Cutting extruded foam board - Model Railroader Magazine

galaxy:

There are small little utility knives that have a blade that is about 4-5 inches long. Designed so that you can break off a piece of blade as the tip gets dull, with a new sharp one and the blade Extended all the way out they should slice right through the rigid foam easily. That is how I cut the foam prior to getting the hot wire cutter.

I've been using the 1/2" and 1" (wide) breakaway utility knives you can get 5 for $1 at the dollar store. You get nice clean cuts and very little debris. You extend them about half-way for 1" foam, all the way out for 2". I also use a couple of filleting knives with 1/2" wide blades for general carving of foam and keep a sharpening stone handy. The real trick is that the blade always has to be razor sharp, you can't cut much with a dull blade and foam will dull a blade pretty quickly.