More evidence that the foreclosure pain that began in overbuilt middle-class communities is moving higher up the real-estate food chain: The share of foreclosures in the most expensive third of housing markets is on the upswing.

Around 30% of foreclosures are coming from homes in the top tier of the housing market, up from 16% when the foreclosure crisis began three years ago, according to new research from real-estate Web site Meanwhile, the bottom one-third of the market now accounts for just 35% of foreclosures, down from 55% in 2006.

The Zillow research compared homes against the median value for their respective local market, and broke each market into three tiers’ the bottom, middle and top third, by value. Zillow then looked at the share of monthly foreclosures in each tier over the past decade.

The Zillow area chart (above) offers a good visual representation of what’s happening. When the foreclosure crisis accelerated in 2007, foreclosures increased among all price levels, but they rose from historically lower levels at the middle and top tiers, increasing the foreclosure share among those price brackets.

This summer, foreclosures began to pick up again, and the high-end share of homes are accounting for a growing piece of the foreclosure pie. “The slope of that curve in recent months is much sharper than it was recently,” says Stan Humphries, chief economist for Zillow.

Foreclosures are moving up the food chain for a variety of reasons (see this WSJ story for more). One key factor: deterioration among prime mortgages to borrowers with good credit. These often included exotic mortgages, such as option adjustable-rate mortgages and interest-only mortgages, that were increasingly used to buy more expensive homes. Borrowers often aren’t able to refinance out of these products because home values have fallen, leaving them with little equity in their homes.

Zillow estimated that nearly one in four homes with mortgages was worth less than the value of the property at the end of June. Mr. Humphries says he doesn’t expect to see foreclosure volumes level off until later in 2010.