"How can we vent our bath to prevent mildew?"
Column #841 04/09/2011
On The Level
Q.Our downstairs bathroom in our Cape Cod has a shower. The ventilation is poor. A ceiling fan in the center of the room vents into the attic. Mold is a continuing problem, despite cleansers and Kilz paint. We are upgrading our bathroom and contractors have proposed two options: vent through the roof or vent through the soffitt. Which is a better option, i.e, the one that will fix the problem? Which is more practical and probably cheaper?
A. I don't like poking holes in roofs unless I really have to-- plumbing vents,chimneys, gas flues and the like are items that must go through the sheathing and shingles. When roofs begin to leak they almost invariably start at a point of penetration, so the fewer penetrations the better in my book. Look towards a side wall or a soffit as a convenient ventilation location. Locating the exhaust point somewhere other than the roof should be less expensive but I don't know how far away that bath is from the side of the house. Following a joist bay to the side of the house would be easiest if that's possible. Talk to the contractors about that.
The problems you've had with that bath aren't necessarily singular in source. The first problem is the fan itself. Bath fans are notoriously cheap and don't move the air they claim to. It's been a building code for as long as I can remember that a bath (or powder room) must have an operable window or a powered ventilator fan. The operable window requirement always amused me because who is going to open the window while showering in the dead of winter? The window glass is cold as are most of the wall and ceiling surfaces and the bathing steam condenses all over the place and when the shower is over everything in the bathroom has a film of moisture on it. Then the bather exits the shower, towels off-- bath door still closed-- then leaves the bathroom turning the light and fan off as they exit the room, sometimes closing the door behind them. Now all the wet surfaces must slowly dry and that, as you've learned, presents a condition conducive to mildew and mold growth.
When considering the new fan shop for a good one and compare the cubic feet per minute air removal rates one from another. Remember the length of the air duct taking the air from the fan to the exit point impacts on the efficiency of the fan so keep that in mind. Also you need to have a fan that is sized properly for your bathroom and there is a formula for doing that.
The calculation is based on the assumption that the goal for bathroom ventilation is eight complete air changes per hour. Take the square footage of the bathroom times ceiling height to get the total cubic feet to be ventilated. Say your bath measures 10 feet wide and 12 feet long with an 8 foot ceiling. 10 x 12 x 8 = 960 cubic feet. Take the cubic feet and divide by 60-- number of minutes in an hour. Take that number and multiply it by 8-- the target is 8 complete air changes each hour. It goes like this: 960 (cubic feet in the bath) divided by 60 minutes is 16 times the 8 desired air changes per hour comes out to 128 cubic feet per minute (cfm).
Bath fans start out at about 50 cfm and are cheap-- I saw one for just under $14. in that range. Iíll bet your old bath fan was in that range because the building code as I read it says ventilate but not how much so if a builder puts in a cheap one he has satisfied the requirement and heís good to go. Youíve seen how good that is. The bath fan youíd need for our example will cost upwards of a couple of hundred dollars.
But the plot thickens even more. There is another aspect to consider when bath fan shopping and thatís noise. Cheap fans are loud. The noise fans make while operating is rated in something called sones. We donít really need to know what just one sone is but suffice it to know the larger the sone rating the noisier the fan. Iíve seen common, off the shelf fans range in sones from 1.3 to 4.5. Pay attention to that too. Noisy fans are irritating.
As I said earlier people tend to shut everything off when they leave the bath. Consider putting the fan on a timer so it can be set to run 15 or so minutes after the bath has been vacated. And lastly, a mildewcide can be added to the paint prior to painting that will hold down any future microbial growths. But a properly sized, good bath vent fan is the first line of defense.
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