By: Doug Wehrwein, Sales Representative – Architectural Products
I find it interesting that so many people in the construction business give expanded polystyrene a bad rap. Perhaps the only expanded polystyrene they are familiar with, or have heard of, is the ‘beadboard’ from decades ago, characterized as soft and perhaps crumbly. Today’s expanded polystyrene seems to be a widely misunderstand. Manufacturing techniques and technological advances have brought forth vast improvements in the overall quality of expanded polystyrene. It should no longer be viewed as a substandard rigid insulation product. In most applications expanded polystyrene is equal to or superior to extruded polystyrene. Let’s take a look at some of the ways expanded polystyrene and extruded polystyrene (XPS) compare.
Both are produced from polystyrene resin— expanded polystyrene is a resin bead that is expanded, then the beads are fused together in a mold while XPS is a resin that is liquefied, then foamed and drawn through a die.
Expanded polystyrene is available in several different compressive strengths: commonly 15psi, 25psi, 40psi, 60psi. Extruded polystyrene the same plus 100 psi. Compressive resistance comparison is very straight forward. After all, which of these two weighs more, a ton of feathers or a ton of bricks ? Bottom-line, both forms of rigid insulation will provide the same support in like psi rated products.
Both expanded polystyrene and XPS rigid insulation products have the same warranted R value. The R-value per inch of ACH Foam’s Foam Control Plus+ 250, 25 psi expanded polystyrene is 4.8 and is warrantied at 100% for 50 years. The warrantied R value of 25 psi XPS is 4.86. XPS manufacturers only warrant the R values they publish at 90%. The XPS manufacturers are aware of the R value reduction, in fact, they guaranty it in writing. XPS manufacturers use blowing agents that boost the R-values of their insulation products. Those R value boosting blowing agents are held captive within the insulation board for a period of time yet. Eventually those blowing agents dissipate out and are replaced by air. This could happen in as short a period of time as 6 months. A review of this article by Richard Roe explains this. Conversely the blowing agent used to produce expanded polystyrene does not linger in the insulation and, therefore, the R values reported at time of manufacture remain constant. It will not change !
Short Circuiting the R-value ! – Have you ever tried to use a damp or wet oven mitt or towel to pull something hot out of the oven ? OUCH ! The thermal resistance that the thick cloth material typically would give you is gone, it is short circuited by the moisture. The same thing happens with XPS when it’s installed in below grade applications and subjected to moisture over time. Testing shows that XPS does resist the water absorption to a point, but after repeated attacks by the moisture, it eventually lets it in; then resists letting it back out. It a study done by Stork Twin City Laboratory in St. Paul, MN, they found that XPS rigid insulation, 15 years in-service working as perimeter wall insulation against the foundation of an addition to their facility, had taken on almost 20% moisture. Just like the damp cloth of an oven mitt or towel, the XPS lost a great deal of its ability to provide thermal resistance. The R value of XPS rigid insulation was recorded at only 52%. About half of what the manufacturer had published ! Conversely, the EPS sample, which was buried adjacent to the XPS, had only taken on about 5% moisture and retained 94% of the thermal resistance over that same 15 year time. Check it out.
The Coup de Gras—Expanded polystyrene is not only a better rigid board insulation than its cousin XPS, but it’s also a better value. Expanded polystyrene rigid insulation can also help to control material costs. An average of 20% less expensive than XPS. Today, as overall project costs seem to be sky rocketing every little bit helps.
An Interesting Tidbit—The other day, I stopped by a McDonald’s on my way to the office for a cup of coffee. When I was handed my coffee through the drive-up window I couldn’t feel the heat of the coffee through the expanded polystyrene cup it was served in. You’ll remember a few years back, a woman successfully sued McDonald’s after spilling hot coffee on herself, claiming she didn’t know the coffee was going to be so hot. Anyway, I got my coffee and went on my way not really thinking about it. Later that same day I had a doctor’s appointment to remove a little skin thing on my arm. The doctor suggested we try to freeze the skin thing off, so I said, “oakey dokey, you’re the boss.” The doctor left the examining room and returned a few minutes later with an expanded polystyrene cup about half full of liquid nitrogen with a cotton swab in it. The doctor calmly held the foam cup in her hand as she worked on my arm for several minutes. The expanded polystyrene cup, with about 1/8” thick walls, was totally protecting the doctor’s bare hand from this dangerous liquid. She said she could not feel the cold of the liquid nitrogen at nearly 325○ F below zero. It’s amazing that the same expanded polystyrene cup material that Foam Control Plus+ Architectural Grade expanded polystyrene is made of could provide complete thermal protection in these extreme conditions. A temperature swing of about 500○ F, from scalding hot coffee to negative 325○ F.