Do you need Vapor Barriers in a Garage? Everything you need to know

Aluminum foil sauna vapor barrier

Whether or not to install a vapor barrier in a garage has been a topic of discussion for many years. Homeowners and DIY’ers are usually the ones that bring this question up. However, even some building experts still haven’t nailed this concept down. So, in this post, we will talk about vapor barriers and whether you need it in a garage.

Most garages don’t need vapor barriers because the amount of vapor that diffuses through garage walls, ceilings, and the floor is not enough to cause wet walls that result in moisture damages. However, if you live in a cold climate zone such as Alaska, it’s a good idea to install a vapor barrier on the warm-in-winter side of the wall.

Here is what to expect in this article. We will take a look at the reasons why vapor barriers are not as important as some people make it seem. After that, we will discuss when it’s important to consider vapor barriers in your garage. And then later in the article, I’ll give you some recommendations and alternatives to vapor barriers.

For now, let’s talk a little bit about vapor barriers, so we are on the same page.

What is a vapor barrier? A summary

The vapor barrier concept was birthed in the late 1970s and started gaining traction in the early 1980s. Textbooks and architectural magazines wrote several articles about it, and that was when the concept of vapor diffusion and the need for vapor barrier in walls and ceilings became widespread.

After a few years, the vapor barrier became a requirement in the International Building Code, based on research by William Rose, an architectural researcher. That was when it became a necessity for every building. 

Here is why vapor barriers became a thing

According to research, water vapor can diffuse through a vapor-permeable material such as drywall. Just as the concept of heat transfer, water vapor transfers from a high temperature to a low-temperature region. This is known as vapor diffusion.

For instance, in the winter, there is a huge temperature difference between the inside of your home and the outside. The outside wall becomes very cold, while the interior of your home is heated. 

Due to this temperature difference, some of the hot, humid air in the living area, or our case, the garage, diffuses through the drywall. Once this hot, moist air hits the cold wall sheathing, it condenses back into liquid, wetting the wall sheathing. And this is a recipe for disaster.

Prolonged exposure of the wall sheathing to moisture can lead to moisture damages such as the formation of molds and mildew on the wall, and reduced thermal efficiency of your insulation. 

This is where vapor barriers or vapor retarders come in. They are materials that don’t allow (or allow a small percentage of) vapor to diffuse through it. It limits the amount of vapor that passes through the walls. 

Vapor barriers or retarders helps keep the wall sheathing dry at all times to prevent moisture damage.

They are installed on the warm-in-winter side of your wall. If your walls are insulated, a vapor barrier covers the insulation from the inside. If they are not insulated, they cover the wall sheathing. 

The most popular vapor retarders are polyethylene plastics or vapor barrier paints such as Barrier Coat Paint (on Amazon)

Why you don’t need a vapor barrier in a garage

I hope the summary gave you a solid understanding of how and why vapor barriers became a thing. Now let’s discuss why you don’t need a vapor barrier in a garage.

Vapor diffusion is not enough to cause moisture damage.

Yes. Everything we have discussed is true. Vapor can move through walls, and it has been proven by science.

However, vapor diffusion, especially in a garage, is not enough to cause moisture damage. Why do I say this?

Firstly, garages are rooms that have very low moisture levels. There are not a lot of activities in the garage that generates a lot of moisture. The garage is one of the driest rooms in our homes.

The activities that generate a considerable amount of moisture in our homes are cooking, bathing, laundry, or even breathing. All of these activities are not done in the garage. Unless, of course, you have repurposed your garage.

Secondly, even if there is a lot of moisture in the garage, only a small amount will penetrate through the garage walls into the wall sheathing. Even in the longer-term, this will not cause any moisture damage to the sheathing.

In essence, vapor barriers are not that big of a deal. You barely need it in your living area, not to talk of the garage. If you don’t have a vapor barrier installed on your walls, don’t worry. Your walls will do just fine. There are instances when you need to install a vapor barrier. I’ll talk about it in the next section.

Air leakages usually cause moisture damages in walls.

You should worry more about air diffusion than vapor diffusion.

Air diffusion or air leakage is when warm, humid air leaks through holes in your wall. Essentially, when there are hole punches in your wall. Holes in your walls may be due to the installation of an electrical box in your garage, a storage rack, air conditioner pipe holes, and the like. You get the picture.

These holes can leak high volumes of moisture, especially when you have a fan installed in the room. A fan forces the moisture through these holes even faster, and in no time, the wall sheathing starts developing molds and mildew.

This is usually the primary cause of moisture damages. Vapor diffusion through walls is just not enough to cause the formation of molds, just as I’ve explained earlier.

