OPINION
FABE, Chief Justice.I. INTRODUCTION
Kristine Owen (formerly Fardig) appeals a modification of custody rights denying her custody of her minor children. She claims that the doctrines of res judicata and collateral estoppel precluded the trial court from *11considering her drug abuse. Moreover, she argues that the superior court improperly weighed the evidence in making its decision. The superior court's decision satisfies the appropriate standard of review and is therefore affirmed.
II FACTS AND PROCEEDINGS
Kristine Owen and Earle Fardig separated in September 1994. They divorced August 16, 1995. Owen and Fardig had been married for twenty-seven years during which time they had eight children together. Of those children, only three-Anna Fardig, born April 9, 1985; Bethany Fardig, born April 4, 1988; and Edith Fardig, born January 12, 1990-are presently under the age of eighteen. Due in part to evidence of domestic violence by Fardig, Owen was granted custody of the five minor children at the time-the above three plus Sarah Fardig, born March 3, 1980, and Andrew Fardig, born November 24, 1982-and awarded appropriate child support.
In July 1996, approximately ten months after the divorcee decree was issued, Fardig moved for a modification of custody, claiming that Owen's substance abuse impaired her ability to care for the children. Based on a report by a custody investigator indicating that Owen "may be abusing alcohol, pain killers and other controlled substances," Superior Court Judge Peter A. Michalski issued an order on December 18, 1996 granting interim custody to Fardig.1 Owen was ordered to undergo an assessment for drug, alcohol, and narcotics addiction.
Superior Court Judge Sen K. Tan denied Owen's subsequent motion for interim eusto-dy on May 7, 1999, although Owen was granted weekend visitation with daughter Edith. At the June 2000 hearing on Fardig's motion to modify custody, the court learned that Owen had moved to California. Judge Tan subsequently entered a final custody decree, giving sole legal and primary physical custody of Andrew, Anna, Bethany, and Edith to Fardig. Only the custody of Bethany and Edith was in dispute, as Owen had agreed that it was best for Andrew and Anna to remain with their father. Judge Tan found that Owen's move to California constituted a substantial change in cireumstances. Owen was granted only supervised visitation with the children in Alaska, with the possibility of summer visitation dependent upon a drug and alcohol assessment.
Owen appeals Judge Tan's custody decree, although she now only contests custody of Edith.
III. STANDARD OF REVIEW
We will not reverse the superior court's decision to modify custody unless there has been an abuse of discretion or the controlling factual findings are clearly erroneous.2 An abuse of discretion is established where the superior court "considered improper factors in making its custody determination, failed to consider statutorily mandated factors, or assigned disproportionate weight to particular factors while ignoring others." 3 A factual finding is clearly erroneous "when a review of the record leaves the court with a definite and firm conviction that the superior court has made a mistake.4 Issues of res judicata and collateral estoppel are questions of law which we review de novo.5
IV. DISCUSSION AND ANALYSIS
A. The Issue of Owen's Alleged Drug Abuse Is Not Barred by Res Judica-ta or Collateral Estoppel.
Fardig based his motion for modification of child support in part on claims that Owen was abusing drugs. Owen argues that this issue was dismissed with prejudice by Superior Court Judge Rene J. Gonzales at a *12July 21, 1995 domestic violence hearing. However, there is nothing in Judge Gonzales's interim order to support this claim. Moreover, Owen's res judicata and collateral estoppel claims lack merit because consideration of Owen's drug use in the context of a motion to modify custody does not relitigate a past decision. Indeed, Fardig points to Owen's drug abuse as a substantial change in cireumstances. In other words, Owen's alleged drug abuse was not being "relit-igated." 6
B. Judge Tan Had Sufficient Evidence To Find that Owen's Move to California Constituted a Change in Circumstances.
Owen contends that Judge Tan should not have placed weight on her move to California in determining custody because neither party raised this issue and because the move was temporary, made for the purpose of helping an ill mother-in-law. A finding of a substantial "change in cireum-stances" is necessary before a custody modification hearing can be held.7 Judge Tan listed Owen's move to California as the "substantial change in circumstances8 that justified the custody modification hearing. We have previously held that a move to another state constitutes a substantial change in Judge Tan commented that although the move was a temporary relocation, it could last for another year to eighteen months.
