What is membrane waterproofing? How does it work?

Membrane waterproofing means putting a continuous water-resistant sheet or coating over a surface so water cannot pass through it. Inside a basement, the sheet most people mean is a wall vapor barrier that catches seepage and channels it down to a drain.

That is the short version. Here is what it actually does in a New Hampshire basement.

A membrane is a layer. Sheet, liquid-applied, or peel-and-stick, the job is the same. Stop water at the wall and give it somewhere to go. By itself a membrane on the wall does not solve a wet basement. Water needs a path out. That is the part homeowners miss.

In our work the wall membrane is one piece of a bigger system. We install a 12-mil wall vapor barrier as part of the Forever Dry System. The barrier hangs on the wall. Any water weeping through the block or stone runs behind it. Then it drops to the floor wall joint, into the perimeter drain, and over to the sump pump. The pump sends it outside. That is the loop. Barrier, drain, pump, out.

Why does water show up at the wall in the first place? Backfill. When your house was built, the crew dug out the hole, poured the foundation, then pushed the dirt back in around it. That backfill is loose. It drains slower and holds more water than the packed ground further out. So water collects right against your wall and pushes in. NH frost heave adds to it, freezing soil shoves walls inward over the winters.

So a membrane catches the seepage. The drain and pump remove it. Skip the drainage and you have a wet wall barrier with nowhere for the water to run.

A note on the exterior kind. Some companies dig down the outside of the foundation and glue a membrane to the wall from outside. We do not do that. In our experience that fix fails in a few years once the dirt settles and the membrane separates. The durable fix is interior drainage, where you control where the water goes and you can service it later.

For a crawl space the membrane idea changes a little. There we do crawlspace encapsulation, not the Forever Dry System. That is a 12-mil wall vapor barrier plus a 20-mil floor vapor barrier, with dimpled drainage matting under the floor barrier, seams spray-foam sealed, a dehumidifier, and a sump. The floor barrier is the big membrane there. It seals the dirt off so ground moisture stops coming up into the house. The Department of Energy notes that up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl, so sealing it pays off in comfort too.

What drives the cost? Wall length, water volume, the shape of your foundation, and whether you need a full interior system or just a section. Basement waterproofing with the Forever Dry System runs $3,000 to $30,000. Crawl space encapsulation runs $3,000 to $25,000. Every quote starts with a free inspection so we measure the real footage before we price it.

You may not need a full system yet. If you have one small wall that weeps after a hard rain and the rest is bone dry, a crack injection or a smaller drainage run might be all it takes. We will tell you that when we look. We would rather fix the one thing than sell you the whole basement.

A few facts worth knowing. The backup sump battery is battery only, and a charged one can move about a residential pool’s worth of water, roughly 15,000 to 20,000 gallons before it needs power back. The Forever Dry System is 100% guaranteed to be dry for life, transferable under conditions. The wall vapor barrier carries a 25-year warranty and the sump pump system carries 3 years, batteries excluded.

Membrane work is one piece. Drainage is the rest. Get both and the wall stays dry.

See our basement waterproofing and crawl space services pages for the full systems, and read the 603 guarantee for how the dry-for-life promise works. For building-science background on foundation moisture, the DOE Building America program is a solid neutral source.

Want to know what your basement actually needs? Call us at 603-610-1770 or book your free inspection and we will measure it and tell you what we find!

Related questions

Is a vapor barrier the same as waterproofing? Not quite. A vapor barrier stops moisture and seepage at the wall, but it only works if water has a drain to run to. Real waterproofing pairs the barrier with a perimeter drain and a sump pump. See basement waterproofing.

Do I need exterior or interior waterproofing? We install interior drainage. In our experience exterior membranes fail in a few years once the backfill settles and the seal separates. Interior systems let you control the water and service the system later.

Will a membrane stop a leaking crack? A wall barrier hides a weeping crack, but it does not repair it. An active crack is best handled with a crack injection. Pricing for that runs $1,000 to $3,000. See foundation repair.

Does a membrane help a crawl space too? Yes. In a crawl space the floor and wall vapor barriers seal the ground off so moisture stops rising into the house. That is crawlspace encapsulation, and it runs $3,000 to $25,000. See crawl space services.