I need to replace an external patio French door. I am being quoted for a 92 mm door with 1.3 u-value in glass. Is 92 mm's strong enough for a French door?
There will be a wide range of strength in 92mm doors. You'll need to talk to suppliers and get more details on the materials involved and structure, any reinfocement, etc.
Three different suppliers. One door 56 mm. One 92 mm. One 114 mm. All doors treated Scandinavian pine. I am told as good as Hardwood. Just thinking that the 56 mm. seems a bit on the thin size? Also I understand that the gap in the double glazing glass is recommended to be wider for better insulation. The 56 mm door might not allow for this?
Saying it's like hardwood is meaningless. There's a vast range of density in hardwoods, many are not suitable for outdoor use. You'll see here external joinery isn't listed as a recommended use for pine, but a lot of manufacturers seem to use it, many cladding the external faces in aluminium for protection from the elements in alu-cald windows/ doors. Any ideas how much heartwood they use?
For insulation performance, ask them to give you the U values.
I hear the "higher humidity" argument a lot, usually as an excuse of builders to not use modern building methods (modern in a sense of "used successfully across Europe since the 70ies").
Wooden windows (or any outside, exposed wood for that matter) always need some care, regardless of humidity levels. Of course, many people don't bother and then things start looking bad or rot.
@OP: 92mm is not bad, I wouldn't go below that for patio doors.