phenolic vs wood carb spacer

Canton has one it calls "phenolic". It looks like "wood". But that's what I've been using for many years with no issues. Whatever the actual material is, it doesn't seem porous. I also use the tapered 4-hole figuring it's a good compromise between a straight 4-hole and open spacer. This would probably have the best heat insulating qualities, certainly over aluminum. It would also appear to be less prone to warping from heat than the plastic appearing spacers. Canton also has other styles and materials.

http://www.summitracing.com/parts/ctr-85-158/overview/