Venting bath fan out soffit vs. side vs. roof

[newowner]

O.K, I posted this question on the shower/bath forum and I have gotten very helpful responses, but since in new home construction it is the HVAC people who handle all the venting, I thought I'd run this by you and see what you thought.

I live in central Maryland in a house built in 1981. The master bath on the 2nd floor has a window and no exhaust fan. It shares a wall with a full bath that has an exhaust fan that is vented by uninsulated rigid metal ducting to the roof and is attached to some sort of cap that doesn't look anything like what I have seen roof vent caps look like. The metal ducting runs at an angle of probably 45 degrees to reach the back slope of our roof.

Anyway, I am having an electrician install some overhead lights in the 2nd floor bedrooms and thought I'd have his crew install an exhaust fan in the master bath while they were here but I am confused about the best way to vent the exhaust. The electrician is an expert in electricity, not venting. He said he could run the duct to the side of our house, which would be 22-23 feet of ducting. Going out the front of our house would only be about 2 feet because the master bath is in the front of the house, but who wants to look at a vent cover on the front of our siding!! I haven't asked the electrician if his crew knows how to vent it out the roof using a roof vent cap. My husband was thinking he could maybe try the roof vent if they say they don't have experience with it, but he is not much of a handyman with no experience of this either, so I am worried about that.

My research indicates people are divided on if venting to the soffit vents under our roof is a good way to vent or not. I got quotes from 2 electricians on my work, and one said soffitt venting is o.k. for central Maryland and the other said he thought some of the moist air would come back in through the soffit intake vents and cracks and gaps in the soffit materials, etc, and cause mold in the attic. The internet has people saying yes it is fine and others saying no, it is not good. Of course it would be the easiest way to vent. I am assuming the electrician means cutting through the soffit perforations and actually putting on a soffit vent, but maybe he means taping the duct to the top of the perforations from inside the attic.

So what do you all of you think? If it truly is not a good idea to soffit vent, then I am wondering if we should we vent out the side, which would be 22-23 feet or to the roof, which would place a roof vent cap on the front slope of our roof, very near the pipe that is on the front slope of our roof which I think is the pipe that vents out the sewer gas. I don't even know if it is safe to cut a hole near that vent pipe besides the fact we then have two ugly things on our roof to be seen from the front of our house.

I talked to a Fantech tech rep about an inline fan system before I discovered our hallway bath was vented to the roof and he said running duct 23 feet would be o.k. since each bath is about 8 X 8. A member on the shower forum brought up the point that 23 feet could cause air resistance, leading to condensation, so I am not sure if I should rely on the fan tech guy. Plus the Fantech guy based his answer on if I connected both baths to share a 230 CFM fan, not if I put a separate 100 CFM fan with 23 feet ducting out the side. Please share your thoughts and experience!

 

[Rancher]

My HVAC guy, said it's done all the time and the less penetrations thru the roof the better.

But this is AZ, a little dryer out here.

Rancher

 

[Sponsor]

 

[newowner]

If it makes a difference, I could make the ducting "only" 18 feet by centering it on the bath ceiling vs. 21 feet if centered in front of shower stall. Cool photo, BTW, Rancher.

 

[Molo]

waiting

I am also waiting for the best solution to this problem. I have 2 bathrooms that need vents. I will not install them until I find out where to vent them. Please someone provide a conclusive answer to this problem
The local hardware owner told me to vent it into the attic!!!

TIA,
Molo

 

[Jadnashua]

Hot air likes to rise...venting through the roof is the shortest path so it has the least possibility of condensation, and shorter runs are more efficient. Fantec makes some nice remote fan systems - virtually silent in operation and you only need one fan for the 2 (or more) bathrooms. What you don't want for ducting is something like a dryer vent - accordian style stuff. Way too much restriction. They do make some flexible dual walled, insulated duct that is nearly smooth on the inside what works pretty well. Whatever ductwork you use, you do want to insulate it.

