There's a tax credit available for making energy efficiency upgrades. However, it now has a lifetime limit of $500. If I, as a single person, claimed the credit in the past, can I, now as a joint filer, claim the tax credit again, for a different set of improvements (different house), given that my wife never took the credit?
This is more comment than answer, but too lengthy for a comment block:
I was intrigued by your question, I don't have a definitive answer, but the instructions for form 5695 mention married taxpayers with more than one home, and how the limit is an individual limit provided they are separate homes. I decided to put a call into IRS tax support who were unable to provide guidance (not a good time of the year for tax questions that aren't straightforward).
Given that the address is different and the credit has never been claimed on a return that your spouse is named on, I think you have a solid argument for your spouse claiming the credit on the new house.
Being a cautious fellow, I would not attempt to claim it without first getting a more definitive answer from the IRS, will update if I get a better response from IRS support.