Thanks for the link about apartment buildings. I was surprised not to see collateral mentioned. Maybe it's just a commercial phenomenon but as I understand it, many loans require additional collateral when the rent doesn't make enough profit over the interest payments.

What this means is it's cheaper from a capital standpoint to leave an apartment vacant instead of rent it at market rate IF market rate hasn't risen with the cost of interest. As I understand it that's a factor which effectively reduces supply and makes the market that much tougher for the renter.