question about moisture migration
Hi all–I have a question about water vapor traveling up from a shower to the attic. Its a 2nd floor bathroom and its not ventilated. Above is a sheetrock ceiling and an attic floor insulated with fiberglass. Above that is plywood, attic space and the rafters have insulation and a vapor barrier. So the questions are: where does the water vapor go? Does it stay with the insulation in the attic floor and eventually rot the joists, even if the temperature is not freezing? Does if go into the airspace of the attic stay there because of the vapor barrier? How necessary is putting a ventilating fan in the bathroom? Dies anybody know?
(I tried posting this earlier, but I didnt see the message so Im sending it again. Sorry if its a repeat). Thanks for your help.
Rob
Replies
Not a moisture expert, but I'd say venting the bathroom is very important (at least here in So. Cal. it would be!). Trapped moisture can lead to lots of problems over time. I would think frequency of shower/bath use and ambient humidity would figure into the equation.
"...never charged nothing for his preaching, and it was worth it, too" - Mark Twain
It goes between the insulation on the attic floor, and the bathroom ceiling (e.g., under the insulation itself).
Plus, the bathroom needs ventilation.
Hey Rob. I guess your from Boston. Fill in your profile and we'll all know for sure.
The vapor goes into the air in the house. Now the air in the house would probably have more relative humidity then the air in the walls and the roof. So, when the hot air rises or is pulled into the wall cavity the exess water vapor will be deposited on the first condensing surface.
This time of year (colder season) that is right in the middle of the fiberglass insulation. The most air goes up through the holes in the ceiling. There are lots of them. Seal them up to help the situation.
The sheetrock itsel is rather permeable and lets a lot of moist air through anyways but sealing all holes and leaks slows that down 90%.
Stu
Stu,
Don't confuse air movement and vapor diffusion. You wrote, "The sheetrock itself is rather permeable and lets a lot of moist air through anyways but sealing all holes and leaks slows that down 90%." Sheetrock is, in fact, an excellent air barrier -- unless we're talking about the sheetrock in a frat house, riddled with holes where the college students punch their fists through the walls. It doesn't let "a lot of moist air through." It is, however, fairly vapor permeable, at least before it is painted. But that vapor diffusion has nothing to do with air movement.
You are distinguishing between vapor that passes thru the material, and vapor that is carried by air passing thru holes... correct?
Excellent JLC article, by the way. Very valuable info for anyone in the business.
David,
Thanks for the kind words. Yes, I'm distinguishing between vapor that passes through the material, and vapor that is carried by air passing through holes. In wall and ceiling assemblies, the most significant vapor transport mechanism is by means of air leakage. However, a lot of builders get confused on the issue -- they talk about "vapor retarders," which, properly speaking, address only diffusion, not air movement, and confuse the function of a vapor retarder with an air barrier. In the case of Stu's post, he said that sheetrock lets air pass through it; he probably meant water vapor. How much vapor passes through the sheetrock depends on how many layers of paint it has, and the type of paint, as well as the level of indoor humidity. Unpainted sheetrock is vapor permeable; however, in most houses in most climates, the amount of vapor that passes through painted sheetrock is very unlikely to be significant enough to cause moisture problems in a wall or ceiling assembly.
How do you recommend sealing around electrical boxes? Also, window openings are usually just cut out and cased, so that would seem to be a big leak. Glue the board to the framing around the window?
David,
It depends upon many factors. What type of wall insulation is being used? Spray foam insulation creates an air barrier, whereas more care is required if fiberglass batts are being used. There are several brands of airtight electrical boxes on the market; some only work in conjunction with a polyethylene vapor retarder, while others can be used without poly. Air sealing details vary widely; many different methods work. Each builder has to decide how persnickety he or she wants to be. Some people don't worry about a little air; other people want the tightest house in the state. If you are following the airtight drywall approach, you're probably already using gaskets. Some people use spray foam between the window and the rough opening; others use gaskets. So, it depends on the materials you are using, the insulation system you are using, the climate, the drywall technique, and your own personal fussiness.
The others have provided good information on the migration and diffusion of moisture in structures and their elements.
Your question "How necessary is putting a ventilating fan in the bathroom?" has yet to be answered. Some building and/or mechanical codes require mechanical ventilation in bathrooms. Some allow an exception with natural ventilation (i.e. a window that opens) present. I consider it necessary in all instances, as a matter of good practice.