Of course I didnt strap the pipe off like I should have (never get around to things on your own house) and I ended up with about a 1/2 gallon of water in the pipe. So just remember to use all the straight runs you can, insulate, and strap the pipe.

[dale rex]

nate, if I go vertical and insulate the pipe will there be any chance of condensation inside the pipe? If so, where does it go, back down into the fan? Just wondering, because logic tells me that a sloped(away from the fan) horizontal run will allow the condensation to run out the wall cap........ Not trying to be a smartass here, just asking why you think vertical is better.

[tucker]

Either way you do it your going to want to insulate and use rigid duct for sure. If you are going vertical consider snow accumulation as well. If the area of the roof where you plan to terminate is an area where snow could possibly drift or accumulate then take the sidewall option and pitch it down towards the sidewall just in case your insulation doesn't do the trick....also position your round duct seems towards the sky for horizontal applications.

[Woodticks are square]

Martin - Thanks for correcting me. This is why I enjoy the site and interaction - its the way I learn better techniques along with conversations with Mr. Parlee. I had removed a 6" non insulated flex line from a house with the foil barrier over blown in fiberglass. It was winter and did have water in the trap, but I attributed it to both the foil and non insulated hose. We removed the foil (which had actual frost on the bottom side of it, and replaced the hose with 6" insulated. Are you saying if the hose went from the fan directly vertical without any traps we would not have any condensation issues? What would have happened to the moisture had I have done it the way you are suggesting? Thanks for your help.

[Martin Holladay]

Woodticks,
I'm not a fan of vertical pipes or roof penetrations, for all of the reasons already provided (the possibility of condensation; the possibility that a roof penetration will lead to a roof leak; and the problem of deep snow, which is a real issue where I live).

Hipped roofs are a problem, since attics in houses with hipped roofs don't have a gable wall. In an attic like that, you need to exit through the roof.

If you do it the way I am suggesting, you extend rigid duct straight up from the fan until you get enough height to provide slope all the way out to the gable wall. You install a 90 degree elbow (or better yet, two 45 degree elbows) once you are high enough. You install strapping or a duct hanger to keep the elbow where it belongs. You install a straight run of rigid duct from the elbow all the way to the termination; the duct slopes towards the exterior of the house. All ductwork is carefully supported and insulated.

Very little water, if any, will condense in the short vertical duct. (The indoor air hasn't had much of a chance to cool off in the first 12 inches of duct.) Any condensation that forms in the long horizonatal run drips toward the exterior of the building.

Martin Holladay, senior editor
www.greenbuildingadvisor.com

[robillard]

Any fans that i install follow this method:

1. direct vented out of the house to a fan vent hood.
2. rigid metal 4" ducts,
3. seams up and taped with aluminum tape,
4. male ends into female ends to prevent moisture seepage,
5. pitched to the exterior,
6. Insulation over ductwork
7. short straight runs and minimal elbows if possible, if not, as last resort I will vent up through a roof vent cap
8. install a 10-20-30 -60 push button timer and encourage the customer to use the most time... clears the room and duct of moiusture.