Investing in New Strategies for Upgrading Lower Attaining Efficiency Buildings Act of 2021 or the INSULATE Buildings Act of 2021 This bill establishes two grant programs under the State Energy Program of the Department of Energy (DOE). Under the first grant program, DOE must provide grants to states for revolving loan funds. States may use the fund to provide loans to eligible businesses and homeowners for energy audits and upgrades to reduce energy consumption. Under the second grant program, DOE must provide grants to states for training energy auditors.