The evidence before the court was sufficient to support the conclusion that the move was potentially long-term. Due to the fact that Owen was requesting summer visitation, the move could reasonably have been interpreted as less temporary than Owen herself contends. Furthermore, when asked how long she intended to stay in California, Owen, who was participating telephonically at the June 2000 hearing, testified first that it might be one to two years and later stated that she was not sure if she will ever come back to live in Alaska. Consequently, it was not clearly erroneous for Judge Tan to find a substantial change in cireumstances meriting a reconsideration of custody.
C. Judge Tan Had Sufficient Evidence To Support a Change of Custody as Being in the Best Interests of the Children.
Owen challenges the allegation of drug and alcohol abuse on the ground that there was insufficient evidence introduced at the June 12, 2000 custody trial to support the allegation and the subsequent change in custody. Owen points to two doctors' letters saying she was not abusing drugs or alcohol and claims that Fardig presented no evidence to the contrary.9 While Judge Tan did *13find evidence of drug and alcohol abuse, it is clear from his oral findings that this was not the only factor relevant to his decision.
Though the written findings are rather cursory, Judge Tan made extensive oral findings of fact to support his order changing custody from Owen to Fardig.10 Judge Tan determined the best interests of the children by discussing each of the statutory factors listed in AS 25.24.150(c).11 The first factor discussed by Judge Tan was the physical, emotional, mental, religious, and social needs of the child. He found that Bethany and Edith, the only children whose custody was in dispute, needed emotional support, a stable home, and unconditional love. For the next factor, the ability of the parents to meet the needs of their children, Judge Tan found that Fardig provided a better environment for the children, noting that Fardig's living situation seemed more stable, that he provided them with sufficient food and clothing, and that he was sending the children to see a therapist. Judge Tan worried that until Owen settled into a more permanent living arrangement, she would not be able to provide the support that the children needed.12
Judge Tan next noted that because Bethany and Edith were only twelve and ten years old he did not feel it would be appropriate to take their preferences into consideration.13 Judge Tan found that the factor of love and affection toward the parent was also inconclusive, since each child expressed love and affection for both parents. For the factor of length and stability of living arrangements, Judge Tan found that both of the children were happy living in Anchorage and had a good group of friends here. Judge Tan next found that each parent had a difficult time allowing the children to have an open relationship with the other parent. Judge Tan found that there was insufficient evidence to conclude that either parent had committed domestic violence.
'Judge Tan next addressed the drug use by each parent. Judge Tan focused on the fact *14that Fardig had been in an eight-month substance abuse program and had attended meetings of Alcoholics Anonymous and Nar-coties Anonymous as evidence that he was getting his drug problem under control. Judge Tan chastised Owen for not making similar efforts. Judge Tan considered the two doctors' letters that Owen submitted to be unsworn statements because neither doctor testified at trial. Judge Tan further noted that the letters were unclear about what medications Owen was taking and what testing and drug assessment she was undergoing.14 Consequently, the trial court found little value in the letters.
Further support for Judge Tan's conclusion can be found in the trial testimony of Janice Bernzott, a therapist who had previously counseled Andrew Fardig and was continuing to counsel Bethany and Edith Fardig at the time of the trial. Bernzott, who has a masters degree in clinical psychology and a nursing degree, testified, based on comments that the children made to her, that she believed that Owen was currently using drugs.15
Earle Fardig testified that Owen refused to enter the same treatment program that Fardig had earlier undergone for addiction to prescription drugs; furthermore, Fardig testified that Owen even denied that she had a problem. Andrew Fardig, their son, testified that his mother was very manipulative and frequently seemed to "be on something." Jennifer Fardig, their daughter, testified that her father did not smoke marijuana or abuse alcohol and that he would not let the children smoke cigarettes. In addition, Jennifer related a conversation in which her mother discussed being beaten by her husband, Bob Owen, and 'using drugs with him. This testimony further supports Judge Tan's finding of possible drug use by Kristine Owen.
