Dry Basements Depend on Durable Vapor Barriers That Isolate Slabs

To prevent cracks, and head off mold and rot in finished basements, a sturdy vapor barrier is still the most important detail.

Creating the perfect slab has always been a challenge for homebuilders. Even when built to code, slabs sometimes crack and fail. What went wrong? Often, sloppy installation is the issue. Seams in the underlayment are not sealed and overlapped properly, or workers punch holes in the plastic--or it simply doesn’t extend fully underneath the slab and vertically up the edge of the buried part of the slab perimeter.


Extra Defense. The specs for vapor barriers such as Moistop Ultra (shown above) are typically applied under the slab but not up the exterior edge. But building science experts such as Joe Lstiburek at Building Science Corp. say they should go the extra distance, and wrap all the way around the slab and come back up to soil grade on the exterior. Above-grade surfaces should then be waterproofed with latex paint to prevent further water absorption.

At other times, local code provisions contribute to the problem. As Building Science Corp’s Joe Lstiburek points out, California has far more slab failures than Florida. But why? Florida is often swampy and damp, while California’s soils are typically much drier. The difference is in one code detail. Both states require a layer of plastic between the ground and the concrete, but California also requires an extra layer of sand on top of the water barrier. This is foolish, Lstiburek says, because the sand becomes a storage layer for moisture. The original reason behind this, he says, was to keep slabs from curling as they dried unevenly. But the same result can be achieved with good building science:  “use a low water-to-cement ratio (less than 0.5) and wet cure the top (wetted burlap works).”

Follow the Water

In most cases, problems with slabs can be traced to the inability of water to escape from the concrete. In a proper installation, the slab is allowed to dry upward, into the building space. Over time, it eventually becomes quite dry, and stays that way. The vapor barrier underneath should cut off any additional moisture inputs from soil. The slab cannot dry downward through the barrier, so moisture has to go up, into living spaces.

This is not necessarily a problem--provided it’s a one-time process not a permanent cycle of wet and dry. Once the concrete cures for a few weeks, it should stabilize and remain at a constant moisture level. Can you finish the floor during that time? Yes, if you proceed with care.

There's some disagreement about whether vapor barriers should be placed on top of finished slabs to keep moisture away from wood or other flooring materials. Many manufacturers of vapor barriers include this method in their installation documents. Lstiburek asserts, however, that a better approach is to protect the bottom of the slab and let the top "breathe"—at the same time managing humidity in the conditioned space indoors.

For example, moisture passes through most carpet, he says, so if you put carpet on top of a slab, and a dehumidifier (or better yet, heat pump system) in the basement, you can avoid problems.  If the moisture level is kept relatively low and stable, odors, staining and warping can be kept at bay.

The Freeze Factor

In northern climates, unwanted moisture in slabs poses a bigger problem. Freezing conditions can cause the concrete to break, even if compression joints are used in the slab at large. A much better solution is simply to install a proper plastic barrier at installation. I would also recommend insulating the edges of all northern slabs. Even if water-protected perfectly, the concrete will typically take about 28 days to fully cure in proper temperatures, and will never become completely "dry" (nor should it, necessarily).

Insulation methods for slab perimeters have been part of the IBC for several years now, under the frost protected shallow foundations xxx section. The key is to isolate the concrete from the cold ground and increase the distance the cold has to travel to get under and into the slab. Go ahead and insulate the above-grade exposed portion of the slab as well. You’ll be saving the owners a lot of wasted heating energy, and insuring the durability of your slab.

Of course, not all homes have basements, and the same rules apply to slab-on-grade floors. Let the concrete dry, and balance the amount of humidity inside the home, and most problems with mold, odors and cracking can be avoided.

To Tape or Not to Tape

One question about vapor barriers is whether all seams need to be taped to achieve full effect. Building experts disagree slightly on this topic, and I haven't seen any definitive research to answer the question. Lstiburek asserts that because air movement is limited under the slab, overlapping the vapor barrier is enough to keep moisture out. But energy auditors say that blower door tests have shown that a surprising amount of air escaping from untaped slab barriers. Much of this leakage is attributed to the edges and penetrations for mechanicals.

Our recommendation would be to spend a little extra and tape the seams, but also tape carefully around penetrations and gaps, especially on the slab perimeter. Keep unwanted water out, and let the slab dry toward the inside of the house, which is really the only way it CAN dry. You and your clients will have a trouble-free slab.


Fortifiber has developed and extra heavy-duty concrete vapor barrier called Moistop Ultra 15 that could make under-slab installations more durable and resistant to jobsite damage.