Best glue for styrofoam?

My hot tub cover (styrofoam core) is shot. I’m reluctant to pay the $600+ for a replacement, so I thought I’d make my own. Unfortunately 4" thick foam costs a fortune, so I plan on laminating inexpensive Lowes 3/4"x2’x4’ styro insulation in an effort to fab my own thick slab. Any suggestion for the best adhesive?

BTW: WTF with my old cover? The styrofoam is completely waterlogged, it weighs a ton! I thought styrofoam was water impermeable.

Great Stuff spray foam.


For future reference.

Glue advice.

Hot melt is good for gluing foam, be careful your not using recycled foam or it will readily soak up water.

Agree. I recently had my crawlsace encapsulated and the process included styrofoam rigid panels installed at the interior crawlspace walls above grade and it was attached using Great Stuff spray foam. This stuff sticks to almost anything!

It does indeed stick to anything. Recommend some throw away latex gloves. Buy em by the box.

Since the OP is trying to laminate styrofoam, there might be an issue using spray foam. It expands when it dries (this makes it great for filing gaps in insulation). Two pieces of 2" stryrofoam might end up being 4-1/2" inches thick. As always, test it first to make sure it doesn’t screw with your dimensions.

any non-solvent glue should work OK. hot melt glue would work, as would urethanes like Gorilla Glue. you could even use 5 minute epoxy, which is what I would do. cures fast w/o air and doesn’t expand.

I would imagine silicon or similar tubes of construction adhesive would work well, depending on whether you want a full area gluing or just to ensure the two pieces stick together. Anything used with a caulking gun has the thickness and volume to adhere to foam; just be sure it does not dissolve the foam. The glue tube should indicate if it works with foam.

Water based contact adhesive works well. But there is no re-positioning, once it’s in place it’s staying there.

And make sure to get the water based, the solvent based will eat the foam.

Pretty much anything that doesn’t dissolve the foam will work. Even some solvents can be used, for instance a light spray of 3M 77 can be used to laminate foam, but a little too much will just start dissolving the foam and leave a void between the sheets, it requires a deft hand and experience.

For the OP’s situation the construction glues meant for use with polystyrene will work, I’d squeeze some thin lines out of the caulk gun and spread and smear them with a notched trowel, but a flat putty knife should do fine also. Water based epoxys should work also, like 5 minute or 1 hour epoxy. Simple white school glue works just fine also.

For some background, we’re talking about expanded and extruded polystyrene. Styra-Foam is a trade name for a variety of expanded polystyrene. Expanded polystyrene is made by expanding little beads of polystyrene using various methods. It is not all that waterproof, it leaves lots of little voids in the material where water can seep in and slowly degrade the material by separating the beads so it is no longer solid. Extruded polystyrene is a much better construction material, still slightly porous but will degrade less over time, and it is also stronger and more dimensionally stable. Insulation sheets are available in the extruded form, and for the OP’s situation is the best approach.

Note that hot-melt glue is a really bad foam adhesive. It’s too hot, and melts the foam.
Cool-melt glue is better, although it is often too hot, too.
Also hot glue is applied in a then line, which is tedious when gluing large sheets.

“project” adhesive tubes at the home supply store of your choice should list if they are good for polystyrene foam insulation. They won’t have the solvents that can dissolve the foam.

Thanks to all for the suggestions, especially beowulff 's link. From that, it appears Glidden Gripper Primer is best suited for my application.