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The Crucial Role of Vapor Barriers in Preventing Moisture Damage in Your Crawl Space

The Crucial Role of Vapor Barriers in Preventing Moisture Damage in Your Crawl Space

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If there’s one area in a house or building that’s most neglected, it’s probably the crawl space.

Little do we know, crawl spaces are the entryway to one of the main causes of property deterioration: unwanted moisture. Fortunately, moisture issues can be prevented by lining the crawl space with a vapor barrier.

Keep reading to find out what is a vapor barrier in a crawl space, what it does, and how it can help protect your building.

What Is a Vapor Barrier in a Crawl Space?

Vapor barriers, also known as moisture barriers, are sheets made of impermeable material, such as polyethylene. They’re placed on the ground, in the narrow space underneath a building, to prevent moisture from rising.

You can install a vapor barrier in your home DIY-style if you have the skills and the right equipment. Otherwise, it’s better to have the supplier or contractor install it in case the service is available.

Do I Need a Vapor Barrier in My Crawl Space?

The straightforward answer is: yes. Believe it or not, even in regions with dry climates, structures are still susceptible to moisture problems.

To help you understand better, this section covers the groundwork of vapor barriers.

How Does a Vapor Barrier Work?

A vapor barrier serves as a membrane. It’s laid directly over the ground to keep soil moisture from diffusing into the crawl space.

Remember, though, that a vapor barrier doesn’t work against water run-off and pipe leaks. Such issues should be addressed separately.

Signs of Moisture Damage

Other than the obvious, visible ones, here are the tell-tale signs of moisture damage you shouldn’t ignore.

Musty Odor

Mold or mildew emits a certain dank smell, like wet wood or paper. Be wary of such odors even when you don’t see the mold.

Sometimes, you’re going to need to look deeper into wooden drawers, behind wallpapers, or behind the ceiling to find the mold.

Misaligned Doors or Windows

If your doors and windows used to open and close just fine, and presently, they awkwardly stick out, this could be a sign of moisture damage.

Deformation or misalignment of doors, windows, floorboards, and even drawers, could sometimes mean the presence of moisture that wasn’t there before.

Why Put Vapor Barriers in Crawl Spaces?

When the vapor is blocked from entering a building, the structural members and objects in it are saved from premature deterioration. Below are some of the advantages of having a vapor barrier installed in the crawl space.

Manage Indoor Temperature and Reduce Energy Use

Humidity, or the amount of water vapor in the air, affects indoor temperature. Since higher humidity makes the air warmer, it’s preferred during winter and frowned upon during summer.

Banning ground moisture from entry keeps the indoor temperature at comfortable levels. Consequently, this helps reduce energy consumption through minimal use of air-conditioning.

Prevent Structural Damage

Trapped moisture inside concrete’s pores can freeze and expand during winter. This, in turn, will enlarge the pores into dimples and result in spalling.

Additionally, accumulated moisture can cause corrosion in the concrete’s steel reinforcements. This is especially worrying because the foundation will be the first to receive the damage, being closest to the ground.

Stop Wooden Members From Rotting

You could extend the service of wooden structural members, such as trusses, drywall partitions, or floor framings, by keeping moisture away.

To clarify, wood doesn’t rot simply because it’s wet. Rather, it’s the propagation of wood-eating fungi that thrives in damp environments.

Thwart Development of Mold

Otherwise known as mildew, wood molds are another concern when wood catches and traps moisture. These molds are commonly found in unfinished wood and are either black or greenish-brown.

Besides being an eyesore, mildew is known to cause allergies, asthma, and other respiratory conditions.

Inhibit Damage on Paint Finish

Vapor could trigger blisters on painted surfaces. Not only is this unsightly, but it also exposes the structure to further damage by removing the coat.

This also means additional expenses for the owner as the building will require repainting.

Prevent Electrical Shorts

Extreme humidity can corrode electrical wirings, fuses, and circuits. Oxidized fuses may impede the flow of current.

However, in a corroded circuit breaker, the current may keep flowing even when the circuit trips. When this happens, the wiring can overheat and result in a fire.

Additionally, moisture can condensate and gather around electrical outlets or switches, and cause electrocution.

Deters Degradation of Fiberglass and Other Insulation Materials

When exposed to high levels of humidity, the insulation materials may lose their functionality.

