How to Install a Basement Vapor Barrier in a Crawl Space

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What You'll Need
Tape Measure
Vapor Barrier

A basement vapor barrier is a very useful piece that helps keep moisture from reaching the walls of the basement before spreading into the rest of the house. A vapor barrier is usually made of a plastic sheet, but it is how that sheet is fitted into the basement that makes the difference between a good installation and an ineffectual one. Installing a barrier in your crawlspace can help keep these areas dry, preventing dampness from reaching the first floor of your home.

Step 1 - Measuring the Crawlspace

A crawl space is different from a large basement in that it is usually constructed with an earth floor and the ceiling is much lower than in a full sized basement. That makes it easier for moisture to accumulate inside the space. You must measure it carefully to ensure that it remains completely dry. Rather than combining the length and width of the house, use a tape measure to get the exact distance. Purchase enough barrier material to cover this area.

Step 2 - Add a Layer of Mastic

You need a layer of asphalt mastic behind the basement vapor barrier, both to assist in keeping the area dry and to provide an anchor to which the barrier can bond to form a watertight seal. Spread the mastic in a portion of the crawl space, then leave it to mostly dry. Once the mastic is tacky to the touch, but not coming away from the base, you can add the barrier.

Step 3 - Install the Vapor Barrier

Once the mastic is ready, you can then add the barrier. Place it down firmly into the almost-dry mastic and fold it around the corners. Leave a 4-inch gap of barrier at each edge. Cut the barrier around obstructions such as support pillars. Your material should be laid so that you have between 6-inches and 1-foot at either end, which is smoothed against the wall. You can use mastic or caulking to keep these ends of the fabric in place. If you do use a caulk, it should be 100% silicone, which will defend the wall against moisture.

Step 4 - Finishing the Installation

Lay the plastic until the crawlspace is completely covered and there is barrier on all of the walls. You can caulk the edges of the barrier together to prevent moisture from forming between the sheets, or you can simply tape them together using some home improvement tape. Apply a layer of caulking and leave it to set. Repeat this process until you have covered the whole crawl space. Add an extra layer of caulk over the entire space once the space has been fitted with a barrier. Payparticular attention to the ends of the sheets where they touch the wall. Leave the caulk to dry.