If you're surfing cold water seriously, the Core-X ARC-Zip range was built for you. But with three thickness options — 4/4, 5/4, and 6/4 — the question isn't which suit is best. It's which suit is right for your water, your sessions, and your surfing.
Here's everything you need to know.
See it in action:
What Makes the ARC-Zip Different
Before we get into thickness, it's worth understanding what sets the Core-X apart from every other cold-water wetsuit.
The ARC-Zip entry system follows a smooth ergonomic arc across the shoulders — away from high-movement zones — rather than a straight zip that pulls and restricts. The result is a suit that seals completely against cold-water flushing while giving you full, unrestricted paddling movement from the first wave to the last.
Every Core-X suit is built entirely from 100% Yamamoto limestone neoprene — lighter, warmer, and more durable than standard neoprene, with a closed-cell structure that retains heat more effectively and dries faster between sessions.
All seams are GBS — glued and blind stitched — then covered with 4-Way Stretch Tape: a thin, comfortable, high-stretch neoprene tape placed over every interior seam to minimise water entry and maximise warmth retention, without adding bulk or restricting movement.
The integrated hood with lip warmer locks in heat around the neck and shields your face from wind chill and freezing water. Flip it up when you need it, tuck it away when you don't.
Across all three suits, the arms and shins are 4mm — keeping movement free and unrestricted where you need it most. The body thickness is what changes between models, and that's determined by one thing: water temperature.
Lunasurf & Yamamoto: Built on Years of Partnership
Yamamoto Corporation in Japan produces the finest neoprene in the world — and Lunasurf has worked directly with them for many years to bring that material into every suit we make. This isn't a supplier relationship. It's a partnership built on a shared commitment to performance, quality, and pushing what a wetsuit can do.
Some brands claim to use Japanese limestone neoprene or market themselves as Yamamoto suits — but use it only in select panels, or not at all. Every Core-X ARC-Zip suit is built from 100% genuine Yamamoto limestone neoprene throughout — no compromises, no blending with cheaper materials, no misleading claims. Yamamoto's closed-cell limestone-based neoprene is softer, warmer, and lighter than petroleum-based alternatives, with superior stretch and faster drying times. It's also more environmentally responsible — limestone is a far more sustainable raw material than crude oil.
That direct relationship with Yamamoto means we can specify exactly what we need — and build suits that perform at the highest level, session after session, in the coldest water on the planet.
Water Temperature Guide
| Water Temp | Recommended Thickness |
|---|---|
| 7–15°C (45–59°F) | 4/4mm |
| 6–12°C (43–54°F) | 5/4mm |
| Below 9°C (48°F) | 6/4mm |
UK and Irish waters typically range from 8–16°C depending on season and location. Norwegian and Icelandic waters can drop well below 8°C in winter.
Core-X 4/4 Hooded ARC-Zip — Cool to Cold Conditions
Best for: Autumn and spring surfing, milder Atlantic swells, water temps 7–15°C
The 4/4 is the most versatile suit in the Core-X range. 4mm body, 4mm arms and shins — uniform throughout, delivering serious warmth without the bulk of a thicker winter suit. Ideal for shoulder-season surfing where flexibility matters as much as thermal performance.
The front chest and torso down to the knees is thermal lined, locking in core heat where you need it most during long paddle-outs and cold waits between sets.
If you're surfing the west coast of Ireland or the UK from September through November, or again from March through May, the 4/4 is your suit. It's also the go-to for Californian winters and those wind chill days in Baja where the water is cool but the air makes it feel colder.
★★★★★ Fine wetsuit!
“What do you want from a hooded 4mm? For me it's got to be properly, absolutely, extremely warm and still not get in the way. Worn this one half a dozen times now and it's a new best friend. Fully snug (can't imagine ever needing more warmth than this offers), not restricting and a decent fit. No water gets in and the hood fits brilliantly. Bloody great. It's like wearing a super power. 💥 If it's anything like the other Luna suits I'll get a few seasons out of it too. ✌️”
— moderatetofresh, verified buyer
★★★★★ No sh*t this suit is unreal
“From the very first moment you put the suit on you realise you're wearing something different. The thought that has gone into developing this new style suit is evident from the first time you pull the hood over your head and zip it up. Once you get out into the water the experience is the same. The new and improved hood closure system is next level — even in the worst wipeouts there is not a drop of water that gets in through the closed zip. Also the new sized hood and chin cover is so cozy. But remember boys and girls: ‘Don’t leak body fluid into your suit.’”