 

[TedL]

I am also waiting for the best solution to this problem. I have 2 bathrooms that need vents. I will not install them until I find out where to vent them. Please someone provide a conclusive answer to this problem
The local hardware owner told me to vent it into the attic!!!

TIA,
Molo

There is no single right answer. The best solution could be side/back wall, roof or through the soffit or fascia, depending on home layout, your tolerance for working in tight spaces, willingness to climb a roob, etc. Notice I said through the soffit, not behind a perfed soffit or into the attic, and not through a front wall.

Properly sized and insulated vent pipe (or flexible duct) should work over any needed run, but longer runs will tend to decrease vent performance.
Minimizing the length of the run is a good thing, but not an absolute rule that must always be followed.

 

[Hube]

NEVER vent moist bathroom (or kitchen) exhaust air into any attic area.
Fr best results, go up and thru the roof .
A soffit vent installation will do a 90 -95 % job that a going up to the roof installation will do.
Cut an opening in the soffit to accomodate the special "T" vent grille. This soffit grille looks like an upside down T .The exhausted air is fed into the single entry and is exhausted sideways into each leg of the T.
Both of these exit legs have a "backdraft flap to avoid any air from re-entering after the fan has shut off.
Most hardware store have them. A 4" diam. smooth metal pipe and fittings is best from fan housing to this T termination point. This T shape grille, when installed will only protrude approx 1" below the soffit, and it looks good too.

 

[PTN]

NEVER vent moist bathroom (or kitchen) exhaust air into any attic area.
QUOTE]
is this really a major problem say if you have a completely empty attic (full length and width of house) with vents at either end?

 

[TedL]

Define "major"......

 

[PTN]

i termed it major because Hube labeled this action as "NEVER". Implies very serious problems to me.....

just looking for clarification for the specific example i listed.

 

[Jadnashua]

In the winter, you can end up with frost when the moisture condenses. In the summer, you can end up with feeding mold, especially if it is directing it on or near the insulation - it could end up saturating it, which also ends up decreasing its effectivness. You really want that concentrated moisture laden air out of the house, that's why you put in the fan, not to relocate it.

 

[Hube]

jadnashua has given some very good reasons why you should NEVER dump this moist(and oderous) air into the attic area
Do it right, either take the best way and thru the roof, or take the next best route and vent it outdoors via the sidewall or soffet.

 

[Bill Arden]

I'm still thinking of venting the bathroom vent into the furnace intake using insulated duct's.

The bathroom fan would not be used and the switch would instead turn on the furnace fan or possibly a heat exchanger fan.

Then again I am in a cold area.

Everyone worrys about the vent and does not ask where the air comes from?

 

[Geniescience]

more facts

H2O in vapor form is the lightest molecule in air, so it rises. Heated air also rises. Warm air rises. Moisture, or warmth, will rise in air.

To respond to the original post: Humidity causes dry rot in your climate. You are not in Tucson.

If you add more humidity into a relatively closed space, you are "playing with fire". Let us create a new expression: playing with moisture. It can make the wood in your structure smell bad and rot. Here is the bigger problem: Anything made with wood pieces and glue (plywood, OSB, lauan, particleboard, and the hundreds of other trade names) can rot through and through, since each cut reveals cell walls (cut cell walls) where the bacteria that are known as "rot" begin their feeding and reproducing. Solid wood holds up fairly well structurally, so that is not the big problem.

For Bill: consider whole-house Air Exchangers with Heat Recovery. I agree with your line of thinking but I cannot say much about the idea you described.


David

 

[Geniescience]

more precise definition

when i said "cut" i meant to refer to each individual piece of wood chip in a board, or each individual piece of wood speck in boards made of small small specks of wood glued and compressed together. I was not referring to the cutting of the board with a saw cut.

 

[jimj]

Is a roof vent still the best if you have snow?

For best results, go up and thru the roof.

