An improperly vented bath fan can lead to tens of thousands of dollars of damage to your home and even create a health risk. How? *Moisture!*
The main purpose of a bath fan is to remove moisture from the bathroom after you take a shower. The reason you want to do that is that all that excess moisture can lead to mold and potentially cause your paint to peel and may even rot out your walls. So why do people dump all that water into the attic? Basically, because they’re too stupid and lazy to vent it properly – straight up and out through the roof.
How bad can it really be? Bad! I have had many clients with moldy attics due to this improper fan venting. But one in particular stands out in my mind. It was a beautiful custom home – no expense was spared. I was called in due to mold in the attic. Usually, this means a little patch of black on the roof plywood. But when I opened the door to this attic, I was greeted by a roof covered with fuzzy mold. Everything was damp. This was a problem!
After searching, I found the culprit – the builder had routed the bath fan duct under the fiberglass insulation over towards the soffit (the overhang where the roof meets the house). Often this are has a little bit of ventilation, so many lazy builders run bath fans to this area. However, in this case, there wasn’t even any ventilation slots in the soffit, so the bath fan was just dumping all the moisture straight into the fiberglass insulation.
The homeowners had to spend thousands on mold remediation. They then decided to re-insulate the entire area using spray foam and had to pay to get added attic ventilation to avoid this problem in the future. All because the builder was too cheap and lazy to add a $25 roof vent cap and run the bath fan to it.
So if you ever consider venting a bath fan into the attic, remember this story. Always vent bath fans straight up and through the roof. Use insulated ductwork from the fan to minimize the chance of condensation in the duct. And use a high quality roof cap, like this one from Broan.
Why not vent out the soffit?
I’ve had some builders argue with me, saying that it’s ok to vent out the soffit. They claim it’s safer because you don’t want to put more holes in the roof because of the risk of leaks.
First off, a high quality, self flashing roof cap like the Broan shown here is very easy to install water-tight. I did two myself and I’m an engineer, not a builder. So scratch that argument – it’s bogus.
Next, think for a moment. What does warm, moist air do? It floats up! Duh. So if you try to vent a bath fan out the soffit, that warm moist air is just going to rise back up through the soffit and back into the attic. It’s hardly better than venting straight into the attic. So under no circumstances should you accept soffit venting of a bath fan. Save $50 on installation today, pay thousands for mold remediation and a new roof tomorrow!
The only acceptable alternative to roof venting is sidewall venting. You might do this when you have a slate roof. In this case, you can route the vent to the nearest exterior sidewall. it’s not perfect, but it will do if it’s not too far away.