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Crawl Space 101

Crawl Space 101

In older buildings, the crawl space is an 18″ to 36″ space between the floor of the building and the earth where mechanical elements like electrical and plumbing are often hidden. This overlooked piece of the house is important to understand because it can cause major issues if neglected.

Your crawl space: it’s dark, it’s scary and if you’re like most of us, you have no idea what’s lurking down there. If you own an old house that was built before the slab on grade craze after WWII, you likely have a crawl space foundation.

A crawl space was sometimes part of a pier and beam foundation where the builder simply had a mason install brick or stone piers spaced evenly across the foundation to support the home, or it is part of a stem wall construction where the crawl space is enclosed, except for regularly spaced vents to encourage ventilation.

Why is the Crawl Space Important?

The crawl space holds a lot of secrets to your old house and can provide a lot of information about past issues and potential future issues. Understanding how it functions best and how to treat it will make you a rock star homeowner and save money and trouble down the line.


Your crawlspace needs to be ventilated. Period. Unless you are building a new zero-energy Passivhaus your crawlspace was likely designed to be ventilated to avoid problems. Ventilation allows moisture from the earth to escape through the vents rather than work its way into your house, potentially causing mold and mildew problems.

Before you listen to the insulation guy who tells you to seal it up and make it airtight, think for a minute about why those vents were installed in the first place.

Mechanicals Access

Unlike those poor sobs with a slab foundation, you have access to all your mechanicals like plumbing, electrical, and HVAC (sometimes) by crawling around under the house. Nobody likes climbing down there, but it’s a lot easier than jacking up a hole in a concrete slab to repair a burst pipe or upgrade your electrical.

Inspecting for potential issues and making repairs in much easier in a crawl space, so count your blessings.

Spotting Issues

Not only are you able to spot issues with your mechanicals, but you can also nip other problems in the bud. Inspections are much easier in a crawl space where you can more easily spot and remedy issues like subterranean termites, wood rot, and foundation issues.

Another plus is getting to play detective. I can tell if there was fire damage at one point, tell what kind of fasteners were used on the foundation, check to see what’s underneath those tile or linoleum floors and if they were original, and a host of other things.

How to Protect Your Crawl Space

There are a few things you need to do to secure and protect your crawl space and fortunately, they don’t require constant work. They are very much a once and done kind of thing and once they are completed, you can avoid sticking your head down there except for the random curiosity.

Keeping Critters Out

Protecting your crawl space from critters like mice, rats, raccoons, squirrels, or the neighbor’s feral cats is more important than you may think. If not sealed up, your crawl space is a warm bed for any critter looking for a home. Those pests tend to chew on warm electrical wires, tear open plumbing pipes for a drink, make nests in you insulation, and leave their messes behind.

Critters in the crawl space are NOT a good thing, and you need to seal things up in a way that keeps them out for good, but allows airflow. My favorite tool for this is 1/4″ hardware cloth that keeps most anything at bay when secured properly. I’ve written another post about How To Keep Critters Out of Your Crawl Space that you can read here.

Beat Moisture

In stem wall construction, moisture continuously evaporates from the earth and makes its way into your crawl space since these are not as well ventilated as the more open design of the pier and beam foundation. To keep the humidity at bay, you need to lay down some plastic sheeting covering the dirt. Black 6 mil plastic works best and is fairly inexpensive. The Spruce has a great how to post on the topic.

Dealing with Radon

Radon is an odorless, tasteless radioactive gas that occurs naturally in the earth and can seep into your crawl space increasing your risk of cancer. It sounds terrifying, but in reality, it is not difficult or expensive to fix. The thing is that you have to have someone test for it to know if it’s an issue.

It is more common in the northern parts of the US, so having a test for peace of mind is definitely worth it especially if you have neighbors with positive tests.

Crawl Space Conclusion

Now you’re a pro about crawl spaces. You know what the potential issues are and you know how to resolve them. It doesn’t take much to get your crawl space in tip top shape, and once you make these few improvements, you’ll be able to forget all about that dark scary place again. Before you switch into “out of sight, out of mind” mode again give some thought to how you can improve that dark corner of your house called the crawl space.

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14 thoughts on “Crawl Space 101

  1. Can I use PVC banners to encapsulate my crawl space? It’s 11 mil PVC, but I don’t know the perm.
    The banners are used and very inexpensive.

  2. What if the crawl space floods? What if it has a sump pump, can you still encapsulate and install the humidity sensor? Doesnt the sump pump section need to remain exposed? I know the water cause needs remediation, its possible gutters and grading. Could take years to trouble shoot all of it.

