Can we claim attic insulation installed 12/31/19, but paid for in 2020?

[DaveF1006]

Yes, you can claim this in 2020 since this is when you paid for it.

[Opus 17]

Technically, you claim this credit when the product is installed, not when paid for.  However, in this case I think you are close enough.  If by some bizarre bad luck the IRS audits you and denies it, you could amend your 2019 return to claim the same amount.

From the instructions to form 5695:

Costs.

For purposes of both credits, costs are treated as being paid when the original installation of the item is completed, or, in the case of costs connected with the reconstruction of your home, when your original use of the reconstructed home begins.