Photo of the interior of a home under construction with exposed insulation partly covered by a vapor barrier and the caption "Do I need a vapor barrier?"

When constructing a house and installing insulation, you might wonder if you need a vapor barrier. While the idea of installing a vapor barrier seems simple, there are a lot of factors to consider.

Vapor barriers protect against mold and rot inside structures by preventing condensation and moisture from building up and damaging building materials. If a home is being built in a very humid climate, a vapor barrier can be an essential source of protection from problems caused by excess moisture.

There are a lot of different parts of your specific situation that can change how and where a vapor barrier is installed. Keep reading to understand how vapor barriers work and how to install one correctly in your home.

What Is A Vapor Barrier?

A vapor barrier, also known as a vapor diffusion retarder, is a material that is used to reduce moisture damage to a building. They can be effective against both indoor and outdoor humidity. Different environments have different rules for vapor barriers to keep your home from being damaged by moisture.

Vapor barriers are made of different materials, each one having a different permeability. The most common materials for vapor barriers are plastic or aluminum sheeting. But depending on your situation, even paint can function as a vapor barrier! The permeability of a vapor barrier refers to how much water vapor can get through. While it might seem like the obvious choice is the least permeable material, different vapor barriers are suited for different climates.

How It Works

Vapor barriers are pretty simple additions to your insulation if needed, but they are very effective in keeping away dampness. A vapor barrier works by stopping water vapor from traveling through the walls of a building. This decreases the dampness in the walls and reduces the chances of damage to the structure from excess moisture.

Interior vs. Exterior

When installing vapor barriers, one of the most important things to consider is whether it goes inside or outside of the insulation. In other words, do you install the barrier first and then the insulation, or install the drywall and insulation first and then add the vapor barrier just inside the exterior siding? Here is some basic information on where to put the vapor barrier:

Because the idea is to keep water vapor out of the building’s main structural elements, you need to install the vapor barrier closer to where the humidity and moisture are going to come in. Generally, in cooler climates, you want the vapor barrier to be on the interior of the insulation. In warmer climates, you want the vapor barrier to be installed on the exterior of the insulation. Why?

Photo of the interior of a home under construction with exposed insulation partly covered by a vapor barrier.
This home is in a cooler climate, so the vapor barrier is being installed right before the drywall, inside the insulation, keeping the moisture inside the house out of the walls. Copyright: ivas76

In colder climates, there is much more interior heating. This means that most of the moisture in the building is going to come from the inside, so you want the barrier to be protecting against that. In hotter, more humid climates, there is air conditioning on the inside of the building, cooling it down. This means that the protection needs to be against the outside.

Unsure of if your climate is considered warm or cool? Have questions about the moisture levels in different climates? Find more details here.

A Guide For You

It is helpful to know how a vapor barrier works and why there might be differences in placement. But you’re probably here because you really want to know if you, specifically, need to install a vapor barrier and where to install it. Right here is a complete guide on all of the conditions that dictate the specifics of a vapor barrier, so you can find out what you need to do in your situation.

Weather

As we briefly touched on before, the weather and climate that a building is in can have a great effect on where a vapor barrier is needed. The different climates that we will cover include cold climates, mixed-humid climates, mixed-dry climates, and hot humid climates.

Cold climate: Cold climates are those with very cold temperatures in the winter, even though they can sometimes have pretty hot temperatures in the summer. Because the colder months require a lot of internal heating in this climate, it is best to install the vapor barrier on the interior of the insulation. There may be more outside humidity during the summer months, but the longer winter will make an interior vapor barrier much more effective.

Mixed-humid climate: A mixed-humid climate has almost equal amounts of time each year with heating and cooling needed. Because it is a humid climate with a lot more water vapor to worry about, a vapor barrier is often required by the building code.

If the climate requires a bit more indoor heating during the year, an interior barrier is your best bet. If the area spends a bit more time in the warmer months, install the vapor barrier at the exterior. Each mixed-humid climate is different, so going with your area’s typical weather will ensure you get the best protection from moisture.

Mixed-dry: Mixed-dry climates make it easier to decide where to put your vapor barrier than mixed-humid climates do. Because this climate is dry and therefore less prone to water damage, vapor barriers are not necessarily required or needed. However, if a vapor barrier is required for the building, the general rule is to put it on the interior. This is the recommended placement because any humidity is more likely to come from the inside of the home than from the outside. Because of cooling in the dry, summer months and heating in the cold ones, the humidity will come from the inside more than the outside.

