WASHINGTON (AP) – July 14, 2014 – Would-be home sellers across the country are grappling with a once-in-a-lifetime problem: They have mortgage rates so absurdly low it would hurt them financially to sell.

Doing so would mean giving up an irresistible rate in exchange for a new mortgage carrying a rate up to a percentage point higher. Their monthly payments would be larger even for a house of the same price. That's discouraging some people from selling, thereby limiting the supply of available homes and contributing to slower home sales.

It's a significant shift from the way the U.S. housing market has worked for the past 30 years. For most of that time, whenever a homeowner decided to trade up to a better home, mortgage rates usually were lower than the last time they had bought. That helped make a new purchase seem more attractive.

But that is changing. The average rate on a 30-year mortgage fell below 4 percent in late 2011 and reached a record low level of 3.3 percent in November 2012. It didn't top 4 percent again until mid-2013. Homeowners took advantage of the lower rates and a refinancing boom ensued.

More than one-third of homes with a mortgage now have rates below 4 percent, real estate data provider CoreLogic estimates. Yet mortgage rates now average 4.2 percent. That is still low by historical standards but up about three-quarters of a point from a year and a half ago. And should mortgage rates rise later this year and next, as many economists expect, even more homeowners will be affected.