In this appeal from a decision by the district court on a petition for modification of a decree for dissolution of marriage, the noncustodial parent primarily challenges the denial of his request to terminate his child-support obligation and to establish a postsecondary education subsidy for his adult son. The district court and the court of appeals both found he failed to show a substantial change in circumstances justifying modification. We granted further review. We vacate the decision of the court of appeals, affirm the decision of the district court in part, reverse in part, and remand for further proceedings.
I. Background Facts and Proceedings
Bonnie and James Pals were married on August 14, 1982. They had two children. Nicole was born June 19, 1979 and was adopted by James during the marriage. Joel was born August 30,1985.
Bonnie and James were divorced on April 1, 1991. The parties stipulated that Bonnie would have primary physical care of the children, and James would have reasonable visitation with them. The court ordered James to pay child support for both children pursuant to the child support guidelines in the amount of $679 per month. Support was to continue in that amount until Nicole turned eighteen or finished high school, whichever occurred later; or married, died, or became self-supporting. Additionally, the decree contained a separate provision that provided:
In the event that the first child of the parties shall continue with any post-secondary education, the required level of support shall continue at the rate of $679 per month until such time as the first child of the parties either completes the post-secondary education or attains the age of 22, whichever shall first occur, pursuant to section 598.1(2), The Code.
Once the obligation to support Nicole terminated, the monthly support obligation for Joel was reduced to $495. The decree contained the same terms for termination of Joel’s support as for Nicole, and also included the same provision to extend the support obligation in the event Joel pursued a postsecondary education.
Nicole turned eighteen in 1997 and began college. Pursuant to the decree, James continued to pay $679 in child support. Nicole married in October 2000, an event under the decree that terminated James’s obligation to provide support for her and reduced his support obligation to Joel to $495 a month. Notwithstanding, James continued to pay $679 in child support each month until May 2004, when he filed a petition to modify the decree.
Joel graduated from high school in 2004, and began college at Northern Iowa Area Community College in the fall of that year. He moved from Bonnie’s house into a rental house he shared with three roommates. During Joel’s first year of college, James *646 paid the cost of tuition and books not covered by scholarships and grants— $1,118.50.
In the modification proceeding, James alleged the following changes since the time of the decree constituted a substantial change in circumstances justifying modification: (1) he retired as a teacher and began receiving retirement benefits on June 1, 2004; (2) the legislature enacted Iowa Code section 598.21(5A), providing for postsecondary education subsidies, in 1997; (3) the parties never submitted a qualified domestic relations order (QDRO) to the court for approval to implement the original decree’s division of James’s IP-ERS benefits; and (4) his income decreased, while Bonnie’s income increased. James asked the court to modify the decree to enter a QDRO dividing his IPERS benefits according to the provisions of the original decree, terminate his child-support obligation, and order a postsecondary education subsidy for Joel. James also sought attorney fees.
The district court entered its judgment on March 1, 2005. The court concluded James failed to establish a substantial change in circumstances to justify the termination of his child-support obligation or the imposition of a postsecondary education subsidy. However, the court found James overpaid $7912 in child support under the original decree by continuing to pay $679 per month after Nicole’s marriage in October 2000. Accordingly, the court offset his child-support arrearage that had accumulated when he stopped paying support after he filed the modification petition in May 2004, and gave James “a net credit of $8457 yet to be applied to the ongoing post-secondary support obligation owed to Bonnie.” Additionally, the court entered a QDRO implementing the original decree’s division of James’s IP-ERS benefits and ordered each party to be responsible for his or her own attorney fees and one half of the court costs.
James appealed, and Bonnie cross-appealed. James claimed the district court erred in failing to replace the child-support obligation under the decree with an educational subsidy, effective August 2004. He also asked that he be reimbursed for any child-support payments made during the pendency of the appeal not paid over to Joel. Bonnie claimed the district court erred in granting James a credit for the support overpayment. She further claimed the QDRO entered by the district court did not accurately reflect the IPERS division in the original decree. We transferred the case to the court of appeals. The court of appeals reversed the portion of the district court judgment that gave James a credit for the overpaid support, but otherwise affirmed the district court decision. James applied for further review, which we granted.
II. Standard of Review
“A proceeding to modify or implement a marriage dissolution decree subsequent to its entry is triable in equity and reviewed de novo on appeal.”
In re Marriage of Mullen-Funderburk,
III. Modification of Support for College-Aged Child
Dissolution decrees may be modified upon a substantial change in circumstances. Iowa Code § 598.21(8) (2003). To constitute a “substantial change in circumstances,” the changed conditions “ ‘must be material and substantial, not trivial, more or less permanent or continuous, not temporary, and must be such as were not within the knowledge or contemplation of the court when the decree was entered.’”
In re Marriage of Rolek,
Prior to July 1, 1997, Iowa did not have a specific statute to determine each parent’s contributions to their children’s- college education. Instead, our legislature defined child “support” under section 598.1(6) to generally include support of a child between the ages of eighteen and twenty-two who was a fulltime college student. Id. § 598.21(6) (1995). This definition permitted the court to impose a child-support obligation on the noncustodial parent in the event the child pursued a post-secondary education as a fulltime student.
