You are correct that taller beds cool/warm sooner as weather changes and in this case metal sides will farther speed it up.

Thanks.

To those with metal beds, do they warm or cool noticeably faster, assuming you have a basis for comparison? Although metal is a better conductor, the mass of metal is much less than the mass of soil in the bed so I am thinking heat transfer to and from the soil may not be a significant factor. Also the vertical galvanized surface will reflect sunlight especially on the southern and western bed sides thus less heat gain than might be expected

bottom line: i don’t want the convenience of tall beds to come at a cost of unanticipated consequences.

Anyone with a better understanding of thermodynamics than me?

Yes metal will cool faster than wood and taller beds will cool faster than shorter ones. I assume your beds are close to ground level now but lifting them up even if they are full of soil will let in more of the cold because the sides are now exposed to wind and cold weather. It is the same as leaving plants to winter in a planter as opposed to planting them in the soil. You may be able to get away with it if you line the sides of your beds with thick Styrofoam. It will lessen the heat loss from the sides. If you get snow shovel it up against the more exposed sides. Loose snow is a great insulator because of the trapped air inside it

Thank you.

I have thought about the Styrofoam as there will be a 3.5 inch space between the panel supports that will get it out of the way or sorts.

I also cover the current 10 inch tall beds beds with removable plastic like a low tunnel and that keeps the soil from freezing. Perhaps it will be enough to keep the top few inches of a tall bed warm enough to prevent significant freezing and allow me to continue to overwinter kale, spinach and carrots.

Excellent advice. The snow thing sound's counterintuitive but were it not true native people in Alaska would never have survived. During my leisure and gardening, I lean toward common sense and experience rather than dwelling on "thermodynamics" and such. Good stuff to think about if you are trying to invent a better wheel but I'm tired and retired.

As for metal reflecting more solar heat than wood,lay a piece of each in the sun then come back 3 hours later to touch the back side of each to draw your own conclusions. :)

I am curious - why are you doubling the depth of the beds? What are you going to grow?

Only Convenience. Less bending over.