Do You Need A Vapor Barrier In Garage Ceiling? (When To Install)

Last Updated on October 9, 2022 by Barry Gray

The use of vapor barriers is often debated in construction circles, and the need for this construction element is usually dependent on several factors, including the climate of the region in which the building is located. Vapor barriers in ceilings are considered necessary by some people and highly unnecessary by others.

Because a garage is generally not used for activities that produce a lot of water vapor – such as cooking, laundry, and showering – the amount of moisture produced in the space is generally not sufficient to cause water damage and therefore warrants the need for a vapor barrier.

While it is generally considered necessary to have a vapor barrier in a wall, the decision to install a vapor barrier in a garage ceiling is an often debated topic that seems to have no definitive answer at the present time. It is purely dependent on the circumstances surrounding the specific ceiling in question.

Do You Need A Vapor Barrier In A Garage Ceiling?

Generally speaking, a vapor barrier is not necessary for a garage ceiling. While it may be useful and helpful in some settings to make use of a vapor barrier in your garage ceiling, there are other instances where a vapor barrier in a garage ceiling may actually do more harm than good. 

Under normal circumstances, the amount of water vapor produced in a garage (and therefore moves through the garage walls, floors, and the ceiling) is simply not enough to cause significantly damp conditions in the structure of the building that would potentially result in moisture damage. 

The activities in a garage are not usually the type to create significant amounts of vapor. Vapor-producing activities would be considered those activities such as bathing, washing laundry, and any other activities that involve significant amounts of water and, therefore, moisture.

Under circumstances where a garage has been repurposed, and the activities that then occur within the garage start to produce significant amounts of moisture, one might consider using a vapor barrier. 

Bear in mind, however, that a dehumidifier can generally absorb the majority of this moisture in a garage, thereby eliminating the need for a vapor barrier. A vapor barrier can have the benefit of keeping the ceiling dry, but it can simultaneously prevent the ceiling from drying out properly.

It is generally agreed upon that if there is good ventilation within the attic space above the garage ceiling, there will be no need for a vapor barrier. This is because any water vapor that makes its way through the ceiling and insulation will essentially remain water vapor while moving through the attic space and eventually moving out through the vents.

Bearing the above in mind, you can still make use of a vapor barrier in a garage ceiling if the space above the ceiling is ventilated. This will also depend on whether the garage space is heated or not.

As the warm air in a heated garage moves towards the cold air in the ventilated attic space above, a vapor barrier will help to prevent condensation from reaching the insulation. Moisture problems in your ceiling insulation will cause major issues in the long term.

In an unheated garage with a heated space above it, such as a bedroom, your focus should be more on ensuring airtightness and proper insulation so that the rooms above the garage are not subject to any draughts moving up through the garage. This is because garages tend not to be particularly airtight spaces.

What Is The General Purpose Of A Vapor Barrier?

Water vapor is able to move through drywall and other such permeable materials. It generally moves from a higher temperature zone to a lower temperature zone through a process called vapor diffusion. A vapor barrier, therefore, limits the amount of vapor that is able to diffuse through a wall or ceiling.

In general, the purpose of a vapor barrier is to prevent the humid air inside a building from moving through the insulation and subsequently condensing as soon as it reaches the colder external temperatures during winter.

Insulation that is constantly exposed to moisture can become destroyed from freezing or from matting as a result of the presence of dew. Resultantly, mold will start to grow, and this is, of course, an extremely undesirable occurrence.

Vapor barriers also serve the function of preventing external air from entering spaces that are air-conditioned. Because air-conditioning is essentially de-humidification, the lack of a vapor barrier in an air-conditioned space will cause major energy wastage. 

This is because the external humidity is able to make its way back into the space almost as quickly as it leaves the space.

When Is It Generally Necessary To Have A Vapor Barrier?

It is generally considered necessary to have a vapor barrier installed (generally in the external walls) in areas with cold climates. Cold climates mean significant temperature differences between external spaces and internal spaces. The inside of the house is generally warm and humid, while the outside of the house is generally cold and dry.

As a result of this significant temperature difference, there is a relatively high level of vapor diffusion that occurs, which can potentially cause moisture problems within the structure of the house.


Generally speaking, the need for a vapor barrier in a garage will depend on several factors. Firstly, the way in which the garage is used, i.e., the activities carried out therein, will play a major role. Secondly, the climate in which the garage is located will also influence the necessity for a vapor barrier.

Another major influencing factor is the type of space above the garage ceiling and whether these are occupied spaces or simply unused attic space. There is much debate that occurs around the topic of vapor barriers, especially when it comes to ceilings. 

When it comes to construction questions such as the necessity for a vapor barrier, it will always be in your best interest to consult with local building regulations in order to understand the best way to go forward in these types of scenarios.

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Barry Gray

Hi, I’m Barry. I’ve loved woodworking and bringing things back to life for more years than I care to remember. I hope my passion for tools comes across loud and clear in everything you read here on The Tool Square.