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.