Ply thickness vs. foam thickness

[TJinPgh]

Ok... I would venture to guess that this has been discussed at some point in time in here (or, perhaps even often, not sure) so forgive me for asking again, but, here goes.

Has anybody come up with a general rule of thumb for what thickness of foam you need to give an adequate strength comparison to a solid piece of ply?

For example, I posted a question in the general forum about a build that uses a solid piece of 3/4" ply for the sides, connected to a wall frame with studs that look to be spaced about every 2'. Unskinned on the inside.

If one were going to do a hybrid foam construction, skinned on the inside with luan and the outside with canvas, would 3/4" foam still be sufficient? Or, would it need to be something thicker?

The walls of that particular build were 6.5' high.

[GPW]

JMHO, I’d go thicker !!! Better insulation too ...

[crumbruiser]

I used 1 &1/2 foam with 5mm underlayment inside and the exterior is partly skined with old bed sheets and partly just coated with a spray on undercoating then painted with oil based paint. Mine is about 66 inches inside [high enough for me to stand] about 78 inches wide inside and 8 feet long. I haven't towed it that much, I'm still not finished, but when I have towed it it feels tight, sturdy as well as when inside working on it.
Frank

[atahoekid]

I went with 2" foam with 5mm lauan underlayment on the interior where the wall is not supported by a bulkhead and then a 6oz fiberglass skin. It's not been towed yet and it's not fully assembled yet, but it feels very sturdy. I reinforced the roof panel (also 2" foam) with 1 x 3 lumber on the flat. Seems real solid. Wish I could say I calculated the loads but I've long ago forgotten that skill. I did it with "seems about right" engineering.