How do we install galvanized metal roofing in bathroom as 4 ft wainscoting and prevent condensation on back side of it?

Question

We are running the galvanized metal roofing horizontally as 4ft high wainscoting in main bathroom. There is a shower in this bathroom and we have no windows but will install a vent fan. We will use drywall above the metal. The other side of this wall is drywall top with 4foot wainscoting of old bead boards (boards over 150 years old)with no stain or coating on them. Aging on the metal will only add character in our opinion. This is in a log house. The ceiling in the bathroom is the 5/4 flooring and log beams.

Answer 1

Consider the physics: condensation happens where surface temperature is below the dew point of adjacent air. Dew point is related to relative humidity, so we control the relative humidity by controlling the amount of water in the air and/or by controlling the temperature of the air and surfaces.

You've planned for a vent fan, which controls the amount of water in the air by expelling vapor-laden air and replacing it with dry air. It isn't particularly easy to calculate how many CFM will be required, but at least we can say "don't choose the smallest/cheapest fan." Also make sure that there's an easy way for make-up air to enter the room -- the fan will be impaired if pulling in fresh air is difficult.

You might have noticed fogging begins at the top of the mirror and progresses downward. That's because the humidity isn't uniform. It is highest near the ceiling and lower near the floor. Fortunately, your project is putting the metal down low where humidity is lowest.

The surface temperature on metal roofing will be the same on both sides -- but the humidity level doesn't have to be. One strategy to control condensation on the back side of the metal is to seal it well so that humidity from the bathroom can't reach the rear side of the metal. I wouldn't recommend this approach though. Water and vapor seals are never as good as hoped or expected, especially when designed and/or constructed by a novice. They're prone to failure over time as well.

Design your construction to deal gracefully with the possibility of condensation. Think about the amount of condensation that might occur. If this is a seldom-used guest bath then maybe no special attention is needed, but if this is the primary bath in a cabin where a family of 10 people may come home wanting back-to-back hot showers then much more care is warranted.

These ideas borrow from concepts that are usually applied to exterior cladding systems:

1. Use a drainage plane behind the metal: roofing tar paper or house wrap.
2. Mount the metal to support drainage: Orient the corrugations vertically so that liquid water isn't trapped. Maintain a gap between the metal and the wall behind it.
3. Use a substrate behind the metal that has some ability to buffer water without damage such as green "wet areas" drywall or tile backer board.
4. Arrange the details at the bottom of the wall so that any water that drains down the back side runs out onto the floor in the room where it can easily evaporate or be wiped away.
5. If there will be a baseboard below the metal, use a material that won't be damaged by a little bit of water (ie not the cheap fiber composite stuff). Use solid pine or other wood, or possibly one of the PVC baseboard materials.