— Ryan C, verified buyer
Shop the Core-X 4/4 Hooded ARC-Zip →
Core-X 5/4 Hooded ARC-Zip — Cold Water Performance
Best for: Winter surfing in the UK and Ireland, water temps 6–12°C
The 5/4 is the go-to winter suit for most UK and Irish surfers. 5mm body, 4mm arms and shins — extra insulation where you need it most, without sacrificing the paddling freedom that makes the ARC-Zip system worth it.
Full front and back thermal lining from chest to knees wraps your core in maximum insulation — keeping your body temperature stable even in the coldest water and windiest conditions.
This is the suit for December through February on exposed Atlantic breaks, or year-round for surfers in Scotland and the Northern Isles where water temperatures stay cold well into summer.
★★★★★ Used it on the coldest days
“I bought the 5/4 hooded for a trip in January to the most northern reaches of Scotland. The suit feels like one piece which really helped with the wind and stopped any flushing in bigger surf. I was most impressed with its warmth — even sitting in a windy session for 3 hours with long waits between sets I was incredibly warm. This suit combined with the Luna 8mm boots and there is no limit to session length.”
— Ross, verified buyer
★★★★★ Best suit ever
“So happy with this suit, it's the best suit I've had yet. Really warm and incredibly stretchy but the best bit is the ease of getting into it and the lack of restriction in the shoulder area. It fits so well and is so comfortable — it feels like you're not wearing anything — incredible suit! And ridiculously warm.”
— Dozza, verified buyer
Shop the Core-X 5/4 Hooded ARC-Zip →
Core-X 6/4 Hooded ARC-Zip — Maximum Winter Warmth
Best for: Serious winter surfing, extreme cold water, water temps below 9°C
The 6/4 is built for surfers who don't check the temperature — they check the charts. 6mm body, 4mm arms and shins, maximum thermal retention. If you're surfing Iceland, Norway, or the north coast of Scotland in mid-winter, this is the suit that keeps you in the water when everyone else has gone in.
Like the 5/4, the 6/4 features full front and back thermal lining from chest to knees — complete core protection that keeps your body temperature locked in no matter how long the session or how brutal the conditions.
Despite the extra body thickness, Yamamoto neoprene keeps the 6/4 lighter and more flexible than you'd expect. Pair it with boots, gloves, and the integrated hood and you have a complete cold-water system.
★★★★★ Warm and toasty
“I'm a surfer from Iceland. I must say I just got my new gear, surfed a six hour session — snowing, hail, cold day right now — I was so warm and toasty throughout the whole session. Love it, highly recommend Lunasurf suits and gear. PS love the new zipper.”
— Robert S, verified buyer
★★★★★ Top notch
“I surf in Iceland and have been waiting for this new design to drop. As always, it did not disappoint. Cannot recommend this enough. Best quality for a good price on the market in my opinion.”
— David, verified buyer
Shop the Core-X 6/4 Hooded ARC-Zip →
Quick Comparison
| Suit | Body | Arms & Shins | Water Temp | Best For |
|---|---|---|---|---|
| Core-X 4/4 | 4mm | 4mm | 7–15°C | Autumn/spring, California, Baja wind chill |
| Core-X 5/4 | 5mm | 4mm | 6–12°C | UK/Irish winter, Scotland year-round |
| Core-X 6/4 | 6mm | 4mm | Below 9°C | Iceland, Norway, north coast of Scotland |
Still Not Sure?
Where are you surfing? If it's the UK or Ireland in winter, the 5/4 covers most surfers most of the time. If you're regularly surfing Scotland, the Northern Isles, or travelling to colder destinations, go 6/4. If you're a shoulder-season surfer or based somewhere with milder winters, the 4/4 is your suit. Or if you don't feel the cold, move a lot during sessions — or pair up with our thicker boots and gloves — many people do full winters in Iceland and Scotland in just the 4/4.
How do you run temperature-wise? If you feel the cold easily, size up in thickness. If you run warm, you can often go one thickness lighter than the guide suggests.
How long are your sessions? Longer sessions in cold water demand more thermal protection. If you're regularly surfing two hours or more in sub-10°C water, the 6/4 is worth it.
Built for Cold Water. Built to Last.
Every Core-X ARC-Zip suit is made with Yamamoto neoprene, GBS seams sealed with 4-Way Stretch Tape, and the ARC-Zip entry system — designed for surfers who take cold-water surfing seriously and want a suit that performs session after session, season after season.
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Yamamoto Wetsuit Boots | 4mm & 8mm Cold Water Surf Boots