When is a vapor barrier necessary?

As I’ve already explained, vapor barriers are not that big of a deal as some experts make it seem. That is not to say they are totally irrelevant. Some situations require you to install a vapor barrier, and that’s what we’ll discuss in this section. Let’s get into it.

If you live in a cold climate zone

If you live in a cold climate zone, it’s good to install vapor barriers, even in your garage. Let me explain.

There is a big temperature difference between your home and the outside during the winter in a cold climate zone, such as Alaska. The outside becomes very cold and dry, while the inside is warm and humid.

Due to the huge temperature difference, there is a high rate of vapor diffusion through the walls. Over an extended period, this might cause moisture damages.

So, I recommend installing a vapor barrier if you live in a cold climate, especially Zone 7 and Zone 8. These are the coldest regions in the US.

If you have repurposed your garage

Gone are the days where we only parked our vehicles in the garage. Now the garage is used for a variety of activities, depending on the homeowner’s preference. 

If you have converted your garage into a laundry room, a kitchen, bathroom (I don’t know why some people do that), or into a space that generates a lot of moisture, it’s a good idea to install a vapor barrier on your garage walls and ceilings.

Another way to deal with the moisture generated in the garage is to install a dehumidifier. These are devices that absorb all the moisture in a room. This is a much better option, in my opinion. That’s because they’ll absorb the moisture before they even get a chance to diffuse through the walls.

Dehumidifiers are also excellent for the winter because it’s not advisable to use an exhaust fan or open the garage windows during the winter. You don’t need to tear down your walls and install a vapor barrier when you can simply use a dehumidifier.


Here are my recommendations if you want to keep your garage walls and ceilings in good condition.

Use a “smart” vapor retarder.

As I mentioned earlier, polyethylene (or simply poly) is the most popular vapor retarder material used today. It has been around for a long time and was once the only option available when the concept of vapor barriers was introduced. However, polyethylene is not the best vapor retarder option out there. Here is why.

During the summer, the outside air becomes warm and humid. Some of the vapor diffuses through the outside walls into the wall assembly or sheathing. Polyethylene plastic will keep the moisture trapped in the wall cavity, keep the wall sheathing damp, and prevent it from drying. And again, this can cause all the underlying moisture damages. 

This is where smart vapor retarders come in. Smart vapor retarders are materials that allow vapor to pass through it when humidity levels are high (mostly in the summer) and block vapor to pass through when humidity levels are low (in the winter). This allows the wall cavity to dry up in the summer and keeps the wall cavity dry during the winter.

CertainTeed’s Membrain “Smart” Vapor Retarder (on Amazon) is the most sought-after smart vapor barrier available today. It can differentiate between high humidity and low humidity seasons and adjust its permeability (the ability for vapor to pass through) accordingly.

Not only is the Membrain an excellent vapor retarder, but it also doubles as an air barrier, blocking all air leakages. I highly recommend it if you need to install a vapor barrier.

Consider installing an internal air barrier.

As I mentioned earlier, air leakage is the major cause of moisture damages in walls. Vapors get access directly to the wall sheathing without the need to diffuse through the drywall. For this reason, it’s extremely important to have an air barrier that blocks all that warm, humid air from condensing all the wall assembly.

CertainTeed’s Membrain retarder doubles as both an air barrier and a vapor barrier, as I mentioned earlier. So, it can be an excellent option for your garage.

Alternatively, you can inspect your garage walls and ceilings thoroughly and seal all holes with a putty filler such as Bondo All-Purpose Putty Filler (on Amazon) and then paint it with latex paint. 

This will prevent air diffusion through the walls and keep your garage walls dry at all times.

Do you need a vapor barrier on a garage floor?

Short answer, no. You don’t need a vapor barrier on the garage floor. There is no temperature difference on the floor that will cause vapor diffusion through the concrete floor or even a wood floor in the garage.

Do you need a vapor barrier in a garage ceiling?

There is not much vapor generated in the garage, as I’ve explained earlier. So, you don’t need a vapor barrier in the garage ceiling. Unless you have converted your garage into a kitchen, a bathroom, a laundry room, or a space that generates a lot of moisture, even for that, a dehumidifier can absorb all the moisture in the garage to keep the air dry.


 We have covered a lot of grounds on vapor barriers in general. In summary, you don’t need a vapor barrier in a garage because there is not a lot of moisture in the garage.

If you live in Climate Zone 7 or 8, then it’s a good idea to install a vapor barrier. You should rather pay more attention to air leakages in the garage than vapor diffusion through the walls.

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