preventing moisture in insulation in bathroom
chuchelo
| Posted in Energy, Heating & Insulation on
I am remodeling a bathroom in an older home, which seems to have been an enclosed porch at one point and had some major moisture issues. After tearing through layers of paneling and plywood, I am down to the studs and staring at the sunlight coming through the cracks in the outside wooden siding. The question is, what would be the most efficient and/or best way to deal with the insulation and moisture issues coming in from the inside and the outside of the walls? It is in zone 6a in Indiana
Replies
Inside, the main thing is to not have the shower leak. Plus you need a good exhaust fan (that exhausts all the way to the outside, not simply into the attic).
Outside you want siding that keeps the rain out, possibly assisted by some sort of drain plane.
Water vapor/condensation
Most bathrooms are high in both temperature and humidity and because most places are colder and drier outside for most of the year - we insulate and install water vapor proof plastic sheet over the inside of the frame, to stop the warm wet air indoors heading inside the walls trying to reach the colder outer wall covering and condensing/being absorbed by the wood.
In your case, as it is often hot and humid outside, you have the added problem of the water vapor outside, heading inwards trying to find somewhere cold and where better than inside your walls where it condenses/is absorbed by the wood, leading to mold and possible wood rot.
Wood that is warm (above the dew point) never has condensation, nor does wood that is varnished or gloss painted, water vapor will settle on the varnish and gloss paint, if the wood is below dew point - but it does no harm, when the sun comes out it dries up - before coming back again as the day warms.
Your task is to varnish the frame etc to stop water vapor damage, and then to insulate the frame by fixing a one inch layer of polystyrene over the room side of the frame followed by a water vapor proof layer of plastic sheet (carefully sealed - no cracks or holes) followed by another one inch thick layer of polystyrene and drywall. Leave some cracks in the wood siding for the warm wet outside air to come in and go out, as it will with the heat of the sun and the cold at night.
This way you will have a more comfortable bathroom, not so hot, not so cold. And the water vapor driven inside the walls by the sun will not be causing any harm.
Where do you get this stuff?
Varnish the framing? Two layers of 1" interior insulation with a plastic sheet between them??
Sounds like your building
doesn't have building wrap, felt, or sheathing on it. If it wasn't originally built as a wall of the house (but was a wall enclosing an unheated porch) then it may be best to remove the siding, install sheathing, and perhaps install rigid foam over the sheathing. Then, insulate the stud bays with fiberglass or cellulose before drywalling.
If you're not going to remove the siding, add sheathing and rigid insulation, and then replace the siding, the best approach is to install rigid foam in the stud bays. I'm guessing you have 2x4 walls, in which case you would get 3" foil-faced polyiso, cut pieces to fit into each joist bay, and then foam all the way around the edges for an airtight seal. Hold the foam to the inside plane of the wall, so there's an air space ~1/2" thick between the outer face of the insulation board and the back of the siding.