In Davis v Davis (2016 ONSC 1589), the Superior Court of Justice made a decision on the issue of whether spousal support should be awarded retroactivity or prospectively. After analyzing the facts of that case, the court noted the expenses incurred by the payor by way of expenses for matrimonial home and decided that any spousal support payments shall only be applied prospectively.
http://www.canlii.org/en/on/onsc/doc/2016/2016onsc1589/2016onsc1589.html