Vapor Barrier Paint

January 25, 2008
vapor barrier paint
You want to install hardwood flooring in your basement but hear you can’t do that because of moisture problems so often found in basements. A vapor barrier can be the solution to that dilemma. And vapor barrier paint is one alternative for basement moisture problems.

Vapor barrier paint is yet another weapon in a homeowner’s arsenal to fight against damage caused by water vapor. Vapor barrier paint is made by many paint manufacturers in different grades, as we will see. There may be reasons you can’t or won’t use traditional vapor barriers for your floors before installing wood or carpets in your basement. That’s one cases when vapor barrier paint is great.

It is important to understand how water vapor acts in order to understand the uses of water vapor barrier paint. In a process called convection, warm air always moves toward colder air. Thus, during cold months, the warm air in your house is constantly seeking to escape outdoors. Warm air also expands, putting more space between its molecules compared to cold air. Expansion allows warm air to hold more water vapor. When this warm air encounters the cooler surfaces of your home, such as exterior walls, it cools down and contracts, squeezing out water that collects on the cool surfaces.

Underslab and in-wall water vapor barriers are designed to keep water out of your home. They defend against groundwater seepage, rainfall, and floods. But having water form inside of your home is also bad. It is here that water barrier paint plays a role in protecting your home’s interior surfaces.

Water vapor barrier paint is often applied to drywall walls and ceilings in attics, where warm air tends to rise. Without water vapor barrier paint, the warm air would release its water inside of the walls and ceilings, where the water would saturate the insulation and framing. This water damage can render insulation useless and cause rot in structural members.

Not every household paint or enamel is an effective water vapor barrier, though. In order to qualify as water vapor barrier paint, a product must attain a Perm rating less than 1.0 under the ASTM D1653 Test Method for Water Permeability of Organic Coating Films. The lower the Perm number, the less water vapor the barrier paint allows to pass during a given amount of time at a given temperature and humidity. When shopping for water vapor barrier paint, ask for it by name and check the Perm rating which should be on the can.

Water vapor barrier paint is often sold as a primer, to be applied under a final coat of a different paint. The primer is generally sold in white. It can be colored to some extent without degrading its water vapor barrier properties. Usually, the can’s label will tell you how many ounces of pigment can be added to a gallon of water vapor barrier primer. The instructions will also state the maximum surface coverage of a gallon, and whether one coat or two are recommended.

Water vapor barrier paint is also used directly on concrete floors. Products such as Aquapel, made by L & M Construction Chemicals of Omaha, Nebraska, penetrate porous concrete and chemically bond with it to seal out water vapor.

Water vapor barrier paint is not a substitute for other water vapor barriers when they can be installed. In some cases, such as cinder block construction, it is impossible to install conventional water vapor barrier sheeting behind exterior walls. In these cases, a high-grade (low Perm) water vapor barrier paint is essential.

Older homes in need of water vapor barriers are also good candidates for water vapor barrier paint. It would be a bad idea to try to pull back the insulation in an old home’s ceiling and try to install plastic behind it. It makes much more sense to paint the ceiling with a good water vapor barrier paint.

Water vapor barrier paint complements other forms of water vapor barriers and can be a valuable tool in insulating older homes.


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