That number is roughly four times the original prediction because new disaster zones were designated and more homes flooded when officials released water from reservoirs to protect dams, according to CNBC's Diana Olick. In total, the number of mortgaged properties in Texas disaster zones is 1.18 million, with Black Knight adding that Houston disaster zones contain twice as many mortgaged properties than Katrina zones, with four times the unpaid principal balance.

Putting the Harvey damange in context, after Hurricane Katrina mortgage delinquencies in Louisiana and Mississippi disaster areas spiked by 25%. The same could happen in Houston, as borrowers without flood insurance weigh their options and decide to walk away from the property. While they will get some federal relief, if rebuilding would cost more than the principal in their homes, they could decide to walk away according to Olick.

What about Irma?