Styrofoam vs Eco Foam: what the word means and what the moisture data really says

Somebody asks whether our board is better than styrofoam roughly once a week. It is a fair question with a confusing answer, because the word is doing two jobs at once and the honest reply depends on which job you mean.

First, the word

Styrofoam is a registered trademark of DuPont, and it refers to extruded polystyrene. The blue board. That is the same material category we supply: XPS, extruded polystyrene.

What most people in the United States mean when they say styrofoam is something else entirely. Coffee cups, coolers, packaging peanuts, the white crumbly bead board at the lumber yard. That is EPS, expanded polystyrene, a different product made a different way.

So “XPS versus styrofoam” is not really a comparison. If you mean the trademark, you are comparing XPS to XPS. If you mean the white stuff, the real question is XPS against EPS, and that one has a proper answer.

Why the moisture behaviour differs

The two materials are the same polymer. The difference is entirely in how the board is made, and that shows up in the cell structure.

XPS is extruded: the polystyrene is melted, mixed with a blowing agent and forced through a die as one continuous mass. It cools into a uniform matrix of small closed cells with no continuous path between them.

EPS is expanded: individual beads are puffed with steam and fused together in a mould. Each bead is closed, but the spaces between the beads are not. Those interstitial gaps join up, and that network is what carries water into the board.

The short-term numbers are not close

The standard test is ASTM C272: submerge a sample for 24 hours and measure how much water it took on by volume.

Our published figure is 0.2 percent by volume. The XPS standard caps every type at 0.3 percent. EPS runs from 2 percent for the denser types up to 4 percent for the lighter ones, so roughly ten to twenty times more water on the same test.

Vapour permeance follows the same pattern. Our board is rated at 0.70 to 0.71 perm at the 2 inch reference thickness, which is low enough that it functions as a vapour retarder in its own right rather than needing one behind it.

Now the part most suppliers leave out

If we stopped there you would have a tidy story and an incomplete one. That 24 hour immersion test measures uptake. It does not measure what happens over fifteen years in wet ground, and it says nothing about whether a board can dry out again.

On that longer question the published field data does not favour XPS. Studies circulated by EPS manufacturers report XPS losing considerably more of its thermal resistance over time below grade than EPS does, on the grounds that XPS takes on less water but holds onto what it takes. Work associated with Oak Ridge National Laboratory has been cited showing below-grade XPS losing anywhere from 10 to 44 percent of its thermal resistance over a long service life.

Two caveats worth applying to all of it. Most of the long-term data favouring EPS is published by EPS manufacturers and their trade association, and most of the short-term data favouring XPS is published by ours. Everybody in this argument is selling something. And “below grade in permanently wet soil” is a harsher condition than most of what we supply into.

So here is where we land, and it is deliberately not a sales pitch: if your assembly is going to sit in standing water for decades, do your own reading and design for drainage rather than trusting any manufacturer’s absorption number, ours included. Moisture performance is a drainage and detailing problem before it is a product-selection problem.

Where the comparison is not contested

Compressive strength. This one nobody argues about.

EPS runs roughly 10 to 25 psi depending on type and density. To reach the strength of an XPS board it has to be made considerably denser, which means more material for the same result. XPS is available from about 15 psi up to 100.

That is the difference a crew feels rather than reads. At 40 psi you can run a power buggy or a mini skid steer straight across the board without wrecking it, which can take the pump truck off the job entirely. The 25 psi board sold at most supply houses does not survive that pass, and neither does standard EPS.

It is also the reason a higher grade is forgiving of an imperfect subgrade. A soft spot under a slab has a far better chance of being spanned by 60 psi board than by 15.

What we would tell you on the phone

• Under a slab, over a properly drained base: XPS at the grade the load calls for. Residential and light commercial is 40 psi, truck traffic is 60, industrial is 100.
• Deep below grade in wet soil: get the drainage right first. That matters more than which polystyrene you pick.
• Anywhere equipment will cross it before the pour: XPS, and not the 25 psi board. This is the clearest case for it.
• Cups and coolers: genuinely EPS, and genuinely nothing to do with insulation board.

One detail that comes up once people are choosing: our board is available with a shiplap edge as well as a plain butt edge, in every grade. The shiplap laps at the joint rather than butting, which closes the thermal gap between sheets and makes the finished wall or slab measurably more efficient. It costs nothing extra to specify and it is worth asking for.

See the three grades and their full test data, or tell us about the job and we will tell you which one you need, including when the answer is that you do not need the upgrade.

Sources

• DuPont, Styrofoam XPS insulation: EPS versus XPS
• XPS Association, The truth about water absorption
• IIBEC, Moisture absorption in polystyrene insulation and effects on in-service design R-values
• Insulfoam, Sink or swim: EPS moisture performance
• Wikipedia, Styrofoam, on the trademark and its genericised use

Eco Foam Solutions product figures are from the published ECO-40, ECO-60 and ECO-100 specification sheets. Styrofoam is a trademark of DuPont and is used here only to discuss what the word means. We are not affiliated with DuPont.