Hydrophobic Properties of Foam

I know this has probably been discussed extensively here, but the search function does not work all that well.

When comparing types of foam used for blanks, which are the least likely to suck up water?

I have long held to the idea that poly foam was adopted as a replacement for balsa due to its resistance to waterlogging.

I have never used EPS or XPS foam but seem to recall that one or both tend to suck water more than poly.

Please shed some light based on hands on experience and let me know if I have this right.


EPS absorbs the most water because it has pores/channels between the foam beads.  (Air and water moves between the beads.)

XPS absorbs almost no water because it is closed cell foam.

EPS bonds better with epoxy than XPS because it has the connected air spaces between beads.  XPS bonds poorly because it has small “closed cells” that absorb virtually no fluid.  Both are polystyrene foams.

Polyurethane absorbs less fluid than EPS.  The advantage of polyurethane is that it bonds fairly well with PE and Epoxy (higher surface energy than polystyrene) and has larger cell structure that will absorb some fluid.

Balsa is significantly more dense (avg. 10 pcf) than polyurethane (about 3 pcf originally) – aprox. 3x heavier.

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I know that both EPS and XPS  are styrene based, hence the incompatiblity with poly resin. I was having a discussion with someone who claims poly foam is like a sponge. They also said that styrene based foams absorb less water. I always understood the opposite to be the case.

XPS absorbs less than 0.3% water by volume.  Less than 0.1% for HD-40 and above.

EPS will soak it up.

Polyurethane less so than EPS.  Unrepaired, PU decomposes over time.

The tight fused, closed cell EPS like US Blanks and Marko use, supposedly soak less.  As their name implies;  The cells(beads) are closed and the beads are so tightly fused together that water does not penetrate gaps between the beads.  The problem is the shaping process , which opens the beads.  Foams that are not as tightly fused absorb quite a lot water between cells.  A tight fused EPS blank,sealed with Epoxy before glassing is a pretty good bet for less water absorption.  My opinion only.  Based on what people at Marko have told me.  In fact I have hauled a few back from Orange County to Oregon in the rain on the roof with no water absorption.  Freshly molded blanks unshaped though.

Polyurethane foam is a tight open cell foam like pvc foam polyimide foam, etc. It absorb water slowly but can take a lot.

Eps and xps are closed cell foam. Each cell don’t take water if not open. In eps there is more or less space between cells that can take water, lot of water by succion when foam enclosed and pressure difference between outside and inside. No space between cells in xps so no water intake. By itself plastic use for all those foams have poor adhesion with resins.

XPS absorbs the least water and shapes nicely, but is the most problematic when it comes to bonding.  It loves to delaminate.  Especially if exposed to even moderate heat.  (I had one board blow up like a balloon when a buddy left it leaning near a space heater.  Aaaaaargh!)  I thought I read somewhere that it has polyethylene added to it to improve the extrusion process(?)  Don’t quote me on that, but my experiences with it make it easy to believe.

The guys at XTR have worked out sourcing for the most user-friendly XPS and figured out a process of perforating the skin to prevent delamination.  They have a pretty loyal following.  As a home-builder in Florida, I won’t bother messing witht the stuff any more.

Sorry to jump into the thread,

But that XPS … I just wanted to ask if someone here has tried other glues than epoxy on xps.

I talked to a technician working for a glue manufacturer a while ago, and he said :

It’s well known that epoxy doesn’t work on thermoplastics …for that purpose it’s better to use 2k polyurethan glue .

Would it be possible to glue a veneer on Xps with that PU stuff and epoxycoat later?

The product he adviced is technicoll 8301…

Don’t know if it’s available everywhere…






For the new people, I have extensive experience in plastics molding.  If I were to rate foams in order of water absorbtion, xps, chemically expanded polystyrene, would be the least absorbtive, polyurethane the next, and finally eps (bead type polystyrene).  Imagine a ball of string wrapped loosly around a liquid core, then imagine the core turning to vapor and the ball of string expanding to three or four times it’s size.  That’s eps.  You break the surface and you have gaps between the strings and in the core.  There is some difference in highly fused eps like in Marko blanks, but it is still more absorbtive.  Bead fusion is a tricky thing.  I know from years of experience.  Marko does a excellent job for surfboard blanks, but the inner beads are still somewhat unfused.  That is because steam is used to fuse & expand the beads and beads closer to the surface seal off the inner beads at a certain point.  I know the people at Marko and highly recommend their product if you want to use polystyrene.  I have used their blanks in the past and will do so again when my customers want a superlite board.  Xps is a nightmare for surfboard blanks.  When Clark closed, I saw many major manufacturers try xps and all never brought it to market.  Surfboard manufacturing is such a small market that I think we are lucky to have people like Marko, US Foam, Millenium and Artic making blanks.  A friend of mine bought a inexpensive collector balsa board and a collector pu/pe 60’s single fin and suspended them from his high ceiling in his kitchen.  A couple of years ago he brought the 60’s board to me asking if I could “fix it”.  Anywhere that there was a pinhole in the glass job, grease vapors and penetrated and colored the foam a shit brown.  The balsa board had no “worm tracks”.  It’s not just the limitations of the core, it’s sometimes the limitations of the build.  Just my 2c…

Yes you can glue wood on xps with pu glue effectively. Often use in diy airplanes, small and rc one’s. Some use it for water toys with success so for me it’s a way to experiment.

