Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
Included in this category are units held for occasional use, temporarily occupied by persons with usual residence elsewhere, and vacant for other reasons.
US. Bureau of the Census, Housing Inventory Estimate: Vacant Housing Units Held Off the Market for the United States [EOFFMARUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/EOFFMARUSQ176N, December 10, 2016.