Whenever you open your freezer, outside air enters the freezer and allows heat and moisture in. This moisture becomes frost inside your appliance, which can build up over time. If your freezer doesn’t have an auto-defrost option, this frost can become ice that covers interior air vents and temperature sensors. This can cause your freezer to work overtime, exacerbating ice buildup. Even if your freezer has an auto-defrost feature, you may want to defrost it if you’re moving or upgrading your appliance.