Bathroom fans Steam from shower

Published on April 25th, 2016 | by Henri


Bathroom steams up in newly remodeled house

Q.  We have a 5-room ranch house built in the 1950s. When we moved in there was no insulation or ceiling fan in the bathroom. We redid the bathroom about 14 years ago. It was gutted and insulated, new walls, floor, ceiling, ceiling fan, etc.

For the first 7 or 8 years it was fine with moisture. Then, about 7 years ago we had the whole house insulated and re-sided, new windows, roof vents, and soffit vents. Ever since then we have terrible moisture in the bathroom when the shower is on. The water just drips from the ceiling and walls. We tried to address this by installing a larger ceiling fan (110 cfm exhaust fan) but the moisture has not diminished. Now around the tub, the wall is starting to rot and the mildew is terrible on the ceiling and walls. We constantly have to wash them down with Clorox and repaint.

We have had 2 contractors who have gone in the attic and on the roof to check and don’t know why we are having this problem. We leave the bathroom window cracked, use the fan, open the door a little, and leave the door to the laundry chute open. We have had everything checked, looking for holes in the vent pipe, checking the outside vents. Please help with any suggestions.

A.  You have done a lot to tighten your house — an important improvement. But in doing so, you have increased the potential for moisture accumulation, as you have cut down the number of air exchanges that used to take place before the improvements. The relative humidity (RH) in your house is now over the top. You may want to buy a humidistat in a hardware store to check it out. The goal is to lower the RH to no more than 30 percent in the winter.

The first thing to look at is the efficiency of the bathroom venting system. If the vent has any sharp bends or droops in one or more sections, there may be an accumulation of condensate that renders the vent useless. I have seen this in condos and houses in which the vent loops down before discharging through the roof or a high point through a gable wall hoping this will catch the condensate so it won’t run down onto the fan. While this is an interesting approach, it is  a serious mistake as the condensate accumulates over time. The bottom of these loops fills with water, acting as a plumbing trap does to keep sewer gases from entering the house.

The best way to vent a bathroom fan is with a rigid plastic pipe having a slight slope down from the fan exhaust to the outside wall jack. Use one or more 10-foot sections of 4-inch schedule 20 drain pipe. Make sure the bell-ends face toward the fan and not the outside. Come up from the fan just high enough to obtain a gentle slope to the outside wall. Place small wood blocks under the pipe, nailed to the floor joists, reducing their size as the slope progresses. Terminate the pipe into an aluminum wall jack as low as possible in a gable wall.

Once the pipe is installed, it should be insulated by placing fiberglass batts tightly on each side and on top to reduce the possibility of condensation.
Bath and kitchen fans should never be vented into attics, soffit or ridge vents, and in climates where heat is prevalent, they should never be vented through the roof. They should be vented through gable walls, and preferably those facing south. You should also leave the bathroom door wide open after showers to help dissipate the moisture into other rooms.

You mention roof ventilation, but did not mention what it was. The most effective attic ventilation is a combination of continuous soffit vents at all eaves, a full-length, externally baffled ridge vent installed on the right size opening at the ridge, and an unimpeded air flow between the soffits and the ridge.  The ridge vent needs to be externally baffled to direct the wind over the ridge instead of allowing it to enter the ridge vent itself and stop all attic ventilation.

The next thing to look at is your lifestyle. Is your dryer vented to the outside — and not clogged — or do you dry clothes on a rack? If you use a humidifier, whether it be a table-top type or integral to your warm air furnace, shut it off. Do you have a lot of water-loving plants? Do you store firewood in the basement? Is your basement (or crawl space) dry? Do you take extra-long or frequent showers? Does your cooking produce a lot of steam? How large is your house and how many people live in it? Do you have pets?

You may also want to consider an air-to-air heat exchanger for the bathroom or the whole house.

It will take a long time for the moisture content that has accumulated in all the building components behind the new exterior siding,  and your furnishings, to dissipate.

All those questions need to be addressed in order to reduce the humidity in the house, and they should be soon. Your house is at risk of significant structural damages.

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