Short answer. The annual average 30-year fixed mortgage rate in 1980 was 13.74%, on its way to a peak of 16.63% in 1981. The weekly high in 1980 was 16.30% (October).
The 1980 annual average 30-year fixed mortgage rate was 13.74% per the Freddie Mac PMMS series. That was up from 11.20% in 1979 — and would climb further to 16.63% in 1981.
Context — why rates were so high
Federal Reserve Chairman Paul Volcker had taken the chair in August 1979 with a mandate to break double-digit inflation. By 1980 the federal funds rate was over 17%, and the prime rate hit 21.5% by year-end. Mortgage rates followed mechanically.
What did this do to home buying?
New-home sales fell from 709K (1979) to 545K (1980) and then 436K (1981) — a 38% peak-to-trough decline in two years. Existing-home sales fell from 3.83M (1979) to 2.97M (1980) to 2.42M (1981).
The combined 1980–1981 housing recession was the deepest since the 1930s, and remains a benchmark for what double-digit mortgage rates do to transaction volume.
Sources
U.S. Census Bureau Survey of Construction; National Association of Realtors Existing Home Sales report; Freddie Mac Primary Mortgage Market Survey; National Bureau of Economic Research Business Cycle Dating Committee.