Insulation materials and fiberglass can trap condensed vapor, therefore, dampening the surrounding materials and other members of the structure.

Fend Off Pests

If these barriers can resist minuscule articles such as vapor, then they can definitely ward off pests, such as bugs and rodents, as well!

Although, this is only applicable to the covered area. In case you want to secure the crawl space thoroughly, seal the sides, or opt for appropriate soil treatment.

Do Vapor Barriers Work in Crawl Space?

As it stands, even in locations with dry climates, properties are still susceptible to moisture damage. Therefore, no matter the climate in your region, you could benefit from a moisture-free crawl space.

Moreover, vapor barriers are not to be confused with vapor retarders. Retarders also protect against moisture, except they’re not as resistant as barriers.

Vapor retarders only minimize the flow of moisture, thus allowing its movement through walls. Therefore, vapor retarders can’t replace vapor barriers in any way.

Choosing the Right Thickness of Vapor Barrier

Vapor barriers’ level of protection, durability, and longevity, all depend on the thickness.

For crawl space vapor barriers, the recommended minimum thickness is 10 mil. This thickness is enough to block unwanted moisture and is durable enough for moderate protection.

15 to 20 mil are the heavy-duty options. These are puncture-proof and are preferred for long-term use.

Furthermore, there are vapor barriers in the market that are less than 10 mil thick, but those aren’t ideal for crawl space areas. Thinner vapor barriers are commonly installed inside walls or partitions, where less moisture protection is required.

Types of Crawl Space Vapor Barriers

To help you choose the right one for your crawl space, this section will go through some handy information about the two common types of vapor barriers.

Reinforced Polyethylene

Reinforced polyethylene is made of laminated sheets of high-strength polyethylene fortified with strings or scrim. The reinforcement between the layers makes it ideal for crawl spaces.

This is the vapor-barrier-of-choice of many for the following reasons:

  • Tear-resistant
  • Low perm rating, or trace amounts of vapor pass-through
  • Service life could reach up to 25 years, even in storage (for 15 mil and up)

Unreinforced Polyethylene

As the name implies, the unreinforced polyethylene barrier is made of polyethylene, but without the scrim reinforcement.

Without the strings, they’re susceptible to tearing, and therefore, not expected to last long in a crawl space setting. They’re usually made of recycled materials as well.

Despite being less durable than its reinforced counterpart, some people still choose this option because of the following:

  • Better market availability
  • Less expensive
  • Good for moderate protection or temporary use

Useful Tips for Installing a Crawl Space Vapor Barrier

Crawl space vapor barriers could use some TLC, too. Here are additional pointers for keeping your crawl space moisture-free.

Before Installation

Before installation, clear the crawl space of all objects for better mobility. Keep in mind that the area should be clean, well-ventilated, and free of any hazards.

Dry out the area in the best way possible, meaning there shouldn’t be any puddles, wet items, or debris. For good measure, the surfaces should be mold-free too.

During Installation

Overlap the vapor barriers and use only good-quality seam tape to seal the joints and edges. Even when using 10-mil barriers and up, it’s best to handle the vapor barrier with care during installation.

Vapor barriers are less likely to tear, but sharp edges could puncture them.


The supply-and-install option usually comes with a warranty, so, it’s less effort on your part. Plus, it gives you peace of mind as your vapor barrier was installed properly.

To boost the vapor barrier’s functionality, it’s recommended to incorporate a waterproofing system and dehumidifier. Make sure that the crawl space area is properly ventilated to prevent rogue vapors from accumulating.

For easier maintenance, see to it that the area is easily accessible. Sufficient ventilation would also be advantageous for the maintenance personnel.

Crawl space vapor barriers have a relatively long service life. In other words, check for punctures from time to time to ensure that it’s not compromised.

Final Thoughts

So, what is a vapor barrier in a crawl space? It’s a membrane that hinders unwanted moisture, early deterioration of property, and unnecessary expenses.

If left unaddressed, dissipation of ground moisture could result in the corrosion of electrical and mechanical fixtures, as well as the rotting of wooden members.

It’s also known to cause health hazards through the formation of molds and dust mite droppings. Fortunately, it can be deterred by fixing up the crawl space with a vapor barrier.


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