my EPS block is waterproof...(WTF?)

I was reading specs about some board producers they’re claiming they have waterproof EPS in their boards now. That got me thinking if the blocks I shape from are waterproof too? -I took an unsealed dry chunk, weighed it, and pinned it into a buket of water, waited 3 days, pulled the lid off, the EPS is still super bouyant; cracked open the block, super dry internally, really cool bead structure, like octogons. Is this test pointless? (i.e. pressure cooker simulation collapses the bead, etc.?). (Also, 2 edges of the test block were hand broken exposing the factory bead structure. I was thinking that hot wired edges tend to seal out water).

Made by ACH Tech. plant is in Denver, 60 miles from where I live. The reseller guy at his warehouse said it wasn’t waterproof, but I think he didn’t know much about the foam except the density. I use 1.5lb moslty because I shape flatwater hulls. I’m assuming the 2 lb and 3 lb is bombproof super tight cell structure…

What do you guys make of this? -surf this foam dinged w/pinholes in my lam or what?


uh…I just did a vaccum & water test on my foam and it’s fused-cell by all indications.  So the “bellows” concept probably applies to the other crap…if they even still produce the other crap (like the Lowes sheets that have the film on–yeah, expect those to suck water…But this ACH stuff rocks, it doesn’t suck water even under high vac pressure. Is it 100% waterproof? -Is anythng? Probably not and ACH might have to state it this way to prevent lawsuits. …I’m stoked, thanks @ nj-surfer for sharing that vid!

@nj_surfer…That vid is pretty damn convincing in the face of what the manufacturer of my foam has to say. I guess I should vacuum test my foam see if it’s fused cell. Surftech has to get it from somewhere unless they own the patent and make it themselves?

makes more sense…so then what about these guys building boards with waterproof EPS? -snake oil or is this real tech that exists?

I’m convinced it exists…


…Uh, yeah just called ACH; they say there’s is no such thing as waterproof EPS. Their 1.5lb EPS has rated water absorbtion 3% by volume, and the 2 lb is 2% by volume. Same rating of 2% for their 3 lb foam. They even furthered this statement by saying XPS isn’t completely waterproof I thought was interesting even though I don’t shape this stuff.

yep, nearly the same. tricky, though becuase the exterior of the chuck has some water sticking on it. ~1-2 grams heavier after 3 days, chunck is the size of a cantalope. exterior water coming out of the bath or does she suck it in you think?


Edit: also, I don’t have the most precise scale, it rounds grams. This whole test was ballparking and getitng a feel if this foam wicks decently or sucks like a wh*#$.

try a week submerged with all edges hand-broken and after the test have the exterior dry then re-weigh? gonna call ACH too and ask what’s the deal. Maybe it’s ERDO, maybe it’s something else.

Stuff doesn’t suck in water anywhere close what EPS seems to be known for…or at least what other’s have experienced in the past. I feel better about surfing this foam in the tropics I know that much.



Dunking and holding isn’t an accurate test for a surfboard.  EPS surfboards actually suck water rather than passively absorb.  It has to do with the shell of fiberglass,  and the expansion and contraction of the shell.  Think of a blacksmith’s bellows.  You do a turn, and the board deck gets squeezed, and pushes air out.  You finish the turn, and the board expands back to neutral, and sucks water to replace the expelled air.  You put the boad on the roof of your car and it heats up, and the expanded air is expelled through the ding.  You put the board in the cold water, and the air inside contracts, and sucks water in.

In surfboard repairs, I’ve seen water get sucked into boards.  I’ve seen a surftech with a  small ding in the nose a full pound or more than it should have been

I’d never use a EPS blank in a board I was traveling with.

Did you weigh the chunk before and after?

Edit: I read you weighted it before, but you do not mention the weight difference.

Did another vacuum test. This time, 1.5lb EPS 1/2" thick slab. No water suction/penetration. Looks like it’s anything but snake oil. If this stuff didn’t smell like a public swimming pool when you shape it, I’d say it’s the perfect foam. I don’t miss PU, looking forward to dinging and repairing AFTER a long surf trip! Cheers!



My guess is that it is freshly manufactured, so the beads are fused better.  Take your sample, and flex it a dozen or so times.  Stress it like it would be stressed in a real board.  Then do the vacuum test.  That could also be why the surf tech video seemed to not suck water.  Surftech used new foam.  Competition is older foam, with a little break down.