Is a roof vent still the best if you have snow? I've read other places that if you live in a snowy climate that a soffit vent is better since it will never get blocked by snow.

Since I have other vents on my roof that must survive the snow in the winter, why wouldn't I be able to get a roof vent for my bathroom that is resistant to snow? Is there a specific type of roof vent I should be looking for, or is soffit actually better when you have snowy winters?

 

[Jadnashua]

I live in NH. Mine is vented out the roof. Now, I don't use it all that much in the winter, since typically, the additional moisure in the house isn't that bad of a thing, but I've not had problems. I think trying to get a good seal in a soffit is tough, and then with the air generally moving into the attic from surrounding vents, isn't all that great of a location since some of that moisture would be sucked back into the attic. Personally, I think out the roof or end wall are best, not out a soffit.

 

[Molo]

More Vent Questions

Hello All,
Here we are in Western New York State, back to another challenging super-humid western New York summer. Reluctantly anticipating another challenging snow-filled cold winter.
There should be college courses dedicated solely to building in a challenging climate like this.

In this post I would like to raise a few more questions about venting. In question is the same house I had questions on before. It is a single story ranch. There are currently no vents in the house (even the stove isn't vented). The Bathroom is on an outside wall, and is adjacent to the kitchen. There are no vents anywhere, and I would like to vent the bathroom, and ideally the cook-stove as well.
Please note: This is a very small living space (600 sq. ft) so therefore I am concerned about pulling to much air out of the house with the vents (especially in winter when it is sealed up tight).
The questions:
1. Is my concern about pulling too much air a valid one?
2. How do I solve this if it is a problem?
3. I have read about bringing air into the house through "outdoor air intake vents" that are simply a 3-4" pipe run from the outside of the house to the furnace replacement air intake, and into the vicinity of the combustion areas for the hot-water tank and the furnace. Is it a bad idea to bring this much cold air into a home in the cold winters we get here in Western New York ?
4. I would like to vent the bathroom and the stove. The vents would only be 10' from one another. What is the best way to do this? Individually, or connect them in the attic and bring them out one roof penetration?

Thanks in Advance for the help,
Molo in a challenging Western New York climate.

 

[Jadnashua]

The Canadians have had to deal with tougher air control requirements than we have for years. A heat recovery ventilator works great at this. Basically, it is a box with a heat exchanger. The incoming air gets tempered by the outgoing air - warming it in the winter and cooling it in the summer. They apparantly are fairly common there, but not here. They are available. Whenever you do something like turn on a flame (gas dryer, stove, WH, etc.) it senses the pressure change of the air going out the flue, and replaces it with the tempered air instead of sucking it through all of the cracks, leaks, etc. in the house. It helps to manage the humidity levels, too.

There are systems that allow multiple bathrooms to be ventilated with one central fan, but I'm not sure they are rated for stove vents, too. The problem with stoves is the grease; you want a system rated for that or you risk a grease fire.

 

[99ctr]

I'm working in the same problem.
During construction of our additional 12 years ago (part was a whole new roof structure), to try and not have yet another two holes cut into the roof, we put them through the soffit. I have gable ends and soffit ventilation in my attic.
During a recent inspection through the attic, I found lots of mold growth on the rafters and plywood above that soffit area.
Sooooo, if you have that common mode of attic ventilation, I can attest to the fact that others speculate, the warm air will make it back in if venting through the soffit.

I have a long long run to get to the peak ends of the house, and I see the long runs to the end of the house being a problem with moisture build up on the runs to the vent.

So I think I'm opting for the roof vent, very short run right up to the roof, and I'll insulate the vent tube. Warm air loves to rise. Get it out quick and it shouldnt me a moisture issue.

Our local code , as I under stand it, says vent to the outside and it must be a metal tube, no PVC or vinyl flex lines.

So this spring, I'll be installing two roof vents for the bathroom fans.

Any idea of how to kill off the mold on the rafters and plywood???

tnx
-Bob