Founder | BPI® Energy Auditor Certified Professional

A vapor barrier can be an effective solution to prevent various problems caused by moisture around, in, and under your house. The most concerning issues are energy inefficiency, humidity damage, and mold.

They aren’t the same as house wraps or crawl space encapsulation, which is typically more complex than the typical barrier installation.

So, how and where should you use a vapor barrier?

Before installing one in your house, there is a lot to know about vapor barriers, so read on for answers to all your critical FAQs in this ultimate guide.

Table of Contents

When Should a Vapor Barrier Be Installed?

A vapor barrier is necessary for several types of flooring and insulation. The crawl space is a significant source of moisture, so it should have an encapsulated vapor barrier. They are also necessary for some external and internal walls.

Does Mold Grow Under Crawl Space Vapor Barriers?

Mold can grow in a crawl space with a vapor barrier, as some moisture will always accumulate underneath it. However, if there’s little to no food or airflow, the mold shouldn’t become a problem.

Installing a vapor barrier in a crawl space can prevent mold growth. However, mold might still manage to grow in several scenarios.

A crawl space vapor barrier prevents the diffusion of moisture from the ground to your property’s floor or subfloor. Still, there will always be some moisture underneath the barrier, whether you’re dealing with concrete or earth.

Since you cannot wholly remedy this moisture problem, mold growth is likely under the crawl space barrier. But the extent or severity of mold growth depends on the airflow over the moist ground.

A lack of or minimal airflow will prevent mold from growing unchecked under a vapor barrier. Mold needs air to grow because these fungi are aerobes: they need oxygen to survive.

They’ll also find it hard to grow if there isn’t any food—in this case, cellulose.

The barrier isn’t the only factor in play here. The encapsulation of the crawl space and the availability of organic matter, such as green plants, paper, wood, etc., will determine the extent of mold growth. A well-encapsulated, organic matter-free crawl space won’t have mold issues.

Should a House Crawl Space Be Vented?

A crawl space should be vented if it isn’t encapsulated. However, an encapsulated crawl space with vapor barriers is better if it isn’t vented. This way, humid air won’t be able to come in. Any moisture that makes its way through can be removed with a dehumidifier or a sump pump.

For decades, many building codes mandated that a house crawl space be vented. But a crawl space encapsulated and sealed with a vapor barrier offers more effective protection from moisture. Of course, you must have a dehumidifier and sump pump for the crawl space.

Suppose you lay and seal a vapor barrier on the crawl space ground. A vented crawl space will continue to have humid air flowing into and over it, so it won’t protect the area above it from moisture.

This is why you should consider encapsulating the crawl space with vapor barriers. Encapsulating differs from laying a barrier on the floor—it covers the entire ground or slab foundation and the load-bearing columns or structural pillars.

Encapsulation seals the entire space under your house. Whatever moisture seeps into the sealed crawl space is eliminated using a dehumidifier, and any water runoff during rains or storms is drawn out using a sump pump.

In a flood-prone area, homeowners can consider using trench drains to redirect surface water. In case you’re worried about groundwater puddling under the vapor barrier of an encapsulated crawl space, consider installing French drains to channel that stream away from your house.

Is Tyvek a Vapor Barrier?

The Dupont Tyvek HomeWrap isn’t a vapor barrier. This bestselling house wrap is a non-woven and breathable material, so it’s permeable. The Tyvek HomeWrap has a permeability of 54 perms.

Two workers installing a Dupont Tyvek HomeWrap on the second story exterior of a home — Courtesy of Dupont

Vapor barriers or diffusion retarders can have a maximum permeability of 10 perms.

Here are the permeability ranges for different types of vapor retarders:

Class I: less than or up to 0.1 perm
Class II: between 0.1 and 1 perm
Class III: 1 to 10 perms

Some house wraps can be vapor barriers if their permeability is lower than ten perms. At 54 perms, it’s clear that the Tyvek HomeWrap doesn’t have the purpose of being a vapor barrier.

Does Fiberglass Insulation Need a Vapor Barrier?

Fiberglass insulation needs a vapor barrier in most regions of the US. Otherwise, you’ll have problems such as ineffective insulation, mold growth, and rotting frames.

Standard fiberglass insulation is unfaced, but you can get kraft-faced fiberglass insulation. The batts have kraft paper on one side, serving as the vapor retarder. You can also install it on exterior walls. However, kraft-faced fiberglass insulation has a few potential issues.

Since the kraft paper is part of the batts, any problem with the latter will also affect the efficacy of the barrier.

This leads to several common problems:

Cracks or gaps in fiberglass insulation due to loose batts or poor framing
Compressed or jammed fiberglass batts affect insulation and protection against vapor
Installing more than one layer of kraft-faced fiberglass batts may trap moisture or vapor

A more effective and practical option is to get a proper vapor barrier that is not part of the fiberglass insulation in your house. This barrier should be flawlessly sealed or taped at all seams.

How Thick Should a Vapor Barrier Be?

The thickness of a vapor barrier should be around 10-12 mils (0.25-0.30 mm). You can use a six mil (0.15 mm) vapor barrier if it is a Class I type with a permeability of fewer than 0.1 perms. However, this thickness may not be viable for crawl spaces or extremely humid regions.

