Crawlspace Moisture Barrier Installation Tips

Drew George asks,

When purchasing my house, my inspector suggested adding a moisture barrier to the floor of my crawlspace. Looking around, there are many different thicknesses to choose from. How thick does a typical crawlspace moisture barrier need to be? Also, does it have to be rolled out as one large sheet, or could it be just as effective if it was made up of several smaller sections?

Our Answer

Your inspector was very smart to recommend a vapor barrier in the crawlspace. Moisture that is allowed to evaporate up into the floor structure can cause the insulation to become ineffective, and allow it to rot and mold. Moisture gets into a crawlspace A vapor barrier is essentially a plastic sheet that lays across the floor of the crawl space. Before you start this project, it’s important that you clean out anything on the crawlspace floor that could pierce the plastic barrier like rocks or wood.

When choosing the plastic barrier, many folks will use polyethylene sheeting, but a better option is to use cross-laminated sheeting. It’s about six millimeters thick, making it much stronger and less likely to let moisture pass through. A reinforced cross-laminated product would be best. It will be more expensive, but worth it. This is the kind of project you want to do once, you want to do right, and not have to do again for a long time.

When you lay down the plastic, you want to do so with as few seams as possible. You can purchase special tape that will allow you to seal the seams together, or you can overlap the seams by as much as four feet. Make sure you go end-to-end or inside-to-inside on the foundation wall so you have continuous coverage. In addition to the crawlspace vapor barrier, if you want to reduce moisture in that space you really should pay attention to a few more things.

Starting on the outside of the house, the most important thing to maintain is the grading and the gutters. The gutters must be clean, free-flowing, and the downspout should charge at least four to six feet from the house. The grading, or angle of soil around the house, should also slope away. It should drop about 4 inches over the first 6 feet, and it shouldn’t be soil that will absorb and hold a lot of water.

For example, topsoil is a bad idea because it’s so organic it’s like laying sponges around your house. You’d be better off improving the grade with clean fill-dirt, then once the grade is established, cover it with topsoil, mulch, stone, or whatever type of material you choose. Just make sure the grade is established first, and don’t put any landscape edging at the outside edge of it that could hold water toward the house. That would be like building a moat around your building, which is something you definitely don’t want to do. Combining these efforts together, you will have a crawlspace that will remain dry for many years to come!