The differences between pink and blue XPS foam insulation board aren't many, as they're both essentially the same thing offered by different companies. Owens Corning, known for its rolls of pink fiberglass insulation, makes the pink foam insulation board, while DuPont and Dow manufacture similar products in blue.
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Foam Board Construction
Any insulation labeled as XPS foam board is made from extruded polystyrene, which is where the XPS term comes from. This type of foam board has closed-cell construction and resists moisture no matter which company makes it. Both pink and blue XPS foam boards come in different thicknesses and with different features designed for specific purposes, such as score marks every 16 or 24 inches for ease of snapping the pieces to size for common construction purposes.
As far as their insulating abilities are concerned, pink and blue are identical, with an R-value of 5 per inch of thickness on a piece of new rigid foam board. R-value means resistance to heat flow, which could also be seen as resistance to allow heat to flow through the material. The R-5 rating applies only to new XPS foam board, as it could lose a little of its insulating ability over time.
Extruded polystyrene isn't the same as EPS or expanded polystyrene foam board; instead, it looks like compressed pellets, similar to an inexpensive foam cooler or a to-go coffee cup. The beads or pellets sometimes have tiny gaps between them, and the presence of the beads is obvious just by looking at the foam. This type of foam board could also crumble where you snap it, as would happen if you did the same to a foam cooler.
From Blue to Gray Foam
Dow and DuPont merged at one point a while back and then separated again, and many of the rigid foam board insulation products in blue may have either the Dow or DuPont name on them depending on the manufacturing date. For instance, the Styrofoam Brand Scoreboard Insulation sold by DuPont is a former Dow product. The classic blue rigid foam board offered by Dow and DuPont is being phased out for a gray color in Canada and the United States.
DuPont notes that the new gray shade reduces greenhouse gas emissions as well as the product's potential impact on global warming. In some cases, both blue and gray XPS foam insulation boards may be sold in the same area, as retailers are likely to keep the blue foam board on hand until it sells out. The new gray foam board has the same R-values and moisture resistance as its blue predecessor.