Those small cracks on basement walls and floors may seem superficial, but basement finishing and waterproofing contractors who ignore them are asking for trouble. In reality, concrete cracks may be portals to poisonous gases and lead to slow, seeping moisture that breeds mold, mildew and fungus. Finishing a basement before repairing the imperfections is like igniting the fuse on a stick of dynamite and then hoping there is no explosion.

Pre-finishing techniques, like low pressure crack injection, have two essential functions. First, they save homeowners money because masking problems with expensive renovations will only lead to more costly difficulties somewhere down the road. Second, they ensure that indoor air quality is not seriously impaired by contaminants such as Radon gas and mold causing moisture, which can cause chronic respiratory and other problems.

"If you've already finished the basement, it's too late. The cracks won't go away; they'll only get worse, no matter how innocuous they may seem initially. More than 50 percent of the air that you are breathing in your home's living space originates in the basement. If there was a problem in the basement, you now have the air quality problem in your living space," says Roy Spencer, owner of Perma-Seal Basement Systems in Downers Grove, Il. Wall cracks leaking behind finished walls may not be detectable but, nonetheless, may be creating an atmosphere for mold support (a humid basement exceeding 50 % relative humidity).

Low-pressure concrete crack injection (20 to 40 psi) is an effective process for basement repair because it effectively leads to the monitoring of permanently filling the entire length and depth of the crack. Epoxy and polyurethane-based injection materials are best, because they adhere to the concrete, preventing further erosion due to moisture infiltration. Also, the material holds a strong seal, even while concrete expands and contracts. This technique is appropriate for poured concrete walls only.

Spencer says high-pressure injection is too forceful for effective residential basement repairs. When used in repair of residential basement cracks, the higher pressure can result in uncontrolled crack filling. This technique is typically saved for repairs of the thick poured-wall foundations used in large buildings and public works projects (such as hydro-electric dams) and to stop flowing water before the repair is completed at low pressure.

Low pressure crack injection has been an accepted technique throughout the Midwest for many years. But basement waterproofing contractors and foundation repair firms in other regions are now embracing this same technique because it is cost-effective, reliable and permanent.

The reason for this is that at pressures less than 40 psi, the injection resin can only rise within a crack after it has filled the crack from front to back and then rising to the position of the port attached above the port being injected. Thus the applicator can be confident that he has filled the crack up to the port above the port being injected. The applicator then repeats the process up the length of the crack, leading to a successful repair.

At pressures above 40 psi, the injection resin can overcome gravity and instead follow the path of least resistance which is commonly on the front half of the crack. As a consequence, material can reach the port above, without having filled the crack and the applicator is unable to determine this. The result is an incomplete filling of the crack, the most common source of crack repair failure.

Most cracks develop within 30 days of the concrete being poured, whether or not leaking during this period. For years, discussion of basement crack repair mostly focused on walls, not floors. Yet through the years it has become clear that cracks in the slab are also an outlet for radon and moisture. Therefore, it is unwise laying down tile or carpet before the floor cracks have been repaired with a polyurea system, the material of choice for slab cracks. Spencer says the only time contractors don't seal a slab is when water is already flooding the basement through the floor cracks. Such a problem suggests the need for a sub-foundation draining system.

"About 95 percent of basement concrete cracks are not a structural threat to the foundation, basement walls or the floor. Most wall cracks only pose a water seepage problem.  But you have to fix them, because the crack is still capable of leaking and affecting air quality of the home." If the leak is trapped inside a finished wall or floor, it may do a lot of damage before it is discovered," says Spencer. The secret to successful pre-finishing of a basement is to repair the cracks at the beginning of your finishing process.