The Crawl Space

Home Inspector with Certified Inspections, PC - Residential & Commercial Property Inspections TN HI #334

The old adage "Out of sight - Out of mind" is not one that you can afford to apply to your crawl space. Even though it is considered dark and creepy by most folks, the crawl space is a very important area of the home, and it should not be ignored.  If the crawl space is not healthy, the house is not going to be healthy.  The most expensive repair work that is required on a home is typically performed in the crawl space.  Water damage, wood destroying fungus, termite infestation or other wood destroying insects can be found in other parts of the home, but they are most often found in the crawl space.  A healthy crawl space is essential for a healthy home and will significantly discourage this type of activity, growth or infestation.

Lets look at the basic requirements for a healthy crawl space by starting at the ground and working our way up.  All organic material and construction debris must be removed.  This means all concrete form boards, grass, shrub stumps, tree roots and anything dropped into the crawl space during construction.  The soil in the crawl space should be flattened out to fill all voids and trenches to prevent moisture from accumulating in these low areas.  It is a fairly common practice (and required in some locations) to then install three to four inches of small gravel over the entire crawl space ground or floor area.  If these procedures are properly done it provides the first step to a healthy crawl space.

The treatment, and proper prep work, at the outside of the foundation wall is also important for a healthy crawl space.  To control surface and sub-soil water movement a foundation drain is required around the perimeter of every home.  The drain is required to gravity flow (down hill) to an acceptable storm water disposal location or be equipped with a means of mechanical drainage.  The drain is covered with a minimum of six inches of gravel and then a soil cloth to prevent soil infiltration.  All portions of the foundation wall that will be below the exterior finished grade is required to be water proofed by an approved method.  This is very important in the over-all control of moisture penetration into the crawl space.  Bear in mind, concrete block will not stop water but acts more like a sponge, transfering water from one side to the other.    

Now lets look at ventilation.  (There is an approved crawl space construction method that does not require ventilation, but it is very seldom used, and it will have to be discussed in a different post.)  Crawl space ventilation is required at a ratio of one square foot of vent area per 150 square feet of crawl space, and one foundation vent is required to be set within three feet of each corner in the foundation wall.  If you have a 1,500 square foot house it would need ten square feet of ventilation, so, if you install ten pre-made foundation vents, you're good to go, right?  No, I'm afraid not, although that is the common misconception.  Less than two percent of the homes that I inspect have proper foundation ventilation, and poor air movement is one of the main contributors to unhealthy crawl space conditions.  Depending on the manufacturer, the common pre-made foundation vent has between 40 and 72 square inches of vent area.  In our home example above we put in ten vents, and if we go with the higher number of vent area provided, we end up with 720 square inches of ventilation, when the required amount is 1,440 square inches.  In reality we ended up with half of the ventilation area required.  In most scoring situations, 50 percent, or half, is called "failing", which is exactly what happens to our crawl space when it can't breath properly.  

Now that the crawl space has plenty of air movement, we go to one of the last items installed in a new home, the vapor barrier.  There are several products that can be used for the vapor barrier, but because of cost and ease of installation, the most prevalent is the black, polyethylene plastic.  It is installed last to prevent it from being damaged and displaced by people working on other aspects of the home.  The vapor barrier should cover all of the dirt or gravel in the crawl space floor.  There is no need to leave a space open around the edges or any where else.  The vapor barrier is designed to catch or hold the inherent moisture that is in the ground below the plastic and to keep that moisture from moving into the air of your crawl space.  It is normal and good if it is "wet" on the underside.  This means that it is doing its job.

In spite of many common misconceptions, the crawl space is not some mystical area with a mind of its own that you have no real control over.  Like all other aspects of home construction, the crawl space is what we make of it.  If you need help with yours, call a professional home inspector.  For a reasonable fee he/she will be able to evaluate the existing conditions and suggest repairs without trying to sell you a product or service.    

Paul A. Perry is an ASHI® certified and TN state licensed home inspector.  If you have any questions about this article, your home or home inspections in general he may be contacted at the following;  Web:

Comments (5)

Matt Grohe
RE/MAX Concepts - Des Moines, IA
Serving the metro since 2003

Paul; In remodeling our home I had to deal with a neglected crawl space, improper venting, debris, no vapor barrier. It took a lot of crawling to get it into shape. I picked up some heavier rubber roofing membrane on sale for the vapor barrier. It resists puncture in case you need to crawl around in there again, which I have had to do. Very informative piece.

Sep 10, 2009 04:43 PM
Baker Home Inspection and Commercial Properties Inspections
Baker Residential and Commercial Properties Inspections - Springfield, VT
Home and Commercial Properties Inspections Vermont

Paul, I was checking to see if you have started being active on your blog here agin or not.

Jul 18, 2013 10:49 PM
Paul A. Perry
Certified Inspections, PC - Residential & Commercial Property Inspections - Crossville, TN
Home Inspector - Crossville & Cookeville, TN

Dale, yes I have started to be active again.  Thank you for being interested!

Jan 08, 2014 02:02 PM
Courtney Cloe

Great article. I'd like to hear about your thoughts on encapsulation?

Apr 26, 2016 01:07 PM
Paul A. Perry
Certified Inspections, PC - Residential & Commercial Property Inspections - Crossville, TN
Home Inspector - Crossville & Cookeville, TN

Thank you Courtney. 

As a home inspector, I love encapsulations (they are so clean) but as an advocate for the consumer (home owner) I am somewhat wary.  First problem: a crawl space isn't going to need encapsulation unless there is moisture penetration.  Encapsulation does not address water penetration, it just keeps the water below the plastic.  This could actually hide serious problems for the home.  Second problem: If a crawl space is not vented to the exterior, it is required to have conditioned air provided for and circulated through the crawl space.  Encapsulation does not address this requirement.  Third problem: If conditioned air is provided for the crawl space there is no need for a giant, industrial sized dehumidifier that frequently costs the home owner $800.00 to $1200.00.  Sorry for the long response; I should probably write another blog.   

Apr 26, 2016 01:41 PM