In most cases, the bubbles that form in carpet are a natural reaction to the heat and moisture that have been injected into the carpet's foundations with steam cleaning. A steam cleaner works by creating an environment of very high pressure and very hot water that has been turned into steam.
The heat of the steam detaches the more stubborn stains and oily dirt from the fibers of the carpet, loosening their bonds, while the high pressure blasts the particles apart and sucks them back up into the cleaner. This works well to remove dirt, but it also has some unintended side effects.
When carpets are first installed, they are attached to the floor via their backing, or the underside of the carpet that interacts with the adhesives used to attach it. Carpet backing is divided into two sections: the structural section, which holds the carpet fibers in place, and the secondary section, which lends support and insulation to the carpet.
Some types of backing are made of natural fibers to promote eco-friendly materials, but many are created of polyurethane, a type of very flexible plastic.
Steam Cleaning Warping
Carpets bubble up or create small spots of swelling carpet for several reasons after the cleaning. Contrary to popular belief, it does not have as much to do with trapped air as it does with the carpet itself and how it is made or installed.
Other damage can occur with steam cleaning, such as carpet warping, but these are more permanent and costly changes. Wool carpets, for instance, can be permanently wrinkled by being steam-cleaned and dried out inappropriately.
The wood underneath carpets might also become warped, leading to permanent bends in the carpet which are different from the temporary swellings.
Bubbles in the Backing
Polyurethane can be fused into many different shapes with a variety of characteristics in a factory setting, using pressure and heat. When exposed to similar conditions again, the plastic tends to shift.
The heat and pressure that the steam cleaner sends down into the carpet often affects the polyurethane backing itself, making it more pliable than it was intended to be and bending it in new forms. As a result, the carpet itself buckles, creating bubbles and pockets where the heat-warped carpet base is no longer aligned parallel to the floor.
As the carpets dry and the heat goes away, these bubbles tend to dissipate, and the polyurethane reverts back to the form it was original cast in. Sometimes, however, some bubbles remain, and in this case professional carpet installers are called back in for a "re-stretch," where the carpet is cut and refitted to the floor, stretching out any warping.