Finally, Judge Tan concluded it was in the best interests of Bethany and Edith to be kept together and with their other siblings because of the emotional support that the siblings provided for each other.
In sum, the evidence does not support Owen's claim that Judge Tan's findings on the statutory factors involved in determining the best interests of the children were clearly erroneous.
D. There Is Evidence in the Record To Support the Trial Court's Finding that Owen's Visits with Her Children Be Supervised.
Under JFE. v. J.A.S., unsupervised visitation by parents is "the norm.16 " A requirement of supervised visitation must be supported by findings that "specify how unsupervised visitation will adversely affect the child's physical, emotional, mental, religious, and social well-being." "" Bernzott testified that Owen should have unsupervised visits with her children only after a psychological evaluation and drug assessment of Owen and even then only after a transitional therapy period involving both Owen and the girls. When Owen confronted Bernzott with the letter from Dr. Beirne stating that she was not using drugs, Bernzott testified that this was not consistent with statements the children had made to her. Bernzott concluded by stating: "My clinical opinion is that the children would best be served by supervised contact with [Owen].17 Although Owen has not previously been required to have supervised visitation with her children, the testimony at trial provides support for Judge Tan's decision to require supervised visitation.
Moreover, we have previously stated that "we prefer that a court ordering supervised visitation also specify a plan by which *15unsupervised visitation can be achieved." 18 Judge Tan outlined such a plan when he directed that Owen undergo a rigorous clinical assessment showing she was clean and sober before he would consider, upon motion by Owen, unsupervised visitation.19 Consequently, Owen has been provided with a means for regaining unsupervised and summer visitation of Edith should she wish to do so.
v. CONCLUSION
Owen's res judicata and collateral estoppel claims lack merit because consideration of Owen's drug use in the context of a motion to modify custody does not relitigate a past decision. Owen has failed to demonstrate that the superior court abused its discretion or was clearly erroneous on a factual determination with regard to the modification of custody. Consequently, this aspect of the superior court's decision is AFFIRMED. The requirement of supervised visitation for Owen is supported by evidence at trial and is also AFFIRMED.
. Sarah Fardig, being no longer a minor, was not subject to this order.
. Valentino v. Cote, 3 P.3d 337, 339 (Alaska 2000).
. Siekawitch v. Siekawitch, 956 P.2d 447, 449 (Alaska 1998).
. Id.
. Renwick v. State, Bd. of Marine Pilots, 971 P.2d 631, 633 (Alaska 1999).
. Owen also asserts that al the July 29, 1996 hearing, Judge Michalski improperly placed the burden of proof on her to show that she did not abuse drugs. Owen is correct that the burden of proving a substantial change in circumstances necessary for custody modification is on the moving parent. S./N.E. v. R.L.B., 699 P.2d 875, 878 (Alaska 1985). However, the custody investigator's report that preceded the interim custody order essentially provided evidence of Owen's drug abuse. Fardig does not claim to be exempt from the burden of persuasion but insists instead that he satisfied it. The evidence, namely the custody investigator's report, is sufficient to show that Judge Michalski was not clearly erroneous in concluding that there was a strong possibility that Owen abused drugs.
. AS 25.20.110(a) ("An award of custody of a child or visitation with the child may be modified if the court determines that a change in circumstances requires the modification of the award and the modification is in the best interests of the child."); see also Barrett v. Alguire, 35 P.3d 1, 5 (Alaska 2001).
