If your house is on low stilts or has a crawlspace, the underside is prone to mold, including toxic black mold. These areas are often dark, poorly ventilated and damp, which are the perfect conditions for a mold infestation. While treating mold is usually a simple procedure, in this case it is complicated by the need to do the job in the probably cramped space under your house, so special care is needed to do the job properly and safely.
Ventilate the underside of your house by rigging an extension cord and a fan so that a regular stream of air blows underneath your house. Treating black mold will release its spores into the air, and blowing these spores from out of the underside of your house is essential.
Mix a treatment solution in a bucket, using 1/4 cup of Borax per 1 qt. of water. Pour this into your spray bottle with the funnel.
Remove any spongy materials that have been infested with black mold, such as insulation or gypsum board. These materials cannot be treated and should be discarded and then replaced.
Saturate the black mold infestation on the underside of your house with the Borax solution, and allow that to sit in place for roughly three minutes. In the case of an infestation on brick, which has a porous surface, wait for five minutes.
Scrub the mold from the moldy areas with your scrub brush. Wipe away the resulting scuzz with a wet rag.
Rig another extension cord and set up a UV lamp under your house, so that the UV rays bombard the moldy area. UV lamps offer a good substitute for mold-killing sunlight. Leave the lamp in place and the fans blowing overnight to guarantee that all spores under your house have either been flushed out or killed.
Return to the underside of your house the next day and inspect the results of the treatment. If there is any black mold remaining, repeat the procedure.