using Dow Blue extruded polystyrene foam

[RAM]

I have never used Dow Blue extruded polystyrene foam.  I want to use it for part of my upperl level.  This will be either 12 or 16 inches wide.  I have one sheet of 1 inch and two sheets of 1/2 inch.  I was wondering if it would work to make a fram of 1 by 1s with 1/4 inch plywood. then put the foam in that.  Do you mount the raodbed right on top of the foam or do you need a sub roadbed.

[brad]

I used 2 inch foam board for my entire layout with only "L" backets every 16-24 inches and have had no problems. I didn't use a sub roadbed, I merely carved roadbed and ditch profiles into the foam and find it no more noisey than my last layout with a  plywood/homosote/cork sub structure.  Your frame of 1X1's and 1/4 inch plywood should be strong enough, although, I would be tempted to make the 1X1 frame out of 3/4 inch plywood as it is more stable than dimentional lumber and less likely to twist, especially with that small of wood.

brad

[RAM]

correct. the foam is 1 1/2 inch and 3/4 inch.  instead of 1 inch and 1/2 inch.