. House v. House, 779 P.2d 1204, 1207-08 (Alaska 1989). Owen correctly points to House to support the proposition that it can be in the best interests of the child to be in the custody of the parent who is moving out of the state. This, though, does not lend any support to her assertion of custody other than to say that it is a possibility.
. Owen further contends thai Judge Tan abused his discretion by failing to give proper weight to a taped conversation in which her son Jon discusses selling marijuana to his father, Earle Far-dig. Judge Tan did accept the tapes as evidence, but apparently did not consider this evidence to be sufficient under AS 25.24.140(c)(8) to award custody to Owen over Fardig as being in the best interests of the children. The statute provides as a factor in determining the best interests of the child "evidence that substance abuse by either parent or other member of the household directly affects the emotional or physical well-being of the child." By invoking an abuse of discretion standard in this particular situation, Owen must *13show that Judge Tan gave "disproportionate weight to particular factors while ignoring others." Siekawitch v. Siekawitch, 956 P.2d 447, 449 (Alaska 1998). While perhaps Judge Tan might have ordered some sort of drug assessment for Fardig, Judge Tan did not clearly give "disproportionate weight" to the relative drug use of Owen as compared to that of Fardig for reasons that will be discussed below.
. Judge Tan commented that because the custody investigator did not testify at trial, the custody report relied upon below was used only as background support. Judge Tan stated that he made bis decision "based on the evidence that has been presented in court and testified to."
. The statute provides:
(c) The court shall determine custody in accordance with the best interests of the child under AS 25.20.060-25.20.130. In determining the best interests of the child the court shall consider ~
(1) the physical, emotional, mental, religious, and social needs of the child;
(2) the capability and desire of each parent to meet these needs;
(3) the child's preference if the child is of sufficient age and capacity to form a preference;
(4) the love and affection existing between the child and each parent;
(5) the length of time the child has lived in a stable, satisfactory environment and the desirability of maintaining continuity;
(6) the desire and ability of each parent to allow an open and loving frequent relationship between the child and the other parent;
(7) any evidence of domestic violence, child abuse, or child neglect in the proposed custodial household or a history of violence between the parents;
(8). evidence that substance abuse by either parent or other members of the household diréctly affects the emotional or physical well-being of the child;
(9) other factors that the court considers pertinent.
. A trial judge is not permitted to condition custody on a parent's ability to relocate. Moeller-Prokosch v. Prokosch, 27 P.3d 314, 317 (Alaska 2001). However, the impact of any potential move may be taken into consideration by the court in assessing the best interests of the child. Id.
. Owen contends that Edith is of an age where her opinion could be asked as to with whom she wants to live. AS 25.24.150(c)(3) requires that when determining custody the court take into account the child's preference "if the child is of sufficient age and capacity to form a preference." Whether a child is of sufficient age to have a meaningful preference for one parent or the other is a question committed to the discretion of the trial court. See Jenkins v. Handel, 10 P.3d 586, 590 (Alaska 2000). Here, a therapist testified that Edith was not mature enough to make a decision about where to live.
. This is not an entirely accurate statement: The letter from Dr. Beirne states that Owen has undergone "periodic urine drug and alcohol testing" and never tested positive. This does not, though, negate the fact that Dr. Beirne did not testify before the court.
. In addition, Bernzott testified that Fardig participated cooperatively in Andrew's counseling, but that Owen only came.to counseling once and chose not to come back again, despite an invitation to do so. This lends support to the conclusion that Fardig was working toward rehabilitation but Owen was not.
. 930 P.2d 409, 409 (Alaska 1996).
. Id. at 413-14.
. Monette v. Hoff, 958 P.2d 434, 437 (Alaska 1998).
. Both Earle Fardig and Andrew Fardig testified that if allowed to take the children to California, Owen would keep the children there. Flight risk could potentially be a justification for supervised visits. See Monette, 958 P.2d at 436. However, this claim was unsupported by anything other than these vague allegations.