CoreLogic released its Home Price Index (HPI) Wednesday, which showed that residential property values in the United States remained flat in July as transaction volumes continued to decline.

According to the CoreLogic HPI, national home prices, including distressed sales, showed no change in July 2010 compared to July 2009. The company revised its June 2010 HPI upward from a 1.4 percent increase to show a 2.4 percent year-over-year gain compared to June 2009.

July was the first time in five months that CoreLogic reported no year-over-year gains, and the company says a growing number of declining markets underscore weakness in the housing market without tax-credit support.

“Although home prices were flat nationally, the majority of states experienced price declines and price declines are spreading across more geographies relative to a few months ago,” said Mark Fleming, chief economist for CoreLogic.

Fleming went on to explain that home prices fell in 36 states in July, nearly twice the number in May and the highest since last November when national home prices were steadily declining.

According to CoreLogic’s study, the five states with the highest 12-month home price appreciation in July, including distressed sales, were: Maine (+4.5 percent), South Dakota (+4.3 percent), California (+3.7 percent), New York (+3.0 percent), and Virginia (+2.6 percent).

The states with the greatest price depreciation were Idaho (-12.6 percent), Alabama (-9.7 percent), Utah (-5.6 percent), Oregon (-4.8 percent), and Washington (-4.3 percent).

Including distressed transactions, the peak-to-current change in CoreLogic’s national HPI (from April 2006 to July 2010) is -27.7 percent. Excluding distressed properties, the peak-to-current change for the same period is -19.5 percent.