On one hand, the meltdown in the housing and mortgage markets have been a boon for apartments, since fewer people are leaving the rental pool to buy a home. On the other, the excess inventory of available housing is applying pressure on traditional units as those residences hit the rental pool. Given the situations, Obrinsky doesn't expect the housing market to correct the supply-demand imbalance for another two years, at best.According to Census Bureau data from midyear, there are 6.2 million for-rent and for-sale units on the market, resulting in a 4.8% overall vacancy rate. Four million of those are rentals, creating a 10.1% vacancy rate for that group.

Between 1965, when the Census Bureau began to track inventory, and 2003, the overall vacancy never surpassed the 4% mark. Yet on the for-rent side, the vacancy is far from record levels, points out Obrinsky. Therefore, the bulk of that overall vacancy surge is attributable to the for-sale inventory, which is at its highest levels since before 2007. Before 2000, the rate never topped 1.6%. Today, available owner-occupied units account for 2.8% of the inventory.

Though the Census Bureau doesn't identify how many of the vacant for-sale units are single-family versus multifamily, NMHC deduced, with data from the American Housing Survey, that single-family units make up 85% of all owner-occupied vacant units, or 2.2 million vacant residences for sale. However, not all of those constitute excess inventory.