How to Line the Floor of a Crawlspace

When it comes to insulating your floor, there is no more effective method than insulating underneath a crawlspace. It may not be an easy job, but it will make a significant difference in the comfort of your home and in how much you spend on heating and cooling.

The only time you need to worry about moisture entering from the ground below is if the ground is damp (not just wet), in which case you can use a thicker polyethylene that’s rated for direct contact with soil. The crawlspace needs to be vented. That’s necessary to maintain sufficient airflow under the house during the summer when humidity levels are high. Vented crawlspaces are not recommended in colder climates because they draw cold external air into the crawlspace during winter months and increase heating costs. 

The best way to insulate under a crawlspace is by lining the entire surface area with insulation material. This is especially recommended if the ground underneath is damp. If you do not want to go through this process, you can still get good results by lining only half of the crawlspace’s surface area. The best way to do this is to install plastic on the ground and seal it all around.

First, you’ll want to dig out the area, so it’s pretty flat. You don’t need a hard level, but try to get it as close as possible. Roll out the plastic sheeting and smooth it flat on the ground. Cut off any excess with a utility knife. Fold over about 1/2 inch on each side of the plastic and staple it down along all four sides of the crawlspace using 10-penny nails driven through small holes in a carpenter’s square.

Materials Suggested for Lining the Floor of a Crawlspace

Vapor barrier. A vapor barrier is a thin plastic sheet that can be rolled out over the earth to prevent moisture from seeping up through the ground.

Dirt floor. If your crawlspace is lined with dirt, spread out 3 inches of gravel on top of it. This will help create air pockets and keep moisture from soaking into the plaster or insulation you lay over it.

Plastic sheeting. Plastic sheeting can be used as an alternative to a vapor barrier for very dry areas. It also helps prevent termites from getting into your home through the crawlspace.

Plywood board. Lay down 2 by 4-foot sheets of plywood over the gravel or plastic sheeting and secure them with screws every 4 feet along the edges and at each seam where two boards join together.

Create your vapor barrier using heavy-duty plastic sheeting that goes over the entire crawlspace. Hammer one stake every 12 inches or so along each side of the crawlspace, driving them through the center hole of a carpenter’s square). Leave 4 inches exposed above ground level so you can pull it out later if necessary. Ensure there are no holes or tears in the plastic, and it is sealed all around to keep moisture from getting underneath.

If the crawlspace is used for storage, you can staple fiberboard or foam board to the bottom of the joists to help insulate them. All areas where air can escape should be sealed and insulated.

This is fairly easy to do yourself with some basic tools, but if you’re not comfortable doing it, contact a professional insulation contractor who will be able to help you with this project.