High indoor humidity from excess moisture in your home can be caused by external moisture entering your home and/or by moisture generated inside your home. Since your home is older, the problem is probably not caused by extreme airtightness—which can be a problem with some new superefficient homes that are being built today.
Check for any leaky plumbing. The wax ring under toilets or the seal between the tank and the bowl sections are common leaks that often go unnoticed. If you hear the toilet running often, it is a good indication that there may be a leak somewhere. Wax rings do begin to leak sometimes, especially if the toilet is not bolted down tightly and rocks easily.
If you don’t find any leaks, try running your kitchen and bathroom vent fans more often when those rooms are used. Children often forget to use the bathroom vent fan, so consider installing one with a motion and humidity sensor. It will automatically start and run only as long as needed.
If, after you check out these areas and fix any problems, you still have the musty odor problem, moisture is probably entering your home through the slab, crawl space, or basement floor and walls. A dirt crawl space floor is an obvious source. The concrete slab and floor are less obvious. Keep in mind that concrete is a porous material, so moisture will readily pass through it.
Start with the crawl space. The floor should be covered with a vapor barrier material, which will block the moisture flow from the ground floor. The vapor barrier film should cover the entire floor and run several feet up the walls to the band joists. Staple it to wood strips and then nail the strips, trapping the edge of the film, to the joists.
Four-millimeter or six-millimeter (0.006 of an inch) polyethylene film is most commonly used for sealing a crawl space floor. It is usually available in 100-foot-long rolls of various widths of up to 20 feet. You can plan on spending about 2 cents to 3 cents per square foot for this type of film.
A much better type of film to use is cross-laminated polyethylene film. It is actually multiple layers bonded together. This makes it much stronger and eliminates most pinholes that allow some moisture to leak through. It costs about 9 cents to 10 cents per square foot. It sounds like a lot more, but it may come to only $50 more overall.
If you have any problem finding this type of film, contact Fortifiber Corp.
Applying a sealer compound to the basement floor and walls is the best do-it-yourself method to stop moisture. It is not as effective as film, but it will stop the majority of the moisture. Use one of the new water-based sealers because they are safer to use than solvent-based ones. Two of the sources for this are Tamms Industries and Laticrete.
Hopefully sealing the basement and crawl space will do the trick. The simplest method to seal the slab, if you still have a problem, is to remove the carpeting and install film over the slab. Reinstall the carpeting.
