Thanks for taking the time to

Thanks for taking the time to report your findings.  just imagine how much ply would have moved around over 3 years!

You're welcome. I made the

You're welcome. I made the error years ago of not putting a gap between the plywood sections of my previous layout. It had some spots that bowed a tad upward. I hung weights underneath those spots to level them. At least I had gaps in my rails. 

I'm glad the foam didn't shrink in depth. 

Plywood movement

I'm amazed that plywood bowed on you, as that is supposed to be one of the benefits of plywood the grain of the different layers running at right angles to each other make it dimensionally stable. Of course you still need proper structure under the ply, I don't know what the spacing should be between "joists." For buildings they are typically 16" or 24", would expect for modules you could go more distance, but you definitely need attachment at the edges of the piece.

Of course all bets are off if it gets wet and delaminates.

Foam Cell

  At temps below 60 degrees, the foam cells can collapse and you will see shrinkage.  I am sort of amazed that you had problems with the plywood.    I used Baltic Birch and had no issues(but my basement never gets below 68 degrees/43% humidity, and most of the time is at room temp like the upstairs.

  Baltic Birch is very stable: and the amount of 'dimensional' lumber one can get out of a sheet, it costs less than buying 'premium' dimensional lumber.  I buy mine at Home Depot, and have them cut it up a 4 by 8 sheet of 3/4" material into 1x4 and 1x2 lumber.  They charge 25 cents per cut - Still very inexpensive.

Jim

Jim B, I still prefer

Jim B,

I still prefer plywood. My previous layout it bowed where the sheets met especially the area water from ballasting got into the ply.  It also put a dent in both walls; I had the plywood in too snug between the walls.

Jim, The foam was in storage

Jim,

The foam was in storage for over 3 years and was exposed to temps ranging from 55 to 85F. However it didn't shrink in thickness. I still prefer plywood though and the next time I use it I will seal it, allow some gaps, and not put it in so snugly between walls. My inglenook is made out of plywood and is sturdy.

Quality = predictability

Thanks Joe, "and plaster

Thanks Joe,

"and plaster drywall (flat areas sandwiched with plywood)."

This sounds interesting, but I don't quite understand.

Taking a closer look at the

Taking a closer look at the foam, I noticed didn't shrink in length as much as I thought. 

In thickness, none as seen here, as it is still flush with the fascia:

In length, just a hair, as seen here at the ends of the modules:

...and the middle parts of the second module, but keep in mind this is where I joined foam together and filled the joined area with spackle; it seems the fill cracked more so than the foam shrinking:

However the homosote roadbed that was glued to the foam with caulking was not affected.

Almost 8 years later, and I

Almost 8 years later, and I noticed just a slight shrinkage in the foam. I noticed it where the track crossed the joints of the module. The joint in plywood.

Good news is, it only shrank by about 1/64 of an inch. One only notices it at eye level if a stiff steam loco speeds over at unrealistic speeds for a branch line. 

Very good to hear!

Thanks Dee. This is an ongoing question in MR. You already know what I'm going to ask. Did you paint it? Did you paint the plywood? What caulk did you use? Lot's of different ways to do what you did and that is what makes or breaks the modules. I do note that your Homasote was not affected. I didn't remember you used Homasote on the new module with the bridge, etc.

The other questions are the

The other questions are the environment. What is the temperature range in the room? does it ever sit in direct sunlight etc etc.

Morgan, On the first two

Morgan,

On the first two modules the foam was only painted on the top side. Only the exposed parts of the plywood were painted. Homasote is in excellent shape.

The bottom sides of the foam were glued to hollow core doors, plus some fasteners in crucial places like around the turntable area.

Yo ho,These modules have

Yo ho,

These modules have experience four moves and has been in storage two times and have been in temps ranging from a hot 40 C to a near freezing 0 C.  They also have been in humidity as high as 60 and low as 27.

https://model-railroad-hobbyist.com/node/39987?page=3

One module also fell off the ramp of a truck.

And has been exposed to direct sunlight for about 4 hours of the day.

Image of cross section. This

Image of cross section. This is a piece I had to cut off to make the layout fit in a smaller room.

And an image showing where the modules meet.

Basically because of the

Basically because of the shrinkage on both side of the two 3/4 inch ply ends, I have a in HO scale a 11 foot long hump that is 2 inches in height at its peak, as there is a grade on both sides to this height.

I've had my railroad running for 5 years now....................

2" foam board over steel studs. Foam board glued to the steel studs and the cork roadbed glued to the foam with LocTite Powergrab (not the heavy duty stuff).  Tomorrow will be my 50th operating session.  No problems with the track work (or benchwork) at all. 

Another well known layout is Bill Darnaby's Maumee, and he pretty much pioneered the use of 2" foam.  His railroad has now hosted well north of 300 operating sessions.  Still runs outstanding. 

I'm more worried about wearing out my motive power and equipment than I am about foam shrinkage.

Regards,

Jerry

Jerry, Any pics to share?

Jerry,

Any pics to share?

I filed the rail where the

I filed the rail where the hump was; I filed the 1.5 inch area down to match both sides of the area.

Then sanded it using a piece of hard wood with sandpaper on it then polished the rail. 

No more hump in the area where the modules meet.

16 years and no issues

I’ve had the layout up since Fall 2005 and used 2” foam with no issues. The tops are painted and have scenery.  I didn’t glue the foam to the plywood bench work but it’s held in place by the backdrop sitting on the back and the facia on the front. If it’s changed dimensions it hasn’t affected the track work. 

Thanks for sharing Charlie.  

Thanks for sharing Charlie.