My regular helmet fits great during the summer but is near its largest adjustment. Anything more than a thin skull cap won't fit underneath. Also the air vents that are so nice at 90 F are freezing my forehead and top of my head now that we are hovering around the freezing mark. I plan to ride even colder as long as the roads are decent so I need to come up with something better.

Does anyone here use a "winter" helmet a size larger to allow for a knit hat underneath? Would the extra padding decrease the effectiveness of the helmet? Are there winter specific helmets with fewer vents? I've seen some helmet covers for wet weather that must be some kind of Gortex or similar material. Would something like this keep your head warmer by decreasing airflow through the helmet?

For similar reasons, I looked a bit at ski helmets. A ski helmet seems to be fit for use as a winter bike helmet. It does not have vents (or only small ones), is insulated and usually is round shaped.

I like my balaclava during the winter. It's warm enough but not so think that it significantly changes the fit of the helmet. Also, my reflective rain cover is water and air proof so that gets pretty warm. The combo of balaclava to keep the wind off my ears and the helmet cover to keep the warm air in does me pretty well until I don't want to go any colder (I don't do much riding below the teens).

Tape up the vents? I was considering doing that this winter

Ski helmet = great idea. Similar speeds and similar needs for protection.

I've got an Underarmor headband that works great to keep my ears warm, but its the top of my head and my forehead that are freezing. I have a thin balkava but I honestly don't need the face protection at these temps. I'll look into a cover for the vents. If that doesn't do it, I may be in the market for a ski helmet. I looked online and some of the Giro and other models look very much like bike helmets with fewer, smaller vents and more protection for the ears and nape of the neck.

I use a ski helmet, but only when the weather is very cold or if the ride is short. Mine is vented, but it can be too warm on a milder day. However, they are ideal in very cold weather.

I use Giro Nine both for skiing and biking between fall and spring.

The Giro Ionos comes with a winter insert that reduces airflow through the vents. That and either a winter-weight cycling skullcap or a balaclava and I'm good to go.

I use the helmet with a rain cover, coolmax do-rag, and a silk balaclava. The balaclava covers most exposed skin then I put on and over the glasses Scott brand ski goggle. The wind and cold take a toll on my eyes and they water, then I can't see. The ski goggle is made to stay clear while exercising so usually it is all right with glasses. You can buy other lenses for night, snow, etc. Don't forget to cover the skin like parts of the nose and lips. Use some form of oil base product. Petroleum jelly is good but some sunscreen can work too.