On DiY airplane build many use xps foam cover with glass epoxy laminate. They fly in their plane so… They use a large cell xps foam from dow, called spider foam or flotation foam and now pib foam if i understand well. I have one small panel, looks like a fine sponge, like pvc foam, adhesion is really strong for sure an effective product, don’t know how to find some. 

Not directed at Lemat or anybody in particular.

FTR the polystyrene in EPS is the same as the polystyrene in XPS.  The reason epoxy bonds better with EPS is the interconnecting pores and channels.  

The generalizations posted at Sways are always amusing.  Factors almost always ignored about XPS are foam density/minimum compressive strength and the effects of perforations (size and number) on epoxy bonding.

Surface energy of polystyrene = 34-36, polyurethane = 38-40 (dyne/cm). â€śTypical cured epoxies have surface energy around 45 dyne/cm. If the substrate’s surface energy is 30 dyne/cm or lower, epoxy adhesives do not adhere well.”

If you want to use epoxy with XPS, use higher densities of XPS with greater minimum compressive strength (at least 25 psi but 40 or 60 would be better).  Multiple “small” perforations will improve epoxy bonding significantly.  XPS is not well suited for “mass production” but most garage/hobby builders are not mass producing surfboards.  


If you live in the U.S. states where winters are mild, most XPS insulation foam available is low density and low minimum compressive strength (15 psi) – a poor choice for surfboard cores.

Adding skins (cork, wood) to XPS helps to significantly disperse impacts.  Don’t leave your glassed XPS board in a closed car or in direct, hot sun on top of your car or on the beach.  Light colors help reflect heat energy from light (red, infra-red).

If you want easy, stick with polyurethane foam.  If you don’t want your board to soak up water when dinged, play with higher densities of perforated XPS (good minimum compressive strength, psi) and take good care of your finished board…


just for clarification:

The higher densities will dent less and delamination won’t occur as frequently. Did I get that right? Or is there another reason for higher compressive strength XPS?


Higher XPS densities (greater compressive strength) are less likely to crush and get bad pressure dings, which lead to delams.  Also the higher densities are less susceptible to tearing which is also part of the delam process.  Finally, for the same reasons stated, epoxy anchors in the perforation holes are less likely to pull out.

Cork skins can compress to 15% of original volume and still recover – high impact absorption.

Yes same analyse for me. Xps delam come from his low elasticity and poor resin adhesion so composit layer delam when it came back after dent and foam don’t move with. A bit of temperature variation and gaz in broken cell puch skin.

A denser foam and stiffer skin reduce this problem, but a stiffer skin on a spongy eps give you strengh/flex at low weight so… 

EPS is the worse for sucking in water, then PU and finally XPS. XPS shapes easily, just like PU foam.

XPS has issues with bonding. Standard glassing techniques tend to have delaminations over time. Heat is a problem and can created huge delams. XTR created a process that uses tiny pinholes in the lam to let air escape. They have a great XPS foam and very good epoxy resin, but they only sell to certified users (at least they used to). I was lucky to buy blanks and resin from our local XTR guy because he was leaving the island and needed to get rid of the stuff he had.

I’ve found that even with very rough sanding, glass will peel off a XPS blank cleanly. One thing I have found that sticks to XPS really good is Gorilla Glue and the other brands of the same type of glue. If you had a full laminate skin like a wood veneer or a layed up and dry fiberglass skin, you could glue it to the XPS and use a vacuum press to hold it firmly to the blank. I think that would produce a sturdy board that will not easily delam. It’s a bit of a hassle compared to making a board with PU/PE.

I like that I don’t worry about dings on my XPS boards, and I don’t worry about pin hoiles when glassing. I have used XPS rail bands on many of my boards. I figure that the rails tend to get the most impacts. I haven’t made a 100% XPS foam board for a while. The last one I did has a huge delam on the deck caused by heat. I plan to strip the deck where the cutlap is and do a fabric insert. May even try using gorrila glue and a vac bag to lam a wood veneer skin, just too lazy to deal with it.

2lb Xps large cells sold by aircraft fourniture shops don’t have delam problems but cost a lot. Guys make DiY small air plane with it and fiberglass for long time.

We made our rc airplane with xps small cell, insulation foam. Like you for us  best weight/strengh/durability was xps core with pu foaming glue wood skin. A friend of rc plane made is kite boards like this with success. He only fiberglass over wood for water protection.

Like you I do some boards with xps rails, I had small delam where dings. Repair as usual no problems. 

I think that my next project, 250 liters+ raceboard windsurf, will be hollow with skin make with pu glue wood and cork over 1 inch shaped xps cover with fiberglass epoxy.