Qualified energy efficiency improvements such as insulation and exterior windows & skylights must be installed on your main home in order to qualify for the Non-business Energy Property Credit. So you don't qualify for the credit by putting them on a second home.
However, these (and other) improvements can be added to your basis in the property, which will reduce any gain you have when you eventually sell it.
Here's a link to the instructions for IRS Form 5695, which covers the residential energy credits (note page 2): https://www.irs.gov/pub/irs-pdf/i5695.pdf