Short answer: pit zips and side vents dump heat mechanically, and on a hard climb that matters more than the breathability rating on the hangtag. A waterproof membrane can only move vapor when the air outside is cooler and drier than the air against your skin. Open a zipper and you skip that physics entirely — you push warm, wet air straight out and pull cool air in. If you routinely reach the top of a climb soaked from the inside, the fix is usually venting hardware and venting technique, not a more expensive laminate.
Below: how the three vent designs compare, how to tell whether a shell has vents before you buy, and how to vent one that does not. Every product detail is quoted from our own spec sheets — where a shell lists no ventilation, we say so.
Why a "breathable" shell still leaves you soaked
Waterproof/breathable laminates move moisture as vapor, and vapor only moves when there is a gradient pushing it. Warm humid air inside your jacket, cooler drier air outside: the vapor migrates outward. That gradient is what makes the membrane work.
Three things flatten that gradient, and rainy days tend to bring all three at once:
- The outside air is already saturated. It is raining. The air outside your shell is close to holding all the water it can. There is much less pull on the vapor sitting against your base layer.
- Your output outruns the membrane. Grinding up a switchback, you generate sweat faster than any laminate transmits vapor. The excess condenses on the inside of the shell, and it feels exactly like a leak.
- The face fabric wets out. When the DWR finish stops beading and the outer fabric saturates, that wet layer further slows vapor transmission. This is why every one of our shells carries the same care line: tumble dry low to reactivate DWR, no fabric softener. We wrote a full explainer on that in What Is DWR? Why Your Jacket Stops Beading Water.
Vents solve a different problem than the membrane does. The membrane handles the slow, steady moisture load at a moderate pace. Vents handle the spike — the climb where you produce far more heat than any fabric can process.
Pit zips vs. side vents vs. pocket vents
Three designs show up on rain shells, and they move very different volumes of air.
| Vent type | Where it sits | Airflow | Trade-offs |
|---|---|---|---|
| Pit zips | Under each arm, running from the bicep down toward the ribs | Moderate — targets the armpit, the hottest, sweatiest zone on your torso | Awkward to reach with a pack on; adds zipper weight and one more seam to seal; the opening is limited by the length of the zip |
| Full-length side zips | Both sides, hem to underarm (the "TorsoFlo" style) | Largest — opens the entire torso, and warm air rises out the top while cool air enters at the hem | The most zipper on the jacket, so the most weight; the panels can flap in a crosswind if you leave them wide open on exposed ground |
| Mesh-backed pocket vents | Hand or chest pockets with mesh instead of solid lining | Smallest — often more marketing than airflow | Blocked by a pack hipbelt; blocked again the moment you put anything in the pocket; nothing to open on the upper torso where heat collects |
| The main zipper (free) | Center front, plus cuffs, hem drawcord and hood | Moderate to large, and it costs nothing | Lets rain straight down your chest if you open it too far in a real downpour |
The reason full-length side zips move the most air is geometry, not magic. Heat rises. An opening that runs from your hip to your armpit creates a chimney: warm air exits high, cool air is drawn in low, and the exchange continues as long as you keep moving. A pit zip is a single opening near the top of that column — useful, but it cannot set up the same circulation.
How to tell whether a shell has vents — before you buy
You usually cannot tell from a photo. Pit zips sit in a shadowed area under the arm and are almost never visible in a straight-on studio shot. Side zips look like a seam when they are closed. Price is no signal either — plenty of expensive alpine shells skip vents on purpose to save weight and reduce failure points.
The reliable method is to read the spec table and look for a ventilation row. Product pages list features that exist. If there is no ventilation line, assume there are no vents — a brand that built vents into a jacket does not forget to mention them.
Here is that test applied to our own rain shells, quoting our spec sheets exactly:
| Shell | Ventilation row on the spec sheet | Weather protection | Stated temp range |
|---|---|---|---|
| FM-202 Foray Tactical Shell | "TorsoFlo full-length side zippers" — opens hem to underarm | 3-layer waterproof/breathable laminate, taped seams, DWR face fabric | Approx. 25–55°F |
| FM-115 Alpha Technical Shell | No ventilation row. Breathability listed as membrane-based vapor transmission only | 3-layer laminate with nylon face, fully taped seams, helmet-compatible storm hood | Approx. 25–55°F |
| Gamma-Shell AR Hardshell | No ventilation row. Membrane-based vapor transmission only | 3-layer laminate, fully taped seams, weather-sealed zippers, helmet-compatible hood | Approx. 20–55°F |
| FM-303 Eco-Tech Torrent Shell | No ventilation row. Membrane-based vapor transmission only | 3-layer recycled nylon, taped seams, DWR; self-stows into its own pocket | Approx. 30–60°F |
One shell in four — roughly what you will find across the category. There is a second tell, too: the FM-202 is the only one of ours whose breathability line reads "membrane transmission plus active venting." The other three say membrane transmission and stop there. When a spec sheet distinguishes passive from active, it is telling you something real.
Do not read the product name as a spec
We have made this point before about UPF numbers and fabric names, and it applies here too: the words in a title or URL are marketing, and the spec table is the document. The FM-202 happens to have "Side-Zip Ventilation System" right in its name and a matching ventilation row — those agree. Do not assume that holds everywhere. Check the table.
How to vent a shell that has no vents
Most people own a shell without vents, and most of the time that is fine — because technique does most of the work. In rough order of how much heat each move dumps:
- Start cold. The single highest-leverage habit in this whole guide. If you are comfortable standing at the trailhead, you are overdressed for the first climb. Be slightly chilly for the first five minutes.
