Preliminary figures from real estate research firm Reis show that the national vacancy rate in the apartment sector dropped 20 basis points in the second quarter and now stands at 4.7 percent—the lowest level in a decade. The figure is 120 basis points below the same time a year ago.

With vacancy rates so low, effective rents may spike, according to Reis.

“With overall vacancy below this level, it appears that rents are beginning to accelerate,” wrote Reis Senior Economist Ryan Severino. “This is only the third time in Reis’s 31+ years of history that the national vacancy rate has fallen below 5 percent.”

Net absorption, or the net change in occupied stock, remained relatively strong, with 25,540 units leasing up, according to Reis. This represents a slowdown from the first quarter’s net absorption of 34,484 units.

“At first blush this seems disconcerting, considering that the second and third quarters of a year typically exhibit seasonal strength and outperform the first and fourth quarters,” Severino wrote. “However, this moderation in vacancy compression is not unexpected. It is imperative to note that the market is in a very tight position. Vacancy has not been this low since the wake of the dot‐com boom more than a decade ago and there is a paucity of available units. As the market tightens and vacancy reaches very low levels, landlords shift their strategy for growing revenue from vacancy decline to accelerating rent increases and that is exactly what is transpiring.”

Effective rents increased at their fastest pace since the third quarter of 2007. Asking rents rose 1 percent in the quarter and are up 2.7 percent from a year ago. Effective rents (asking rents net of concessions) rose 1.3 percent in the quarter and are up 3.5 percent from the second quarter of 2011. The gap between asking and effective rents is also narrowing with landlords offering fewer concessions.

Brightening the picture further is the fact that only 9,452 apartment units came online in the second quarter.

However, developers are beginning to accelerate the pace of construction. Reis expects about 70,000 units to come online in 2012. That is about double the rate of supply growth in 2011. Even more units are slated to come online in 2013, somewhere in the order of 150,000 to 200,000 units in the 79 main markets that Reis tracks.