The storage unit industry's on fire. Rents are rising, units are occupied and competition is tame.

And investors are tuning in by bidding up shares of industry leaders like Extra Space Storage, whose stock is up 33%, crushing the S&P 500 (down 2%).

It's a good industry to be in. Extra Storage Space reported 94.5% occupation rates at the end of Q2, and Public Storage reported a 7% increase in rental income year-over-year.

Big firms are getting in on the action too; Carlyle Group is spending about $80M to build new facilities around the West Coast, targeting areas where the amount of storage space per capita falls below the national average, Rob Stuckey, the firm’s head of US real estate, says.

What's so exciting about self-storage? Actually, it's what's not exciting. The industry is powered by the four D's—death, divorce, downsizing and dislocation—which don't really follow the economic cycle. It's not "recession-proof, but it is recession-resistant," Green Street Advisors analyst Ryan Burke says.

Unfortunately for smaller firms, there's a tight supply on facilities due to long construction times and the fact that people rarely move from a unit. [WSJ]