- Hood down first. You lose a great deal of heat through your head and neck. Dropping the hood costs you nothing in rain protection as long as it is drizzling rather than pouring, and it is instant.
- Drop the main zipper. This is the biggest opening on any jacket. Open it to the sternum on a climb, and if rain is driving in, open it from the bottom if your shell has a two-way zipper — you get the chimney effect without a channel straight down your chest.
- Open the cuffs and loosen the hem. Both of our vented and unvented shells have adjustable hems and cuffs. Loosening the hem drawcord lets air enter at the bottom, which is what makes an open front zip actually circulate rather than just sit there.
- Change your pace before you change your clothes. Slowing down by a small margin on a steep section often costs less time than stopping to strip a layer, and it prevents the sweat in the first place.
- Take the shell off. Light rain plus hard effort frequently means you are drier in a fleece than sealed inside a hardshell. There is no rule that says the waterproof layer stays on because it is raining.
Or: do not wear a hardshell at all
The most common venting mistake is wearing far too much jacket for the conditions. Two alternatives cover a large share of "it is kind of raining" days:
- The Norvan-Shell SL Ultra-Light Wind Jacket weighs about 175 g in size M and stuffs into its own left chest pocket. Its spec sheet is explicit that it is windproof and water-resistant in light rain, not waterproof — use a hardshell for sustained rain. For wind and drizzle it breathes better than any membrane because there is no membrane to fight.
- The Aero-Gamma SL Technical Softshell is DWR-treated and, again per its own page, water-resistant, not fully waterproof. Rated for roughly 35–65°F with 4-way mechanical stretch, it handles cool, breezy, lightly wet days without the sauna effect.
If you are unsure which of the two categories you actually need, Windbreaker vs. Rain Jacket and What Is a Softshell Jacket? break down the decision in more detail. Browse the full range in rain gear for stormy conditions and jackets.
When side zips are worth paying for
Venting hardware costs money and adds a little weight. It earns its place in some situations and not others.
| Your situation | What matters most | Where to look |
|---|---|---|
| High output in sustained rain — steep climbs, fast hiking, working in weather | Active venting. Your sweat rate is the limiting factor, not the laminate | FM-202 Foray, the one shell with a ventilation row |
| Cold, exposed, mostly static — alpine belays, ridgeline weather, standing around in sleet | Seal quality and hood coverage. You are not producing enough heat to need vents | Gamma-Shell AR or FM-115 Alpha, both with fully taped seams and helmet-compatible hoods |
| Commuting, travel, uncertain forecasts | Packability. The shell that is in your bag beats the one at home | FM-303 Eco-Tech, which self-stows into its own pocket |
| Wind and drizzle, high aerobic pace | Low weight and no membrane to trap vapor | Norvan-Shell SL, roughly 175 g, or gear from fast-and-light trail running |
Sizing and layering affect venting too
A shell cut tight over a fleece cannot circulate air even with every zipper open, because there is no air gap to move. Both the FM-202 and the FM-115 advise sizing up if you plan to wear a thick mid-layer underneath — pick the size that lets your intended mid-layer sit inside without compressing it, because a compressed fleece insulates worse and vents worse. For how the whole stack works together, see Fall Hiking Layers and How to Layer for Winter Hiking.
Keep the DWR working, or venting has to do more
Once the face fabric wets out, vapor transmission slows and open zippers are doing all the work. Every shell page above carries the same care line: machine wash cold, tumble dry low to reactivate the DWR, no fabric softener. Softener leaves a residue that attracts water — the opposite of what DWR is for, as covered in Does Fabric Softener Ruin Moisture-Wicking Shirts?
Frequently asked questions
Do pit zips and side vents leak?
Closed, they are a sealed zipper like any other on the jacket. Open, water can obviously get in — that is what an opening is. The practical answer is that you open them when the trade is worth it: light rain plus hard effort means you are wetter from sweat than you would be from a little spray coming through a vent. In driving rain on exposed ground, close them.
Are side zips better than pit zips?
They move more air. A full-length side zip runs from hem to underarm, so warm air exits at the top while cool air enters at the bottom — a continuous exchange rather than one opening. Side zips also make the jacket much easier to pull on over layers. Pit zips are lighter and do not flap in crosswinds, so they are the more common choice on minimalist alpine shells.
How do I know if my current jacket has vents?
Look at the spec sheet for a ventilation row rather than at photos, because closed zippers under the arm are nearly invisible in product shots. On the jacket itself, run your fingers along the underarm seam and down each side seam feeling for zipper teeth under the fabric. If the spec sheet does not list ventilation, it almost certainly does not have it.
Can technique replace vents?
Mostly, yes. Starting cold, dropping the hood, opening the main zipper, loosening the hem and moderating your pace will handle the majority of days. What technique cannot match is a full-length side zip during sustained high output in sustained rain — that is the specific case where the hardware wins.
Should I size up if I want to vent well?
Size for the layers you actually plan to wear underneath. Our shell pages recommend sizing up if you will wear a thick mid-layer beneath, and that guidance helps venting as well: air needs somewhere to move. A shell stretched tight over a fleece will feel clammy no matter how many zippers you open.
The short version
Breathability ratings describe how a fabric behaves when conditions are favorable. Vents describe what you can do when they are not. If you climb hard in wet weather, look for a ventilation row on the spec sheet and treat its absence as a real answer — then learn to use the main zipper, cuffs, hem and hood you already have, because those work on every jacket you will ever own.
Start with our stormy-conditions rain gear if you want a shell built for weather, or the wider jackets collection if you are still deciding which layer the job calls for.