2 or 3 inch extruded foam insulation?

[mike_lawyer]

Hey guys, I am starting construction of my new 9x4 foot layout in a bonus room on our second floor.  For this layout, I am planning to use a sheet of plywood on the bottom of the table surface, with extruded foam insulation glued to the top of the plywood.  I can find 2 inch extruded foam insulation fairly easily here at my local hardware stores, but 3 inch is hard to find and I will need to special order it.  I would think 3 inch might be preferable, but does anyone have an opinion on whether two inch or three inch extruded foam is better for creating scenery "under" the main level of the track?  Thanks for any help you can provide!

[Ed Kapuscinski]

You are FAR better off, in my opinion, with two or three layers of 1" foam instead of one layer of thicker stuff.

Thinner stuff is easier to work with when cutting hillsides and other features. Plus it allows you to "build up" to 0 instead of "digging down".

This scene was done this way:



You can see the construction here:


Even using 1" foam, sloping things gently down towards the creek on the more level areas was a pain and required some ingenuity on how to make a hot wire "scoop".

I can't imagine trying to carve all of that down into 2" foam, let alone all of the "waste".

Build yourself some layers my dude.

[Viperjim1]

Not to say much easier to plan your rockwork and mountain in layers and progression.

[oakcreekco]

Planning ahead does help and saves foam too.

I built my 4 x7 using 2" and 1" foam, layered as needed. Bread knife to carve close. When I was happy with the rough in, I put a thin layer of Sculptamold over everything. Love that stuff.

Ain't no real right or wrong, just plan what you want and start adding layers.

Good luck and it's fun.

[ednadolski]

You are FAR better off, in my opinion, with two or three layers of 1" foam instead of one layer of thicker stuff.

What do you use to glue the layers to each other?

Ed  (N.)

[Ed Kapuscinski]

What do you use to glue the layers to each other?

Ed  (N.)

Usually liquid nails for projects, but I just got one of those Ryobi glue guns so I'm gonna try that out.

[nkalanaga]

I used "store brand" Liquid Nails-like stuff, and a lot of nails.  Seriously.  Where I knew I would never have to poke a hole through the foam, I took ordinary wood nails, two inches long or so, and poked them through the foam.  Stuck in at various angles, not straight in, the layers can't come apart.  If they try to slide up one nail, they're going against the ones next to it!

[Bobster]

Good morning,

I always check the liquid nails tube to make sure it says "foam safe" or "for foam".  I failed to check one time.  Melt down.  Fortunately it was just a small section.

My $.02

Bobster

[C855B]

I should probably note here that if you're depending on XPS for any strength characteristics, such as using it for the primary structural surface (not over plywood), 2" or more has to be your choice. 2 x 1" does not have nearly the strength or rigidity of solid 2" regardless of adhesive. This admonition applies to both whole sheets and cookie-cutter applications.

I personally recommend against considering 3" only because it is a special-order item; most suppliers will insist on a full skid as the minimum order.

[Ed Kapuscinski]

I should probably note here that if you're depending on XPS for any strength characteristics, such as using it for the primary structural surface (not over plywood), 2" or more has to be your choice. 2 x 1" does not have nearly the strength or rigidity of solid 2" regardless of adhesive. This admonition applies to both whole sheets and cookie-cutter applications.

I personally recommend against considering 3" only because it is a special-order item; most suppliers will insist on a full skid as the minimum order.

That's a good point. I'd also argue, from experience, that XPS foam is not structurally reliable on anything of any size. Covering a 1x4 box frame? You'll probably be ok.
Anything more? Forgettabout it.

Mostly because it fails in ways you wouldn't expect, like being able to be reliably butt-joined.

[C855B]

The difference really is surprising. You know I've done a little bit of cookie-cutter stuff. 1" is a wet noodle which doesn't change when laminated, 2" is amazingly rigid. As to butt joints with the cookie-cutter work, I'll use 1/4" dowels pointed on one end with a pencil sharpener as pins to secure the join. I really need to demonstrate this, but the next phase of benchwork won't be for several months.

[MK]

Curious as to why pointed on just one end?  How do you insert the blunt end?

[C855B]

Poke holes to the desired half-depth then turn the dowel around, apply adhesive, then push together. That way you know the dowels are even across the join.

[Ed Kapuscinski]

Poke holes to the desired half-depth then turn the dowel around, apply adhesive, then push together. That way you know the dowels are even across the join.

Jeez. Where was THIS idea 10 years ago when I was stringing christmas lights and trying to keep parts of THIS together?
https://conrail1285.com/tag/ncr-shelf-layout/

[wazzou]

The difference really is surprising. You know I've done a little bit of cookie-cutter stuff. 1" is a wet noodle which doesn't change when laminated, 2" is amazingly rigid. As to butt joints with the cookie-cutter work, I'll use 1/4" dowels pointed on one end with a pencil sharpener as pins to secure the join. I really need to demonstrate this, but the next phase of benchwork won't be for several months.

I always made lap joints, typically 1" depth and 2" width. 
This way, there were no issues with carving the foam at some point if necessary and striking a connector of some sort.