How to Install a Vapor Barrier in a Crawl Space (A Step-By-Step Guide)

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As a homeowner, moisture can be your house’s greatest enemy. Not only does it damage the walls and foundation, but high humidity causes mold growth.

Even when using a dehumidifier, excessive moisture from crawl spaces results in all those issues.

Luckily, installing vapor retarders can prevent water from seeping into your home. Continue reading this article to learn how to install vapor barriers in your crawl space in six simple steps!

Should You Install a Vapor Barrier in Crawl Space?

Yes! It’s recommended to install a vapor barrier in your crawl space. The former brings many benefits that’ll save your house from water damage. Those include:

• Prevent soil moisture from creeping into the flooring and walls of your house.
• Stops mildew and mold growth by keeping the crawl space dry.
• Slows down rust, corrosion, and wiring mishaps due to high humidity.
• Improve energy efficiency by creating a drier environment and reducing the workload on your HAVC system.

What Size Vapor Barrier for the Crawl Space?

Proper polyethylene, or vapor barrier, sheet size is crucial for preventing moisture buildup in your crawl space. Now, there are two measurements to look for when buying the former: mils and perms.

As the name implies, mils refer to the plastic film’s thickness. Perms measure the rate at which vapor molecules pass through a material. The lower the rating, the better.

Typically, 0.6 mil is the minimum thickness of polyethylene sheets allowed according to residential codes. As for the permeability, it should be equal to or less than 0.1.

That said, you should go thicker, around 10 mil or higher, for crawl spaces and other concrete structures.

That’s to provide better insulation properties. Not to mention, thicker polyethylene is puncture-resistant.

It’s worth mentioning that the former is a class I vapor retarder, which isn’t permitted in certain climate zones, like 1 and 2. So, it’s best to check with your local codes before installing those plastic sheets.

Where Does the Vapor Barrier Go in a Crawl Space?

The short answer is: everywhere. Vapor barriers help prevent moisture from seeping into the floor and walls. If that happens, you can expect mold and other water damage.

Nearly all crawlspaces contain soil as flooring. Water from the former can drive up to your house’s foundation, damaging it.

So, you should install the plastic sheets anywhere you suspect vapor can enter. That includes the ground, walls, and other concrete structures.

How to Install a Vapor Barrier in a Crawl Space

To install a moisture barrier, measure the crawl space, clean it, and attach the insulating sheets.

It’s worth mentioning that it’s better to leave it to the professionals when installing a vapor barrier in a worn-out crawl space. Sure, DIY can help you cut costs.

However, water damage could occur if you’ve improperly mounted the insulators. The cost of fixing that can be more expensive than simply hiring an expert.

Whether you choose to do it on your own or hire a professional, typically, installing a vapor barrier follows the following steps:

Step 1: Inspection

Before summoning your inner crawl space professional spirit and installing the plastic sheets, you first need to assess the foundation. That’s to ensure the sheets adhere well.

Check for groundwater and plumbing leaks. Oftentimes, those issues lead to troublesome mold infections, which pose many health risks.

Such serious problems require a specialist to prevent further damage to your property. However, you can repair minor damage, such as debris or worn-out old insulation.

Step 2: Clean Up the Crawl Space

For the vapor barrier to work efficiently, you need to make sure your crawl space is dry and clean.

Leaving the floors to dry on their own can take hours or even days since there is little ventilation down there.

Using dehumidifiers, fans, and pumps can help facilitate the former process. Additionally, make sure to remove any wet material.

As for cleaning, remove any wood debris, stones, or any material that’ll prevent the plastic sheets from adhering. You can also use a broom to remove pesky cobwebs from the roofs and piers.

Once you’ve completed the clean-up process, level the floor. Now, it doesn’t have to be completely even. Simply use a garden rake to make the ground as smooth as possible.

Step 3: Measurement

Taking the crawlspace measurements is crucial for two reasons: it’ll help you know how much vapor barrier you need and the dimensions you should cut it into.

The process is straightforward. All you need is a measuring tape and a piece of paper to record the readings.

Here’s how to do this:

1. Using the tape, measure the length, width, and height of your foundation.
2. Add 6-8 inches to all the dimensions for overlapping.
3. Record other features, like support pillars.
4. Map out your crawl space.
5. Multiply the length and width to get the area of a square or rectangular foundation.
6. For an L-shaped crawl space, divide it into two rectangles and add their areas.
7. Add 10% to the total for measurement errors.

Step 4: Seal the Vent Holes

Look for all vents and entrances, and seal them. Those openings can bring in a great deal of outdoor air, causing the same problems mentioned earlier.

Not to mention, this air can damage your newly installed vapor barriers quickly. This step is essential if you’re going to install a dehumidifier or a ventilation system.

Simply use insulation boards to cover up the holes and mount them to the wall using a zip system flashing tape or spray foam. However, the latter is hazardous, and you should only use it after taking the necessary safety precautions.

Step 5: Roll out and Cut the Vapor Barrier

Pre-cutting the polyethylene sheets can save you a lot of effort. Rolling those sheets in a narrow space can be a hassle.

Just roll out the film on the porch or in the garden. Then, cut out the necessary measurements for the walls and the ground.

Make sure to add the extra 6-8 inches for overlap. You can even go as far as 2 feet.

As for the concrete piers, you can put the vapor barrier on the longer side to directly take the measurements.

Instead of cutting and repeating the same process for the remaining sides, mark the first measurements on the plastic film. Roll it out on the wall again and label the dimensions of the second wall. Repeat the previous steps four times for each column.

Add 6-8 inches and then cut down. If done correctly, you’ll have a long plastic sheet to wrap around the pillar.

Step 6: Install the Sheets

Once you’ve prepared the plastic sheets, all you need to do is install them!

Start with the supporting pillars. Wrap the film around the columns and keep it as smooth as possible. Use duct tape to mount it temporarily. Once properly attached to the wall, use a zip system tape to secure the vapor barrier well.

Repeat the same process for the walls. Now, all you need to do is cover the ground.

To do that, roll out the plastic sheets and make sure they’re as flat as possible. Tape the point of attachment between the floors and walls first.

Use vapor barrier stakes to mount the film on the ground. Repeat the same process throughout the entire crawl space perimeter, and make sure to tape overlapped sheets on the floor.

Final Thoughts

Knowing how to install a vapor barrier in a crawl space can save you from future headaches. The former prevents moisture-related problems, protecting your house from damage.

The best part is that you can mount the insulating sheets yourself. All you need to do is clean your crawl space, take the measurements, cut and tape the plastic film, and you’re done!

While DIY installation is an option, it’s best to consult a professional if you have any water damage in your home. That way, you’ll prevent moisture from creeping into the foundation and safeguard the structural integrity of your house!

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