I’ve always wondered what would happen if you vacuumed a sealer like really thinned future acrylic, rather than water into an EPS blank.  Would it then become waterproof?

Count me as a skeptic.

Even PU soaks water over time, so to say that EPS doesn’t soak at all?

Also, did you notice how much open foam there was?  And how long did the test last?  a second or two?  All that test proved was that in a new surftech blank, air sucks easier than water.

Try it again, where the surftech foam is sealed exept where the water is and where the vacuum hose is attached, so all the vacuum force is sucking the water.  That would be closer to a surfboard ding submerged under water.

How much pressure is the board under during a wipe-out or even a duck dive?  How deep can you swim before you need to equalize you ears?  A few feet?

Smoke and mirrors.


I ride HWS’s, short and long.  If the air is warm, and the day sunny, and the water cool,  when I open the screw vent out in the lineup, it will suck in trememdous amounts of air. On the L board, I can remove the vent entirely and it can take 3 seconds to equalize through s 3/8" opening, giving you an idea how much the temperature can contract the air within.


I usually open the vent twice per session so that any micro water suckers are have less pressure pushing water into them.  Unscrewing the vent is even more important after leaving the water, or the warming expanding air within can split the stringers rendering the board a wall hanger.


Of course, no silver bullet here, and long-term stress test is a good idea too. I’ll do another flex test with the 1/2" piece…ACH tech explained that the cells themsilves are closed and the beads are fused together. The bead looks different that traditional styro. But like they said, it’s ~2% by volume. That surftech you repaired could’ve seen trauma where it was seeping water, where the cells decompressed & the cell structured changed; that could happen to any board. But all in all, I’m impressed with a 40 second high vacuum test. Sure the foam is new; coupled with a good seal, quality lam, it selttles my nerves that perhaps this newer type of EPS has waterproof qualities that make the argument splititng hairs…internal/external pressure is anopther thing I’d prefer not to argue. I don’t use vents, I know a guy that swears by them. Who cares I guess. they’re just vents made for…venting I guess.


…yep, sure as s#%t, man, this foam either holds its beads tightly on a heavy flex or it breaks entirely. I also dented it and tried to suck water through it. Nothing yet. My guess is this foam seeps water only when there’s huge cellular damage or fissures that create a void for water to enter. Flexing and trying to breakdown this bead doesn’t coax water though it. For 1.5 lb, I’m impressed.

The other issue to consider is not just the foam… but the bond between the foam and the skin or composit skin. Whether or not THAT is waterproof is something to consider. Water seeping in between the layers can present serious problems, too.

Also… if there is no chemical bond between foam and skin, and the bond is purely physical, would structural integrity benefit from SOME intrusion? I believe so, to a degree.

Break a peice of that foam. What gets compromised the most, the beads themselves, or the bonds between the beads (the beads tear apart)? My guess is that the beads will tear more than break apart. Because PU foam is rigid and crumbles, the cell walls crack and allow water to soak in. I think this stuff will fatigue less, and resist soaking up water to a much greater degree than even the best PU.

Great thread!


Very interesing insights guys, thanks for that!


…yep, I see more “tearing” on this EPS than a “crumble” or bead break. This foam has a nice flex characteristic pre-shape as well; I don’t hot-wire my rockers anymore. I just bend 3" slabs and g.g. white it to my stringer; or, if I go stringerless, I take rocker bars and set the bars to press the specified rocker into the foam w/wax paper and scrap glass & resin under the bar plate. ~3 hours later I have a bent piece of foam with no rocker waste. Parabolic tape the edges as desired, glass that bitch and go surfing.

 went to the surftech site to look at their FEPS data (Fused Cell EPS); they say they’re the only ones using it…Maybe in AU. They don’t mention a patent, so I gotta assume the EPS industry in general knows how to fuse the cells better w/steam now. I mean, they build boats and commercial docks with this foam.

Instead of saying it’s waterproof, Surftech says “virtually waterproof”. This is basically the foam I have --made by a USA company: ~2% water absorption by volume, doesn’t exhibit sucking characterisitics. Nice quality for such low density, bright white, hand-shapes beautifully. We’ll see how it behaves dinged under heavy pressue (wipeouts, etc.)…thanks for everyone’s input.