  3. To all that came for information that is useful. My company seals crawlspaces at the rate of 1 / day. A $20 wireless temp/rh sensor goes in every single crawlspace we do. It’s often there before we start. We have over 2300 sensors in Crawlspaces and some of those people opted to have their neighbor install one in their vented Crawlspace for comparison.

    While Waterproofing + Drainaige + Gutters are very important things for many reasons, we have several Crawlspaces that take on significant puddles 5 to 10 times per year and do not have any elevated humidity levels because of it. We have not failed. The job of a vapor barrier is not to keep bulk water out it is to keep moisture vapor up or down but it can not handle the water-table rising above it.

    What I am saying is if you want to save money, be more comfortable, and healthy, you don’t want unlimited humid air rushing into your crawlspace through the vents between May and October and you don’t want cold air coming in October thru May.

  4. This article is incorrect in stating that crawlspaces should be vented which is a regional issue. In the southeast this is 100% false. This is not an opinion of mine it is science. While a well sealed crawlspace with traditional fiberglass between the joists will fluctuate between 5°F and 10°F annually virtually eliminates the delta T, a vented crawlspace will fluctuate between 30° and 90° (much less so if ducts are leaky and poorly insulated).

    I have a mountain of REAL DATA that shows vented crawlspaces have escalating moisture in the wood from MAY – OCTOBER.

    While a sealed vapor barrier is a useful and important aspect of a sealed crawlspace that accounts for about 5% -10% of the moisture benefit.

    There is a structural benefit to the sealed vapor barrier which forces that the humidity levels within the soil under your humble stay constant instead of fluctuate annually.

  5. Thanks for the informative post Scott. This has always been an interest. (Hey, some people study the mysteries of the cosmos and some study the finer points of crawl spaces!)
    I think we can all agree that a vapor barrier over the soil is required regardless of passive ventilation.
    For the DIYers here, getting the vapor barrier right isn’t easy. It’s well within the reach of a reasonably competent person. But, taping the joints and the barrier to foundation walls will take a lot more time than you might first expect.
    If floor insulation is exposed, it should be enclosed. As for the rest, I’ve spoken with passive-house engineers, architects waterproofing guys and anyone else who might have a sound opinion. Everyone has an opinion and there’s no consensus!
    My two cents is that if there’s no obvious moisture problem, install an inexpensive WiFi enabled humidity sensor. Once there’s a record of the RH in the crawlspace you’ll have a much better sense of what (if anything) needs to be done.

  6. Yes, “hodgepodge” is NEVER good!

    But a knowledgeable “insulation guy” will absolutely improve the performance and comfort of your home when PROPERLY sealing and insulation a vented crawlspace.

  7. Thanks for the information. Part of my crawl space has no ventilation but is dirt floor is covered in thick black plastic. How much ventilation is necessary? There is a small single pain ‘basement’ window in the crawl space so there is some ventilation. Is more needed? The size is approx 8×8.

    1. I’m not sure how to calculate the size of ventilation required by the air volume of the crawl space, but I believe there are smarter people than I who can figure it out.

      1. The Spruce has info on how large the vent should be, 1’-0” x 1”-0” I believe. The link is in Scott’s article

  8. I think the vapor barrier on the crawlspace floor is the key in any situation. The goal should be preventing the moisture from the earth from entering the crawlspace in the first place. Just like keep water away from the outside of the foundation and you wont have water issues inside the basement. In Midwestern markets, vents are a dated approach set in place before understanding the house as a system. Maybe it is different in Florida, but based on the article below referencing North Carolina, I wouldn’t think its different by much.


  9. I always enjoy reading your articles, but I just must respectfully disagree with your blanket statement that crawlspaces need to be ventilated. Think about why those vents were installed in the first place? Because they were ignorant of building science principles. I have a 1926 Craftsman and also RESNET and BPI certifications. I have sealed and properly insulated my crawlspaces bringing them into the conditioned envelope of the home. No longer are they overly humid mold factories in the summer nor freezing cold in the winter making the rooms above FAR more comfortable year round.

    1. Dave I agree that a crawlspace that was originally designed to be conditioned space should have no issues but what I find are people simply sealing vents or some other hodgepodge method and hoping for the best. I may have overstepped about saying ALL.

      1. Simply sealing the vents alone is not the answer. But if a ground vapor is correctly installed and the walls of the crawlspace are correctly insulated, their isn’t a crawlspace on the planet that won’t perform better completely sealed.

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