Steam cleaning carpets is perhaps the best known way to clean carpets but in my experience, it has not been favored by customers after the application. Reasons are many from it took too long to dry to it didn't get stains out or remove odors we wanted out. Lets take a look at steam cleaning and if it is really a good choice in terms of green cleaning and cleaning in general. Perhaps there is another reason not to use steam extraction that nobody has considered.
When it comes to getting out just normal soiling as in tracking dirt in from outside, Steam Extraction may work just fine. In this case, using very little water should do the trick and not lead to any of the down falls associated with steam cleaning. When heavy soiling does occur, steam extraction is less effective especially in getting out pet stains and their odors. Our company has been called to remove stains that steam cleaning could not get out. Most stains are organic in nature and contain oils. These oils and water do not mix so a detergent is needed to surround the oils in stains forming what is called a micelle so that water can pull the micelle away from the fibers it holds on to (see pic below)
Steam Extraction companies do use detergents but at a weak dilution so are not as effective as they could be at a stronger concentration. Some use no detergents at all and rely on the heat of water (steam), pressure at which the water is introduced and the power of water extraction (amount of vacuum). To break up soils, a very good amount of water is needed to remove the dirt. So if a detergent is used, it is not 100% extracted and leaves a soap residue. This is not good (see blog on use of shampoo on carpets) as the residue mats down fibers and attracts more dirt. If there is no detergent, than the use of so much water becomes problematic at best. Depending on the pad used under the carpet this pad can become saturated and will not dry effectively and will invite the growth of mold and mildew. Foam padding is most likely to allow this. (see below)
Carpet pads with a vapor barrier would reduce the possibility of mold/mildew but very few people order such pads to keep cost down.
Besides saturating your pad with water, Steam extraction is poor at lifting long standing stains and can be one reason why carpets buckle or become delaminated. See below.
Delaminated carpet will need repair and re-stretching. The only alternative to Steam Extraction is low moisture enzyme applications. Various enzymes can break up all the organic stains that show up on and in carpets. The micelle creation by enzymes are far smaller and therefore break up soiling in a far superior way without any of the drawbacks that steam extraction can produce.