What to wear under a ski helmet, and what not to

Why anything underneath is a fit question

A helmet works because it sits in a specific relationship to your skull: level, about a finger width above the eyebrows, in contact all the way round. The sizing page covers how to check that. Putting a compressible layer between your head and the shell changes two things.

First, it takes up room, so a helmet that fit is now tight, or a helmet that was loose is now temporarily snug in a way that disappears the moment the hat compresses. Second, a thick knitted hat is exactly the kind of squashy layer that lets the shell move relative to your head during an impact, which is the opposite of what the fit is for.

This is not a reason to ski with a cold head. It is a reason to solve the warmth problem with something thin and designed for the job, and to solve the actual cold spots, which are almost never the top of your head.

What actually goes under a helmet

A thin merino or synthetic skull cap is a few millimeters thick, does not compress much, and adds warmth without changing where the shell sits. The same material logic applies as everywhere else on this site: wool or synthetic, not cotton, because a cotton cap holds sweat against your scalp and then cools it. The base layer page covers the reason in full.

The balaclava is the better answer

If it is cold enough that you are thinking about a hat, a thin balaclava is the better purchase, because it solves the gaps that are actually making you cold: the neck, the ears, the cheeks, and the strip of forehead between the helmet brow and the goggle frame.

Two fitting rules matter. It goes under the helmet at the head and under the goggle foam at the face - never tucked up inside the goggle frame, because your breath then becomes the goggle’s humidity source and it will fog within minutes. The anti-fog page is largely a page about this one mistake. And it should be thin enough that the helmet still fits the way it did without it.

The face protection page compares balaclavas, neck gaiters and face masks, and covers the one hard rule: no scarves, ever, because a loose scarf on a lift is a genuine hazard.

Where the heat is actually going

A helmet is a foam shell. It is already a decent insulator, which is why most people find a helmet warmer than the hat it replaced. If your head is cold in one, work through these before adding thickness.

• The vents are open. Most helmets have adjustable vents and most people never touch them. Closing them on a cold day is free warmth, and opening them in spring is free cooling.
• The ear pads are out. They come out for warm days and for audio drop-ins and then never go back. The audio page covers that trade.
• There is a gap at the forehead. Between the helmet brow and the goggle frame. It is the coldest exposed skin on your body and it is a fit problem between two products; the fit page covers it.
• Your neck is open. Warm air escapes upward out of a jacket collar. A gaiter closes the chimney.
• You are damp. A sweaty head under a helmet on a cold chairlift is refrigeration, and the fix is a wicking liner rather than a warmer one.

The other direction: warm days

On a spring afternoon the problem reverses. Open every adjustable vent - the Smith in this ledger publishes eleven vents with five adjustable, and the OutdoorMaster publishes fourteen - and take the ear pads out, which is what they are removable for. Ear pads are not part of the certified impact structure, so removing them changes warmth and hearing rather than protection.

What not to do on a warm day is take the helmet off. The snow is no softer in April, and spring snow that has frozen overnight is harder than midwinter powder, not softer.

Liners, sweat and washing

A thin liner has a second benefit that nobody mentions: it keeps your sweat off the helmet padding. Helmet pads absorb everything and are much more annoying to wash than a skull cap, and a liner you can throw in with the laundry is the easiest way to keep a helmet from developing a smell over a season.

Most helmet pads are removable and washable, and the helmet care page covers doing that without damaging the liner - which mostly means cool water, mild soap and air drying, and never a machine or a radiator.

Children

Same rules, with one addition: check the fit with whatever they are actually wearing underneath, on the day. A child who wore nothing under the helmet in December and a balaclava in February has a helmet that fits differently in those two months, and a child will not tell you it has become tight.

Use the dial to take up or release the difference rather than changing what they wear, and re-check the fit every few outings, because a child’s head grows through a size band over a season. The kids helmet chart has the measurement bands, and the kids layering page covers the rest of keeping them warm.