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I've seen it quite a few times when companies fasten or tack the liner to your crawlspace wall in the encapsulation and call it a day. Funny thing is, when some of those same companies use the clear liner, instead of our preferred white liner, it allows you to see ALL the moisture behind the barrier. And what's happening, with an unsealed liner, is that moisture is evaporating up INTO the crawl space. If you have a dehumidifier already installed, great, but it's going to significantly cut into its efficiency and potentially create moisture issues that will have to be addressed at some point. Preferably, you always want to seal that liner to the wall.
In order to get a good seal, we recommend using a product specifically marketed for crawl spaces. There's a good number of places you can source that from, a simple google search should yield decent results. Another product you might rely on is Liquid Nails, it tends to have a solid performance is creating that strong seal you're looking to get. You can find that at any chain home improvement store.
In our encapsulations we look to create a 360 degree seal, all around the crawl space walls. After all, that's the name of the game and that's the purpose of an encapsulation, sealing that moisture out. We take the steps to seal all liner to the walls and tape every seam on the ground. I highly suggest a very thoroughly executed plan if you intend to take on a home encapsulation yourself. Hope this was helpful. If in doubt, reach out to a local crawl space professional to assist you with all your crawl space needs.