Dimensional Properties of EPS

I’ve been having an interesting convo with a colleague on Linkedin recently, the subject: dimensional stability of Expanded polystyrene.  To see the original post, please click this link:  https://www.linkedin.com/feed/update/urn:li:activity:6418502244446007296/?commentUrn=urn%3Ali%3Acomment%3A(activity%3A6418502244446007296%2C6418665654492205056)

I wished to respond directly on Linkedin, but as we are limited to a certain number of characters, I chose to post here which gave me the opportunity to shamelessly promote my own website!  ( shame shame shame!)

My convo was with Richard Beyer, a consultant who researches safety issues with foamed plastics.


Dimensional stability is one of the properties tested for in order to meet the S701-11 standard in Canada, and the ASTM C 578 in the U.S. There is a max dimensional change allowed, 1.5%  I believe. Most manufacturer’s test results generally exceed this requirement by a large margin, but that’s another discussion.  

Now, if you read that link that you sent me, and I quote ” All samples stored at a higher temperature (60°C) reached dimensional stability by the end of this study. Thus, air diffusion into EPS foam, encouraged by the high temperature storage, was found to play a significant role in post-molding shrinkage.”

Take note of that, and this quote :”However, EPS also experiences post-molding shrinkage; it shrinks dimensionally from its molded size after processing. This means parts must be stored in warehouses until they are considered stable by the industry standard, DIN EN 1603. This often takes 11–18 weeks and is thus very timely and expensive.”

I started out in EPS selling Insulated Concrete forms, back in 1999, when I knew absolutely nothing about any foamed plastic. Part of the certification of these products calls for a “drying period” of 30 days minimum because we knew that EPS would shrink. That’s why ICF’s, and any molded product for that matter, is actually slightly over-molded, so that when it does shrink, it does so to it’s intended finished size. Typically, Thermofoam, ICF’s, other molded construction products, are produced, and then stored to allow for the “drying” and dimensional shrinkage as required.  ( newer manufacturing methods/equipment actually remove a larger amount of the moisture used in the process, reducing this “drying time” ).  Now to the point about storing at higher temperatures (60 C): There is one industry that requires “heat curing” of the EPS, and that’s the EIFS industry. This is because this industry absolutely must have a stable substrate, otherwise there is potential for system failures. Some manufacturer’s have adopted this practice to also speed up the drying time for the other eps products they produce. ( Large billets of foam, other molded parts) 

Second quote about 11-18 weeks to dry. Most of the older, larger producers will mold the large blocks that are used to fabricate eps products during the “slow period”  ( a.k.a.  winter! UGH!) and let them sit on the floor for future modification. I have seen this with my own eyes. They sit on the floor during the lower humidity months of winter ( up here in the Great White North) , and then when approaching the busy season, they are used to create stock and packaged up for whatever product is desired.

So sir, you are correct about the shrinkage of EPS, however, in rebuttal ( respectfully of course), the EPS in the pic shown has two great things going for it:  It is tongue and groove, therefore even if there was shrinkage or movement, the fit of the joints would compensate for that; secondly, the certification of the product dictates that it must be allowed to “dry”, meaning that this product was produced and stored for the required drying time before it was permitted to be shipped for use.  Most importantly, I have a very happy and satisfied client who saved money while meeting the building code requirements!