My brother-in-law has been helping his girlfriend remodel her house. The big project was to install a shower stall where there had only been roughed-in plumbing before; he made a workmanlike job of it, but now there’s a dilemma.
The bathroom fan. The bathroom is roughly in the center of the house, in the basement. His only real choices for ducting a fan to the outside world are west, in which case he has to drill three-inch holes through a large number of floor joists, or south, in which case he can run along between the joists.
In either case he’s going to have roughly twenty feet of 3" diameter horizontal duct that he’s trying to push damp warm air through. I think that he’d have a problem with condensation, and eventually with mildew, since a horizontal pipe isn’t going to drain very well.
If he tried to run straight up and through the roof, he’d have twenty-five feet of duct, plus the only logical wall choice is chock-full of plumbing and electrical stuff already.
So. Questions for those of you with some expertise:
Is a twenty-foot horizontal run of 3" duct going to have condensation & mildew problems?
Is a standard bathroom fan going to do the trick, or is a some oversized special fan going to be needed?
Is there anything else he can do to eliminate moisture from the bathroom after taking a shower that could eliminate the need for a fan?
Do they have a building permit? Is it required? Will a plumber/electrician sign off on the work done? My reason for asking is potential liability if something goes wrong, they decide to sell the house somewhere down the road and a home inspection reveals under par workmanship, or worse, there is a leak or even a fire.
You might want to look up the required building codes for the minimum standards. Then visit a local Home Depot or Lowes for their books. You might find several that describe in detail how to install a bathroom with the appropriate accouterments (plumbing, electrical, HVAC).
The trick is going to be making sure it’s pitched one way or the other and that it doesn’t sag anywhere. As long as you pitch it (and it doesn’t sag) any condensation should drip back to one side or the other. Also, insulating the pipe will help keep the steam warm throughout it’s journey.
To expand on what Duckster was saying. If you got a permit for this, you could certainly ask the inspector for his thoughts on the subject.
Also, is there any advantage to running perpendicular to the joists? I’m curious as to why you consider that an option?
A decent dehumidifier can be had for under 200 dollars, would that be an option?
I have an unvented bathroom in an old “Florida style” concrete-block apartment- with terrazzo floor. Dampness was a terrible problem, especially in winters when the weather was too cold to run the air conditioning, but not cold enough to run the heater much or at all. Rapid mildew growth, towels wouldn’t dry, etc. I finally bought a smallish dehumidifier and set it to run intermittently, and the problem is completely solved.
I would run the fan exhaust south with more than 3" duct. 3" seems to be a bit small for an exhaust duct. I wouldn’t make the duct through the beams cutting 3" holes through each one. Seems that it would weaken the upper floor. Beams are there for the purpose of strengthening that floor. No experience, just my view on this construction.
In the last house I live in we had two bathrooms with fans that we only assumed had been vented. Turned out the fans just sucked the air up into the interstitial space between the floors. We never had a problem with humidity or condensation. Do you anticipate this bathroom being used alot?
Agree. At least a 4" duct, maybe 5" or 6" if space allows. Rigid metal duct, not flexible duct, and I’d use aluminum over steel.
Once the duct configuration is determined, you can determine the static pressure that the fan will be operating at. A more restrictive duct means a higher static pressure. This pdf showsthe effect of static pressure vs airflow for a bathroom fan.
This was the thing that jumped out at me, too. Is it completely impossible to run the duct parallel with the joists so that it can run inside one of the bays?
If he runs the duct south he can be parallel to the joists. But he’d still have a good twenty, twenty-five feet of duct needed to get to an outside wall.
The bathroom would be used daily for showers; the girlfriend wants to use the basement as a mother-in-law apartment.
There are remote fans that go inline with the duct. Rather than mount to the ceiling of the bathroom and pushing all the air all the way, they mount partway between the bathroom and the exterior and so are pulling partway and pushing partway. I wonder if that would be a possible solution.
You certainly can put holes for mechanical in some (most) modern floor joists. Not dimensional lumber joists, but most new construction uses engineered joists, of various depths, often with pre-stamped knockouts for exactly this purpose. The manufacturer’s website will have pdf’s that give all the directions and limitations.
Proper pitch requirements may cause it to get too close to the flange by the end of the run. It can be difficult or impossible to fish the duct through and cutting out all the holes may be a lot of work; if space allows it may be easier to run it under the joists and box it out.
Fans should have limitation on total vent run listed in their specs.
Code usually requires a window or ventilation fan in a bathroom - humidity is the enemy of wood construction. A dehumidifier is probably not a reliable solution
i an i-joist you can knock out all the stamped utility holes without consequence. these are in the center and about 1 1/2 inch diameter. other holes have to be cut following manufacturers instruction for size and location.
If you do this, make sure you test-run the system before closing in any ceilings. I know of a case where, despite being ducted in accordance with the fan manufacturer’s specifications (and these were not cheap fans), the draw was grossly insufficient.
the plumbing vent will have sewer gas passing through it, you don’t want that to come into ventilated air space in the house which people would breathe.