IDRick Respectfully speaking, I did have separation at seams between foam pieces and separation between fascia and foam. The 5x9 benchwork followed the specs for a tabletop layout in Wescott's Model Railroad Benchwork. The top was 1" extruded foam over 1/2' plywood. I followed a MR article on how to build benchwork with a foam top. I don't recall which adhesive was used, it was either Liquid Nails for projects or carpenter glue, probably the LN. The top was weighted with college textbooks and left that way for a week. As far as I can tell, I did everything correctly but still had the separation develop over time. The layout was in a basement with temperature varying only a few degrees throughout the year and low humidity. My bottom line is I want to use foam in my next layout but want to do all I can do to prevent separation along the fascia. I will be building the benchwork in sections and the foam top will be a single piece, thus no separation between foam seams within a section. How do I prevent separation between the foam:fascia and foam:endplates? The anecdotal article (mentioned in OP) suggested that using two foam layers with 90-degree difference in seam orientation would prevent/minimize movement of the foam with benchwork expansion/contraction. Seemed plausible to me but then I'm a dairy scientist not an engineer, carpenter, or experienced layout builder. That's why I asked here...
My bottom line is I want to use foam in my next layout but want to do all I can do to prevent separation along the fascia. I will be building the benchwork in sections and the foam top will be a single piece, thus no separation between foam seams within a section. How do I prevent separation between the foam:fascia and foam:endplates?
The anecdotal article (mentioned in OP) suggested that using two foam layers with 90-degree difference in seam orientation would prevent/minimize movement of the foam with benchwork expansion/contraction. Seemed plausible to me but then I'm a dairy scientist not an engineer, carpenter, or experienced layout builder. That's why I asked here...
A number of years ago I had a 2 inch slab of pink foam shrink on me. It buckled the track that was glued to it. The foam was not on plywood. I'm building a new layout now and as a precaution I'm cutting saw kerfs across it every 2 feet. They are cut about 90% through from the bottom side. The foam is on OSB this time. It may not be the answer but it makes me feel better.
George
A lot of research has gone into foam shrinkage by the modular folks who used framed foam modules for ease of construction and light weight. Over the years, there is mounting evidence that extruded foam does shrink as off-gasses after manufacture. Manufacturers admit it, and I've seen the modules where the glue joint has broken, the foam sheered, or the frame actually became curved where the glue and foam held.
It's not the wood because even 36" long modules using plywood-only frames have experienced the shrinkage pulling things apart while other modules using pine frames stay intact.
Most of the shrinkage occurs in the first 18 months after manufacture, and the majority of that most occurs in the first 6 months. But the cumulative effect can be enough to be noticed (and be destructive), particularly if you bought and used the blue or pink stuff right after it was made.
The only "cure" is to age the stuff before you use it - from experience the foam needs to age for at least 6 months for the shrinkage to be minimal and of no consequence from that point on.
For what it's worth, extruded foam is like many plastics that shrink with age. The cracked plastic gears in our model locomotives are another example of shrinkage with age. The plastic gear shrinks but is blocked by the metal shaft. Eventually the shrinkage stress cracks the gear.
As an aside, that "new car" smell is the plastic in your car off-gassing.
Fred W
Some good points made Fred.
I would say I have to agree with you. Come to think of it I do remember researching extruded foam when I started my layout. I thoroughly went through the Owens Corning specs from their technical data sheet. I do remember reading about shrinkage due to outgassing after it is manufactured. The amount it shrinks is quite minimal.
The reason I never had any problem with foam on my layout is because it sat in my trailer a year after I completed a job before I started my layout.
There are actually building scientists that have devised methods of constructing buildings, commercial or residential. These fundamentals have been mandated into building codes over the years. Expansion and contraction is one of the biggest issues these codes were devised for. Temperature infiltration is another big one but I'm not going to get into that.
Vinyl siding has oval holes in it and roofing nails are used for fasteners not to be driven all the way tight so the siding can float on the side of the building as it expands and contracts from the fluctuation of temperature and air conditions. Extruded foam that is installed underneath the siding has to be installed with plastic ring nails usually manufactured in fluorescent green or orange so the inspectors can see them easily. These Fasteners allow the foam to float underneath the siding as the wood framing the foam is fastened to expands and contracts.