In 1997, the legislature amended section 598.1(6) to remove the postsecondary-sup-port clause from the definition of support, redefined support to terminate at age nineteen, and enacted a separate statute to provide for a postsecondary education subsidy by both parents. 1997 Iowa Acts ch. 175, §§ 185,190;
accord In re Marriage of Mullen-Funderburk,
We considered the operation of the amended statute and subsidy provision in
In re Marriage of Sojka,
In this case, the district court and the court of appeals rejected all of the grounds James raised to support a modification of the child-support provisions of the original decree, including the ground that the enactment of the postsecondary-education-subsidy statute and the subsequent declaration by the legislature that the statute applies to decrees entered prior to July 1, 1997, supported modification. Interpreting our recent holding in
In re Marriage of Rosenfeld,
In
Rosenfeld,
we determined that the postsecondary-education statute applied to a modification action brought after July 1, 1997 to establish support for a college-aged child when the original pre-1997 decree did not establish any level of college-aged educational support.
See Rosenfeld,
We think Bonnie’s argument misreads
Rosenfeld,
and fails to properly apply the postsecondary-education-subsidy statute to permit modification of the child-support provisions of the decree in this case. In
In re Marriage of MullenFunderburk,
we explained that the postsecondary-edu-cation-subsidy statute applied in
Rosenfeld
because the original pre-1997 decree did not establish any level of college-aged educational support, and the issue was being decided for the first time after the enactment of the statute.
In re Marriage of Mullen-Funderburk,
We recognize the language of the statute provides that the postsecondary education subsidy applies retroactively to modify decrees entered prior to July 1, 1997 that provided for support of a child for college “expenses.”
Id.
Thus, the subsidy statute can only be used to modify child-support provisions in decrees for college expenses. The original decree in this case did not specifically designate that the continued support after high school was for college “expenses.” However, we think the support provision was nevertheless the type of provision the legislature wanted to be covered under the retroactivity provision, subparagraph (e). Provisions in a divorce decree that extend a child-support obligation while the child is in college necessarily consider expenses associated with attending college.
See id.
§ 598.21(4)(a) (stating that child support is for the “reasonable and necessary” expenses of a child). This approach is also consistent with our pronouncement in
In re Marriage of Goodman
that the retroac-tivity provision applies to pre-July 1, 1997 decrees “providing for support of a child in college.”
See In re Marriage of Goodman,
We therefore reverse the judgment of the district court insofar as it denied modification of child support by refusing to apply the postsecondary-education-subsidy statute, and remand the case to the district court to eliminate James’s child-support obligation and to determine whether there is good cause to establish a postseeondary education subsidy for Joel under the law and the facts now in existence.
See
Iowa Code § 598.21F(1) (Supp.2005) (“The court may order a postsecondary education subsidy if good cause is shown.”);
see also In re Marriage of Mullen-Funderburk,
We recognize that the application of the postsecondary-education subsidy statute necessarily results in a termination of the prior support obligation. The legislature intended the standard under section 598.21(5A) and the new definition of “support” under section 598.1(9) to apply retroactively to pre-July 1, 1997 decrees. Thus, if the district court on remand finds no good cause to establish a college subsidy, the prior decree must nevertheless be modified to eliminate the existing child-support obligation under the decree. If the district court finds good cause for a subsidy, then the terms of the subsidy *650 modify and replace the existing child-support provision of the decree.
James has asked that the termination of his child-support obligation be made retroactive to August 21, 2004, three months after he served Bonnie with his original notice and petition for modification. See Iowa Code § 598.21(8) (stating child-support obligations “may be retroactively modified only from three months after the date the notice of the petition for modification is served on the opposing party”). We leave this issue to the district court on remand. We also leave it to the district court to determine any claim of reimbursement for support not given to Joel by Bonnie.
IV. Overpayment
One final issue remains for our consideration. The court of appeals determined that James was not entitled to a credit for the overpayment of monthly support after Nicole married, and it reversed the district court modification decree insofar as it granted James a credit of $3457 against future support payments.
“Courts generally do not allow a credit to the obligor spouse for voluntary expenditures made on behalf of the child in a manner other than that specified by a decree.” 24A Am. Jur.2d
Divorce and Separation
§ 1067, at 474 (1998);
accord Harner v. Harner,
James testified he overpaid because he “wanted [his] son to continue to have the same standard of living he had had before.” It was undisputed that James knew Nicole married, knew his support obligation was $495 per month after Nicole married, and he knew he was overpaying his support.
See Palagi v. Palagi,
V. Conclusion
The district court and the court of appeals erred by concluding that a postsec-ondary education subsidy was not permissible in this case. By its terms, section 598.21(5A) allows for a postsecondary education subsidy. We therefore vacate the decision of the court of appeals and reverse the judgment of the district court *652 insofar as it denied termination of support and establishment of a postsecondary education subsidy. We also reverse the judgment of the district court insofar as it granted James a credit for overpayment of support. We remand the case to the district court for proceedings consistent with this opinion.
DECISION OF COURT OF APPEALS VACATED; DISTRICT COURT JUDGMENT AFFIRMED IN PART, REVERSED IN PART, AND REMANDED.
