Vapor Barrier
Credit: Opensource

Moisture in a home can lead to serious issues, but where does it come from?

Believe it or not, our normal activities produce the majority of humidity and moisture in the air inside our homes. Things such as cooking, baths and showers, fish aquariums and even breathing can inject a lot of moisture into the indoor atmosphere.

This moisture can pass through the ventilation system and walls and saturate existing insulation in the walls and ceilings reducing its effectiveness. If the insulation in the ceiling becomes soggy, the added weight can eventually cause your drywall ceilings to sag, or the dry wall tape to become detached.

Moisture in the home can also damage your property causing any plastic or paint to weaken and flake, cause mold and mildew issues, or lead to structural damage from weakened wood supports if the moisture issue is serious.

Effective vapor barriers and good ventilation are important for a well-insulated home. A vapor barrier is a material such as asphalts coated paper, aluminum foil or long sheets of 4 mil plastic that stops airflow through the walls of a home.  

Vapor Barriers

If you are considering installing a vapor barrier in the walls, attic or crawl space of your home, it must be placed on the warm or living side of the existing insulation.

Vapor Barriers in Walls

For an external wall, you want the bat insulation between the wall studs to go in first, then seal the wall with a 6 mil plastic vapor barrier with the seamed closed off with tape. Then apply the dry wall with dry wall tape covering the seams, then cover with a latex paint.

If your home already has an existing vapor barrier in the walls or attic and you would like to add additional insulation, do not put it over the vapor barrier sandwiched between the older insulation and the newer bats you are about to install. You should first remove the vapor barrier before adding new insulation in this scenario.

If the vapor barrier is attached to the inner side of the insulation, simply slash though it with a knife.

Vapor Barrier in Attic

If you are going to install a vapor barrier in your attic, it should be placed underneath any blown-in insulation. If your home builder failed to install a 4 mil plastic barrier first, this type of DIY project is going to be a bit more extensive because you will need to remove all of the existing insulation first, then staple sheets of 6 mil polyethylene between each ceiling joist. This is no small task when dealing with blown-in insulation. Vapor Barriers in WallsCredit: Opensource

Plastic barriers in the attic aid in the ventilation of the home and reducing excess moisture. They should be used in combination with proper soffit vents in the eaves of your roofline and a ridge bent at the peak of the roof to allow the home to breath.

If your home does not have a vapor barrier in the attic, it will require more ventilation on the eaves, typically a continuous soffit vent.

Vapor Barrier in Crawl Space

When installing a barrier in a crawl space, the process is a little different. Most professional companies that seal crawl spaces will place a 6 mil plastic sheets covering the ground and extending up the masonry walls. Each joint is sealed with tape.

The plastic is tucked under bat insulation that is installed in between each floor joist. You do not want to place a vapor barrier behind this insulation as it will produce condensation on the inner side of the barrier allowing water to pool in the plastic.


Vapor Barrier and Insulation
Credit: Florian Gerlach via Wikimedia Commons

Plastic barriers go hand in hand with good ventilation in the attic and crawl spaces. Vent openings in the attic and crawl spaces should be placed across from each other to allow for cross ventilation from one area to the other. Combined with ridge vents at the peak of the roofline, cross ventilation is designed to expel moist and hot air from the attic by convection which is the process of hot air rising over cooler air.

If this type of passive ventilation is not effective or practical with your existing home, you may consider installing an attic fan which can help circulate the air allowing for convection to occur.

In colder climates, the purpose of ventilation in the attic is a little different. Proper venting is needed to keep the attic consistently cold in order to prevent ice dams on the roof caused by melting snow that can clog gutters and lead to damage. A warm attic in the winter is typically caused by heat loss from the living space. This type of leakage is normally caused by a missing vapor barrier above the ceiling or duct work located in the attic that has not been sealed properly.

If your home is built over a crawl space, proper ventilation is equally important.  Most building codes require that foundation cross vents should be spaced ever 150 square feet of area.

During hot weather, the foundation vents should be kept open to allow cross ventilation. However, during colder months, the vents should be closed to prevent cold air from circulated under your home.

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Proper Home Ventilation


Proper ventilation inside and outside the home in the attic and crawl spaces is imperative to allow a house to breathe and reduce energy costs.

It is imperative that you have quality exhaust fans installed in each bathroom and in the kitchen above the stove. This will remove excess moisture from the home through vent stacks in the roof or through the siding of the home.

Additionally, if you own a large aquarium, you might consider adding a dehumidifier in the room where it is located to reduce moisture in your home.

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