A vapor barrier roll, staple gun, and some staples resting atop a piece of plywood

If budget isn’t a concern, you should go for 15 mils (0.38 mm) thickness for a Class I vapor barrier. Apart from the thickness and permeability, you’re also getting an incredibly reliable material.

The proper thickness for your barrier depends on why and where you need it. For example, a vapor barrier in a crawl space is exposed to more moisture without assistance from any other fixture, so you’ll need a thicker and less permeable material.

Compare that to a vapor barrier between the insulation and drywall. Again, you may be alright with a bit thinner or more permeable barrier in most areas, although it might not be enough for humid places like bathrooms and kitchens.

How To Install Vapor Barrier on Exterior Walls

A good rule of thumb is installing a vapor barrier on the outer face of an exterior wall in hot and humid climates. In colder climates, the vapor barrier should be on the internal side of an exterior wall, typically between the insulation and drywall, plasterboard, or wallboard.

A vapor barrier installed in a wall before the insulation and drywall is added.

This approach needs to be personalized depending on your needs, such as how much moisture and relative humidity you anticipate in your house. The home’s orientation also matters—for example, north-facing exterior walls tend to get more sun in the winter than in the summer.

Regardless of the specifics, you should install a vapor barrier on an exterior wall without any air gap. Air gaps can defeat the purpose of vapor retarders by trapping moisture during the installation process.

Additionally, it would be best if you considered installing vapor barriers on the interior walls of incredibly humid places, such as:

Basement
Bathrooms
Kitchens
Laundry closet
Utility areas
Water features

The usual way to install a vapor barrier is to call a professional. Expect the price to be between $1000-$1400. Of course, the rate will go up if you have a lot of surface area to cover.

If you have the necessary knowledge and tools and only need to cover a small area, you could do it yourself and bring the price down to $500.

What Is Stego Vapor Barrier?

Stego Industries, LLC. is among the leading brands manufacturing vapor barriers in the US. They offer premium-quality products in different thicknesses.

A glimpse of the Stego inventory includes:

Stego Wrap Vapor Barrier – it comes in various thicknesses: 10 mils (0.25 mm), 15 mils (0.38 mm), and 20 mils (0.5 mm). The barrier has Class I and II (A and C) variants. The permeance is as low as 0.0086 perms for a Stego wrap.
Stego has several barriers made specifically for crawl space encapsulation. The variants range from 6 mils (0.15 mm) to 15 mils (0.38 mm) in thickness. Stego’s Crawl Wrap, Tape, and Term Bar are all you need to encapsulate your crawl space.
Industrial-grade below-slab barriers. These are not only effective in preventing moisture permeance but also soil gas.

You’ll find at least one Stego barrier for every need.

Most Stego products are made of prime virgin resins and additives in a proprietary blend that exceeds all commercial construction standards—most notably, the ASTM E1745. Stego barrier rolls are usually available in two standard sizes.

Do You Need Vapor Barrier on Plywood Subfloor?

Most plywood subfloors need a vapor barrier. Of course, you can choose a plywood grade less prone to moisture damage, but a barrier is almost always necessary and recommended.

Using a vapor barrier on any plywood subfloor isn’t enough, though. The installers should be careful not to cause any damage to the vapor retarder while installing the floor. Puncturing any roll or sheet of vapor barrier will likely defeat the purpose of the preventive fixture.

This problem is prevalent for installations that involve nailing the hardwood onto the plywood subfloor. The nails can penetrate the barrier roll or sheet and the sealing tape.

Do You Need a Vapor Barrier With Rockwool Insulation?

Rockwool, also known as mineral wool and steel wool, doesn’t necessarily need a vapor barrier. Generally, Rockwool insulation is moisture-resistant, vapor-permeable, and water-repellent.

Close-up of worker hands in white gloves installing mineral wool insulation staff in wooden frame for future walls for cold barrier.

Rockwool or mineral wool insulation is superior to fiberglass, and that’s not limited to the higher R-value. While Rockwool may get wet if the insulation is exposed to water, the material doesn’t wick. Instead, the water will drain, and any moisture will permeate out.

In other words, Rockwool or mineral wool won’t absorb the moisture, and the insulation will work without any degradation once the Rockwool is dry. In addition, this type of insulation can be effective even in ambient humidity, which isn’t the case with fiberglass.

However, consistent exposure to a lot of moisture or extreme humidity for most months of the year calls for at least a Class III vapor barrier, if not II.

Do I Need a Vapor Barrier Under Laminate Flooring?

Laminate flooring has an underlayment. This padding-like underlayment is rolled out on the subfloor before the laminate flooring is installed. You don’t need a separate underlayment if your laminate flooring planks have it attached as an integral part. But you will require a vapor barrier.

You need a barrier or retarder under laminate flooring regardless of the subfloor, which is usually concrete or plywood—both materials are porous.

Final Thoughts

Moisture problems aren’t the same for every climate or every type of construction. Your house may have its own unique issues, such as exceptionally humid areas that aren’t the usual suspects. This variance is why choosing a suitable vapor barrier is so important.

We hope this guide has cleared all your doubts surrounding vapor barriers. From materials to thickness to a couple of brand names, now you’ll know what the installer will be talking about when they come to install barriers in your home.

And who knows—maybe you’ll dare to do it yourself!

The Ultimate Vapor Barrier Guide (Your Top 10 FAQs Answered)