Even OSB roof sheathing is manufactured 1/16 of an inch shy of 8 feet. Sheathing installers tap-in 16 penny nails between the length of the sheets on the roof rafters temporarily as they put the next sheet on to leave an 1/8 gap for expansion and contraction of both the sheathing and roof framing. The inspectors look for the mandatory H-clips that are put between the width of the oriented strand board sheets installed so the roof sheathing does not buckle under temperature fluctuation.
I may be a little anal retentive sometimes but I built an inverted quarter inch Luan criss-cross frame for my layout benchwork. I used 90-degree elbow brackets and anchors in the foam for the fasteners so the foam can float on the bench work. The benchwork for our layouts is a much smaller scale as buildings but still has the same factors involved.
The crib was a good idea and has worked out well. Cantilevering it from the wall was a very bad idea. I had commercial slides so I could slide it two feet out from the wall, duck underneath and work on the other side of the layout. I got sick of ducking under it and nothing ever got done on my layout. I later built different benchwork on wheels that should have been done in the first place.
I have been known to think backwards at times but it seems to me the fascia of a layout is most firmly attached to the sides of the benchwork frame or crib as some call it, and the foam floats in between. If the benchwork crib contracted, the fascia would be pulled tighter to the foam. If the benchwork crib expanded, the fascia would be pushed away from the foam leaving a gap.
Good hardwood floor installers know the winter time is the best time to install a hardwood floor when the air is dry and after the flooring acclimates to the air in the room. Installing a floor when it's humid, the floor shrinks the next winter leaving gaps.
Expansion and contraction are definite considerations to consider when building a layouts benchwork.
Especially Shrinkage
TF
IDRickshould I use a Kreg pocket jig or an air nailer on the butt joints of the open grid?
The true key to rigid benchwork of superior strength is WOOD GLUE! Properly executed glue joints will actually be stronger than the wood it joins when cured. Wire brads or screws alone can never equal the strength and rigidity of glue joints. However, the brad nailer will prove to be a significantly faster means of assembling your benchwork. Put glue on the wood where it will create a joint and nail it together with the brad nailer. Just don't put ant strain on the brads until the glue cures. This is how I assembled my around-the walls single and double cantilevered benchwork with ope spans as long as 13 feet. No sagging or warping in over 10 years!
Hornblower
We are really talking two types of foam, extruded (pink and blue stuff) and expanded (beaded stuff). The extruded stuff will shrink much more than the beaded once cured. Seen the pink stuff curl up a bit even when held flat, not so with beaded stuff. All foam shrinks when first manufactured, once cured shrinkage is min. on beaded, not so much on extruded. With expanded this is due to shrinkage happening right after manufacture and many places age there stuff for a few days to make sure. Extruded has a more gradual process.
Lastspikemike Yes we are but extruded and bead are both closed cell foam structure. To shrink the sheets then the cells themselves must be gas permeable which seems odd to me. Extruded just creates the closed cells in continuous flow rather than popping up the individual cells and gluing them together. Conceptually I mean. The bead style are just expanded foam pieces stuck together after popping up and structurally the same as extruded, within each bead. https://www.buildingscience.com/sites/default/files/migrate/pdf/PA_Foam_Shrinks_Correction.pdf Newer foams are less gassy. Aged in storage for three months makes a difference apparently.
Yes we are but extruded and bead are both closed cell foam structure. To shrink the sheets then the cells themselves must be gas permeable which seems odd to me.
Extruded just creates the closed cells in continuous flow rather than popping up the individual cells and gluing them together. Conceptually I mean. The bead style are just expanded foam pieces stuck together after popping up and structurally the same as extruded, within each bead.
https://www.buildingscience.com/sites/default/files/migrate/pdf/PA_Foam_Shrinks_Correction.pdf
Newer foams are less gassy. Aged in storage for three months makes a difference apparently.