high to low, and low to high works for both pressure and temp, but the effects of temp on actual height above sea level or standard datum plane, is very small compared to the 1" of HG per 1000ft.
To answer the original question though, density altitude IS NOT A HEIGHT REFERENCE! Its an indication of aircraft performance. Engines and Lift work better in colder/denser air. So on a hot day, your 172 wont climb as fast as it would on a cold day. The actual value of DA is in FT yes, but it is comparing how your AC would perform against a standard day or whatever.
EX. Density altitude is 3000ft at an airport at sea level. That means your performance on the ground will be as if you were already at 3000ft.
Clear as mud?