Wis. Stat. § 767.531
Subject to s. 767.511 (6m), a party ordered to pay family support under this section, 2019 stats., shall pay simple interest at the rate of 1 percent per month on any amount in arrears that is equal to or greater than the amount of child support due in one month. Subject to s. 767.511 (6m), if the party no longer has a current obligation to pay child support, interest at the rate of 1 percent per month shall accrue on the total amount of child support in arrears, if any. Interest under this section is in lieu of interest computed under s. 807.01 (4), 814.04 (4), or 815.05 (8) and is paid to the department or its designee under s. 767.57. Except as provided in s. 767.57 (1m), the department or its designee shall apply all payments received for family support ordered under this section, 2019 stats., as follows:
Wis. Stat. § 767.531
This section does not limit the authority a trial court would otherwise have to consider imposing interest on unpaid maintenance arrears. Cashin v. Cashin, 2004 WI App 92, 273 Wis. 2d 754, 681 N.W.2d 255, 03-1010. The circuit court must separately calculate child support and maintenance as a condition precedent to calculating family support. If the court applies the percentage guidelines when setting child support, it must set family support at an amount that results in a net payment, after state and federal taxes are paid, of no less than the child support as calculated under the guidelines. Even if a court makes detailed findings as to all of the factors for family support, the court erroneously exercises its discretion if it neglects to provide a rational explanation of how its findings lead to the support award. Vlies v. Brookman, 2005 WI App 158, 285 Wis. 2d 411, 701 N.W.2d 642, 04-0315. It is evident from the statutory framework and the purpose of family support that at least a portion of family support ordered in any case involving minor children is child support. With respect to child support, any provision in a marital settlement agreement that purports to limit a child support payee's ability to seek a support modification upon a substantial change in circumstances is against public policy and cannot provide a basis to estop the payee from seeking a modification. Huhn v. Stuckmann, 2009 WI App 127, 321 Wis. 2d 169, 772 N.W.2d 744, 08-3102.