If you pedal with any sort of effort, you'll just overheat and sweat inside the ski helmet. I rode in temperatures down to around 0 degrees F this winter and the bike helmet with ear covers was fine by itself most of the time. Only when it got under 10 degrees did I install the rest of the winter kit to plug some of the vents.
My helmet is a Bell Metro, which has a reasonable amount of venting and can be equipped with a snap-attached rain cover, visor-mounted mirror, and winter kit with ear and vent covers.
I only used the rain cover one time and it was worse than getting rained on because of how much my own perspiration condensed inside the helmet. It made me every bit as wet and hot and uncomfortable.
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