Hot-humid: Hot humid climates also make it very easy to know where to install the vapor barrier. This is another humid climate, so vapor barriers are often required in most areas. Because it is mostly hot and very humid outside the building, the vapor barrier should be installed on the exterior of the insulation. This is recommended to keep the outside moisture from getting into the less humid, cooler interiors and damaging the building’s structure.

Photo of the exterior of a home under construction with a vapor barrier installed, waiting for exterior cladding.
In a hotter, more humid climate, the vapor barrier goes on right before the exterior cladding of the house, keeping humidity outdoors. Copyright: pashapixel

Cladding Type

Now that we have covered all of the climates that could affect whether a vapor barrier is installed first or last, we can talk about how cladding type can affect the position and necessity of the vapor barrier you choose.

Cladding is kind of the “skin” of the building. The reason the cladding material in the building is important is that some of them are made with absorptive materials. In fact, in 2009, more than half of all new homes were built with absorptive cladding materials. The most common absorptive cladding materials are brick, stucco, wood siding, fiber cement siding, and stone.

The reason absorptive cladding materials can be problematic is that they retain moisture, keeping the interior of the structure damp. They can also release vapor from retained moisture that can cause all sorts of moisture issues.

When considering which side of the insulation to install your vapor barrier on, the cladding type can affect the decision, especially in cold and mixed-humid climates. If you have an absorptive cladding type in a mixed climate that’s more cold than hot, you might consider putting the vapor barrier on the exterior of the insulation anyway, to protect your cladding. In a dry climate, where vapor barriers are not necessarily needed, an absorptive cladding might make you second guess leaving out a vapor barrier, while a cladding that is not absorptive can further strengthen your decision to not install one.

Wall Location

In general, vapor barriers are going to be installed in exterior walls, where the inside of the building is on one side, and the outdoors is on the other side of the wall. In a mixed-dry climate or a cold climate, though, it might be worth considering whether you want to put a vapor barrier in every wall, exterior and interior.

In all other climates, it’s the ratio of outside to inside humidity that’s the concern, not the humidity of separate rooms, necessarily. That said, having vapor barriers on interior walls can enhance moisture protection for the building overall.

Permeability

The last important factor to consider regarding vapor barriers is the permeability that you want it to have. We have briefly mentioned that you shouldn’t just get a vapor barrier with the lowest permeability to keep out any water vapor. The trick is to find one that fits the ratio of interior to exterior humidity. This will keep moisture out of the wall while letting enough through so that it never gets trapped.

In warmer climates and humid climates, you want higher permeability, so more moisture can flow through without getting trapped. Because there is more humidity in warmer weather, you don’t want that moisture building up in your walls. The interior moisture needs to escape, and the best way to do this is with a highly permeable vapor barrier.

A vapor barrier with low permeability is better suited for dry climates. While you always want some permeability in the vapor barrier, a less permeable barrier works well in dry climates because it does not have as much moisture to manage. It can let the little moisture on the inside out while keeping any more moisture from getting into the walls.

Warnings

The word “warnings” may sound a little extreme, but vapor barriers are not perfect devices. They come with their downfalls that you need to watch out for so that you can keep your home safe.

While the idea of a vapor barrier is meant to solve excess moisture problems, there is always going to be moisture present. Because of this, sometimes vapor barriers can contribute to mold and other moisture damage. They might accidentally hold moisture in the places that you do not want it to be, allowing it to cause problems.

This is why, if in doubt, it is usually better to err on the side of a highly permeable vapor barrier. While the idea is to keep moisture out, trapping it in can become a much bigger problem. Making sure any water vapor can get out is much better than trapping it inside where it gets stagnant and moldy. Proper installation is imperative.

Conclusion

Installing a vapor barrier can bring a lot of benefits to a home or building. It might take some extra work, but in some locations it’s required by the building code. And it may be advisable, even if it’s not required.

Using the information we have provided about different climates, cladding materials, and wall locations, you should be able to make your own judgment on whether you need a vapor barrier. If you think interior vapor barriers might be right for your home, a professional contractor can help you decide.

Now that you understand vapor barriers, how they work, when to use them, and where to apply them, you can keep your home better protected from water vapor and the problems that moisture can cause.

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