Can title 24-2008 compliance be used in lieu of ASHRAE 90.1-200?

Leticia, You can use T24 in California. I assume you are pursing the energy modeling option. Check out the NC EAp2, EAc1 forums. There is a lot of good info there.

Unfortunately, no. You will need to prove that you are 15% better T24-2008 or ASHRAE 90.1-2007 to achieve the minimum of 5 points for this credit. Using Title 24-2010 (although it is more stringent) is not approved to prove that you are 15% better than ASHRAE 90.1-2007. Unless you are using efficient systems and creating an energy model then I would suggest attempting Option 1.

If you are going to prove 15% better than T-24 2008 or 2010 than you need to create an energy model and go the "Performance" route. Prescriptive - Bare minimum measures to meet code (Eg. insulation is bare minimum, window to wall ratio meets current code, systems are per code minimums, installing only the code maximum amount of lighting.) One way to look at it, is that this is the worst building you can legally build. Performance - Proving through an energy model that your building will perform 15% better than a code minimum building. (Eg. your insulation is much better than code, but you might use more windows than allowed by code, you can reduce the amount of lights you are using and use high efficiency HVAC systems and your total builiding energy use for a year will be 15% less than a code building.)