Bathroom fans dump into attic - Contractor Talk - Professional Construction and Remodeling Forum

robertc65 · 03-13-2006, 11:25 AM

I don't know if this is really the domain of electricians or not but I have seen over the years bathroom fans that dump into the attic. The house I moved into last May has the fans installed like this. Now I know it saves the home builder a few bucks and if the code does not call for exterior venting than many builders are going to do this.

I don't know for sure but I suspect that this is not the best way to install a bathroom fan. I plan in the future to replace these fans with a single inline panasonic whisper quiet fan to service both second floor bathrooms.

My question is why do builders do this and what problems does it create? One concern of mine is that in the winter lots of my warm air has easy escape route through the fans and into the attic.

MJW · 03-13-2006, 01:28 PM

It definitely puts alot of wet air into the attic, and it is not good for the sheathing and/or shingles. Get it changed ASAP.

HVAC is to blame for it. They were probably a little lazy.

robertc65 · 03-13-2006, 01:51 PM

So are you saying that typically it's the HVAC guys who install the bathroom fans?

Thanks

Rob

SDLSteve · 03-13-2006, 02:13 PM

Anyone can do it but the fact that the fan simply blows into the attic is a sign of laziness. You can either run it through the soffit or install a roof vent.

Grumpy · 03-13-2006, 02:16 PM

Have a roofer install a bathroom vent on the roof.

robertc65 · 03-13-2006, 02:46 PM

Well I'm not a roofer, electrician or an HVAC guy but the only person I would trust to cut a hole in my house and seal it up tight would be myself. Instead of a soffit or roof vent I plan to cut a hole in the side gable and install a typical metal exhaust vent. I did this in my basement already. I have an inline panasonic whisper quiet fan that services the bathroom and my workshop. I used a metal exhaulst hood. In order to create a tight seal when the fan is off, I glued a rubber gasket on the vent flap and a magnet which holds it tightly closed. I installed the magnet so that it's pull could be adjusted. I adjusted it so that the force of the air could blow the flap open, but when the fan turns off the magnet pulls the flap creating an air tight seal. When the flap is open the magnet is far enough away that it does not impead the air flow. It's works quite well. I'm sure a lot of the pros would frown upon it simply because some DIYer did the install, but it's real clean and hard to tell that it's not OEM.

Rob

Zatol · 03-13-2006, 03:08 PM

Today, the bathroom vent must be routed to the exterior of the house. I have no idea how long that has been code... We have the HVAC company put them in during their rough-in.

Grumpy · 03-13-2006, 04:07 PM

good luck with that.

BIGRED · 03-14-2006, 04:04 PM

I was just in an attic where the vent came out of the fan , ran on the joists, and 90'ed up and was hanging on a nail. Where the 90 was, there was at least a quart of water.

Mike Finley · 03-14-2006, 04:06 PM

I did not even know code allowed for a fan to be vented into an attic???

747 · 03-14-2006, 09:21 PM

speaking of bathroom fans. Can someone tell me what CFM stands for they range everywhere from like 50-240. How do you know how many CFM is rite for you. I thinking about puting one in one of my bathrooms. The other already has one.

SDLSteve · 03-14-2006, 09:28 PM

CFM= Cubic Feet per Minute. The higher the CFMs, the more air the fan will draw out in a given amount of time. You want to match the CFMs to the size of the bathroom basically. The bigger the bathroom, the more CFMs you'll want to use to draw humidity out of the room.

I think what happens is that installers save time by just venting the fan into the attic with the assumption that HO's don't go up there. Humidity and high temperature are not good combinations.

Steve Unkie · 03-14-2006, 09:57 PM

Code does not allow this, but I've seen it alot on remodels. The moisture condenses on the rafters and sheathing and causes rot if not remedied. It's easy to fix though. If you do go through the roof , make sure that you install your vent to flash properly.

Steve Unkie.

Mike Finley · 03-14-2006, 10:45 PM

747 - you're gonna think I've lost it but here goes -

The HVI (Home Ventilation Institute) recommends 8 air changes an hour for a bathroom. (kitchens are 15 and the entire house is 6).

Length room x width of room x heigth of room = Cubic feet of the room
8 ft x 12 ft x 8 ft = 768 cubic feet

cubic feet / 60 gives you the CFM (cubic feet per minute)
768/60 = 12.8 CFM

Multiply the CFM x the recommended air changes per hour
12.8 x 8 ACH = 102

So for an 8x12 bathroom you would want a minimum of a 102 CFM fan.

There is also an additional calculation for the actual duct taking into account the bends. But if you just round up the CFM you come up with by an additional 25% (125 CFM) you are going to be golden.

Ridiculous I know this isn't it! If you want to know even more...

http://www.rmchi.com/bathrooms/how-t...xhaust-fan.htm

ron schenker · 03-14-2006, 10:50 PM

You're right Mike. You've lost it....maybe too much coffee

just kidding, keep up the good work!

Mike Finley · 03-14-2006, 11:41 PM

I can do the math for a fan but don't ask me to pick out a tape measure for you! http://www.contractortalk.com/f40/tape-measure-question-7908/

Grumpy · 03-15-2006, 09:55 AM

Quite often a customer will call us for a roof leak in winter time and when we inspect inside the attic, as aprevious poster said, there is standing water at the bends.

I once saw a flex tube which wasn't trimmed down to the proper height so it had some loose play in it. Anyways there was basically a J trap in the hose with a block of ice.

This whole house was MESSED up in terms of ventilation. It was in a subdivision and this house was a model. You can see where they first started framing the roof pitch, then stopped and increased the height and this blocked the intake, also skylight tunnels blocking intake. It was a mess. In the end the only thing that worked for us was to install a fan with a humidistat so it would kick on at high humidity levels, even during the winter.... the bathroom fan was only a small part of their ventilation problems.