I have an interest in metal boats and in the 70s-80s it was common to spray foam these vessels. This pretty much always caused premature end of life due to corrosion.
There's a whole class of corrosion in metal structures called Corrosion Under Insulation or CUI. This is a problem for any insulated metal structure, but it is way worse with spray foam because the foam traps moisture very effectively, when it breaks down it becomes acidic, it is difficult to remove for inspection or repair, and it makes hotwork dangerous because it is flammable.
When I started seeing wooden houses and roof decks get spray foamed, I could only think that these structures will likely be rotten and useless in 20-30 years. I would only sprayfoam something if the substrate is completely impervious to moisture, like if it was fiberglass or something. Even then you're likely looking forward to a pretty evil concoction under that insulation in a decade or so.
Some fibreglass boats are still filled with spray-foam, and it has the known effects of eventually causing rot to any wood that might be embedded in the fibreglass. Embedded plywood is much less common nowadays though.
(The foam cores in modern fibreglass construction is PVC-based, moisture safe, and can not be sprayed.)
There's a whole class of corrosion in metal structures called Corrosion Under Insulation or CUI. This is a problem for any insulated metal structure, but it is way worse with spray foam because the foam traps moisture very effectively, when it breaks down it becomes acidic, it is difficult to remove for inspection or repair, and it makes hotwork dangerous because it is flammable.
When I started seeing wooden houses and roof decks get spray foamed, I could only think that these structures will likely be rotten and useless in 20-30 years. I would only sprayfoam something if the substrate is completely impervious to moisture, like if it was fiberglass or something. Even then you're likely looking forward to a pretty evil concoction under that insulation in a decade or so.