2 or 3 inch extruded foam insulation?

[davefoxx]

I generally go with 2" foam, because I like the rigidity and less seams when stacking for terraforming.  To me, there's not much difference in carving 2" foam over the 1" foam, because I build up the landforms first and carve the final shape after the adhesive (LokTite Power Grab) has cured.

DFF

[nickelplate759]

I used 2" foam for my last layout, supported on a wood grid (no plywood underlayment).  It was plenty strong once the foam was glued to the grid - until then everything was pretty wobbly.  The longest unsupported span for the foam was about 18".

Downsides were:
1: 2" foam isn't easy to cut cleanly without making an awful mess.
2:  the foam I used (Blue, Dow I think) was not entirely uniform in thickness, so my "flat" surfaces undulated slightly - maybe only +/- 3/16", but enough to matter.
3: it's not practical to mount things like Tortoise motors and circuit boards directly to the foam.  I glued small pieces 1/8" plywood to the underside of the foam where I needed to do that.

[C855B]

Downsides were:
1: 2" foam isn't easy to cut cleanly without making an awful mess.

Proper tool is a sabre saw with a foam blade. These blades are a wavy knife that resembles a bread knife. Zero dust: https://www.therailwire.net/forum/index.php?topic=31540

2:  the foam I used (Blue, Dow I think) was not entirely uniform in thickness, so my "flat" surfaces undulated slightly - maybe only +/- 3/16", but enough to matter.

I do not recommend "blue" foam. Aside from the poor dimensional uniformity, it is considerably less rigid than the same thickness in "pink" (Foamular) XPS.

3: it's not practical to mount things like Tortoise motors and circuit boards directly to the foam.  I glued small pieces 1/8" plywood to the underside of the foam where I needed to do that.

I mount circuit boards to the framing. I use servos for switch motors that are attached to the track: https://www.therailwire.net/forum/index.php?topic=40197

[nkalanaga]

I used 1 inch blue, but haven't had any problems, probably for three reasons:

I used a lot of joists under it, and
I overlapped the few joints, and
most of the areas with more than just a mainline aren't on foam.  The mainline that is is also in balsa-strip "spline" roadbed, on quarter-inch spacers above the foam.

Much of my layout is narrow enough that the are no joints.

[motofavorite]

Is 1/4-inch plywood sufficient beneath 1-or 2-inch pink foam for a rigid 12" cantilever?

[nkalanaga]

For a 12" span it would probably be enough.  Anything beyond that I'd probably go with half inch.

[PRB]

Guys,  I use 3M Polystyrene 78 Spray Adhesive. It's made for this application and works great for me.

[aikorob]

around here, the DIY stores only carry 1".
For 2", there are masonry specialty places that carry it as insulation on the outside basement walls.
If you really want the 3"---someplace like that may work-------depending on your area

[porkypine52]

I'll use any type of foam board that I can get my hands on.  1"  2"  or 3" foam board when available.  I will use WHITE expanded/beaded foam for places that won't be seen.  Just as a filler.  Check around your local building site for leftover pieces.  Ask Foreman for permission to take.   I have traded a little site pickup time [weekends] for foamboard scraps.  White beaded foam can be had at companies that deal with shipping LARGE delicate items--piano stores and appliance dealers come to mind.  YES  white foam pieces can be hassle to cut, beads go EVERY WHERE, so almost all cutting is done OUTSIDE. Again since I am only using the white beaded foam to occupy space, not for scenery carving, who cares?    I have a HOT-WIRE foam cutting tool that does the job.
When buying CUSTOM ORDERED foam board sheets:  Many places, one pallet load is the minimum order.  Make sure you are talking about the right QUANITY.